Cub Cadet MTD 90 User Manual

Professional Shop Manual

78/83/90 Series Horizontal Shaft Engines

(277/357/420 cc Engines)

NOTE: These materials are for use by trained technicians who are experien ced in th e service an d re pair of outdoo r po wer

equipment of the kind described in this publication, a nd are n ot intende d for use by un trained or ine xper ien ced individu als. These materials are intended to provide supplemental information to assist the trained technician. Untrained or inexperienced individuals should seek the assistance of an experienced and tr ained p rofessional. Read, understan d, and follo w all instructions and use common sense when working on power equipment. This includes the contents of the product’s Operators Manual, supplied with the equipment. No liability can be accepted for any inaccuracies or omission in this publication, although care has been taken to make it as co mpl ete and accu rate as possible at the time of publication. However, due to the variety of outdoor power equipment and continuing product changes that occur over time, updates will be made to these instructions from time to time. Therefore, it may be necessary to obtain the latest materials before servicing or repairing a product. The company reserves the right to make changes at any time to this publication without prior notice and without incurring an obligation to make such changes to previously published versions. Instructions, photographs and illustrations used in this publication are for reference use only and may not depict actual model and component parts.

Table of Contents

Chapter 1: Introduction

Professional Service Manual Intent. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Fasteners. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Assembly instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3

Model and serial number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Air filter (summer engines) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Oil type and capacity. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Changing the oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Fuel filters . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Valve lash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Cleaning the engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13

General torque specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Maintenance Chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .14

Frequently used specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .15

Chapter 2: Basic Troubleshooting

Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Steps to troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Define the problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Identify factors that could cause the problem . . . . . . . . . . . . . . . . . . . . . . . . 18

Repairing the problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23

Prime test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24

Leak-down test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .24

Compression test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

PCV testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

Chapter 3: Air Intake systems

Heat box (snow engines) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29

Air filter (summer engines) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30

Air filter base and control panel (summer engines) . . . . . . . . . . . . . . . . . . . 32

Carburetor and Insulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34

Chapter 4: The Fuel System and Governor

Inspecting the fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37

Test fuel for alcohol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38

Fuel tank vent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 39

The fuel filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

Inspect the fuel lines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 40

The fuel tank . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

Evaporative (EVAP) emissions system . . . . . . . . . . . . . . . . . . . . . . . . . . . .42

Troubleshooting the EVAP System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .44

Charcoal canister . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

Roll over valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

Choke . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

Primers (snow engines) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50

Carburetors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Inspecting the carburetor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Disassembly and rebuilding of the carburetor . . . . . . . . . . . . . . . . . . . . . . .52

Fuel shut-off valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .56

Governor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Governor arm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 58

Governor shaft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 59

Governor cup and the governor gear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 60

Governor adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 61

Chapter 5: Lubrication

Oil type and quantity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63

Oil dip stick . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .64

Dip stick tube removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65

Lubrication system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 66

Positive crankcase ventilation valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 67

Chapter 6: Starter and Charging Systems

Recoil Starter Removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69

Starter Cup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 70

Starter Rope . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71

Starter pulley and recoil spring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73

Electric starter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75

Charging system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77

Charging system testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78

Stator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79

Chapter 7: Ignition System

Troubleshooting the ignition system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81

Stop switch . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .82

Remote (ignition) stop switch (snow engines) . . . . . . . . . . . . . . . . . . . . . . . 83

Test for ignition that won’t turn off . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84

The module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 85

Module removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86

Installing the module and setting the air gap . . . . . . . . . . . . . . . . . . . . . . . . 87

Flywheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 88

About the spark plug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Cleaning the spark plug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Inspection of the spark plug. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Spark plug removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 89

Chapter 8: Exhaust

Summer engines . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Spark arrestor (if equipped) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91

Muffler removal/replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 92

Muffler removal/replacement (snow engines) . . . . . . . . . . . . . . . . . . . . . . . . 94

Chapter 9: Cylinder Head

Cylinder head removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97

Cylinder head installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100

Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101

Chapter 10: Crankshaft, piston and Connecting Rod

Crankshaft inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108

Piston Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .109

Connecting rod inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

Cylinder inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111

Balance Shaft (483 & 490) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112

Reassembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113

Engine specifications chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 115

Engine torque values chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 119

Chapter 11: Failure Analysis

Abrasive Ingestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

Insufficient lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 124

Engine Overspeed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 125

Overheated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126

Mechanical Breakage/ Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

Detonation/preignition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127

III

Introduction

CHAPTER 1: INTRODUCTION

Professional Service Manual Intent

This manual is intended to provide service dealers with an introduction to proven diagnostic and repair proce-

dures for 78/83/90 series MTD horizontal shaft engines. Disclaimer: The information contained in this manual is correct at the time of writin g. Both the prod u ct an d th e inf or -

mation about the product are subject to change without notice. About the text format:

NOTE: Is used to point out information that is relevant to the procedure, but does not fit as a step in the proce -

dure.

• Bullet points: indicate sub-steps or points.

! CA UTION! CA UTION

! WA RNI NG! WA RNI NG

! DANGER! DANGER

1. Numbered steps indicate specific things that should be done, and the order in which they should be done . 1a. Substeps will be lettered and nested within steps. Two or more substeps may be combined to describe

the actions required to complete a step.

Disclaimer: This manual is intended for use by trained, professional technicians.

• Common sense in operation and safety is assumed.

• In no event shall MTD be liable for poor text interpretation or poor execution of the pro cedures described

in the text.

Caution is used to point out potential danger to the technician, operator, bystanders, or surrounding property.

Warning indicates a potentially hazardous situation that, if not avoided, could result in death or serious injury.

Danger indicates an imminently hazardous situation that, if not avoided, will result in death or serious injury. This signal word is to be limited to the most extreme situations

• If the person using this manual is uncomfortable with any procedures they encounter, they should seek

the help of a qualified technician or MTD Technical Suppor t.

Safety

This Service Manual is meant to be used along with the Operator’s Manual. Read the Operator’s Manual and familiarize yourself with the safety and operational instructions for the equipment being worked on. Keep a copy of the Operator’s Manual for quick reference. Operator’s manuals may be viewed for free at the brand support website. It will be necessary to have the complete model and serial number for the equipment.

78/83/90 Series Horizontal Shaft Engines

• Be prepared in case of emergency:

! CAUTION! CAUTION

Keep a fire extinguisher nearby Keep a first aid kit nearby Keep emergency contact numbers handy

• Replace any missing or damaged safety labels on shop equipment.

• Replace any missing or damaged safety labels on equipment being serviced.

• Grooming and attire:

! WARNING! WARNING

! CAUTION! CAUTION

Do not wear loose fitting clothing that may become entangled in equipment. Long hair should be secured to prevent entanglement in equipment.

Jewelry is best removed.

• Protective gear: includes, but is not limited to

Clear eye protection ................................ while working around any machinery

Protective gloves ..................................... where necessary

Armored footwear.................................... when working around any machinery

Hearing protection ................................... in noisy environments

Chemically resistant gloves..................... when working with chemicals or solvents

Respirator................................................ when working with chemical or solvents

Appropriate tinted eye protection............. when cutting or welding

Flame resistant headgear, jacket, chaps. when cutting or welding

• Remember that some hazards have a cumulative effect. A single exposure may cause little or no harm, but continual or repeated exposure may cause very serious harm.

• Clean spills and fix obviously dangerous conditions as soon as they are noticed.

! DANGER! DANGER

• Lift and support heavy objects safely and securely.

• Be aware of your surroundings and potential hazards that are inherent to all power equipment. All the labels in the world cannot protect a technician from an instant of carelessness.

• Exhaust fumes from running engines contain carbon monoxide (CO). Carbon monoxide is a colorless odorless gas that is fatal if inhaled in sufficient quantity. Only run engines in well ventilated areas. If running engines indoors, use an exhaust evacuation system with adequate make-up air ventilated into the shop.

Introduction

Fasteners

• Most of the fasteners used on the MTD engine are metric. Some are fractional inches. For this reason, wrench sizes are frequently identified in the text, and measurements are given in U.S. and metric scales.

• If a fastener has a locking feature that has worn, replace the fastener or apply a small amount of releasable thread locking compound such as Loctite® 242 (blue).

• Some fasteners, like cotter pins, are single-use items that are not to be reused. Other fasteners such as lock washers, retaining rings, and internal cotter pins (hairpin clips) may be reused if they do not show signs of wear or damage. This manual leaves that decision to the judgement of the technician.

Assembly instructions

• Torque specifications may be noted in the part of the text that covers assembly. They may be summa- rized in tables along with special instructions regarding locking or lubrication. Whichever method is more appropriate will be used. In many cases, both will be used so that the manual is handy as a quick-reference guide as well as a step-by-step procedure guide that does not require the user to hunt for information.

• Lubricant quantity and specification may be noted in the part of the text that covers maintenance, and again in the section that covers assembly. They may also be summarized in tables along with special instructions. Whichever method is more appropriate will be used. In many cases, the information will be found in several places in the manual so that the manual is handy as a quick-r eference g uide as we ll as a step-by-step procedure guide that does not require the user to hunt for information.

• The level of assembly instructions provided will be determined by the complexity of reassembly, and by the potential for damage or unsafe conditions to arise from mistakes made in assembly.

• Some instructions may refer to other parts of the manual for subsidiary pr ocedures. Th is avoids repeating the same procedure two or three times in the manual.

78/83/90 Series Horizontal Shaft Engines

Model and serial number

The model and serial number can be found on a white sticker with a bar code. The sticker is located on the right side of the engine at the bottom of the block. See Figure 1.1.

NOTE: The serial number will always start with the

model number.

MTD H oriz ontal Engine Model Desi gna tors

Front of engine

Model /serial number

Figure 1.1

Starter/Alternators

1=Recoil st art 2=Electric start 3=E. start/alt. 20W/20W 4=E. start/alt. 3A DC/5A

Bore Dia. (mm)

MTDEngineSerialNumbers

1P65FH/0510271A0023

1 6 1 - S H A

Major Revision Change

Compliance

U United States (50 State) H Europe C California 0 (Zero) 49 State G U .S.(49) and Europe W U.S.(50) and Europe

End Product

C Chipper/Shredder J Snow/No tank L L ogsplitter R T iller (slow reverse) SSnow T Tiller V Verticutter

Modelnu m b er

Mont hYea r

ProducingLine #

Date

Engi ne number

A=1stshift

sh i f t

B=2

Maintenance

Introduction

The information in this manual applies to the MTD engine. Some basic principles may apply to engines produced by other manufacturers.

As the saying goes “an ounce of prevention is worth a pound of cure”. The same can be said about preventive maintenance on outdoor power equipment. By changing the spark plug and oil at recommended intervals many failures can be avoided.

NOTE: Please refer to Chapter 7: Ignition for the complete

service instructions on spark plugs.

1. The spark plug used in the MTD engine is a F6RTC gapped to 0.026” - 0.030” (0.65 - 0.75 mm). See Figure 1.2.

Figure 1.2

2. Wear rate will vary somewhat with severity of use. If the edges of the center electrode are rounded-off, or any other apparent wear / damage occurs, replace the spark plug befo re operating failure (no start) occurs.

3. Cleaning the spark plug: NOTE: MTD does not recommend cleaning spark plugs. Use of a wire brush may leave metal deposits on the

insulator that causes the spark plug to short out and fail to spark. Use of abrasive blast for cleaning may cause damage to ceramic insulator or leave blast media in the recesses of the spark plug. When the media comes loose during engine operation, severe and non-warrantable engine damage may result.

4. Inspection of the spark plug can provide indications of th e op er a ting c on di tion of th e en gine .

• Light tan colored deposits on insulator and electrode s is nor mal.

• Dry , black deposits on the insulator and electrodes indicate an over-rich fuel / air mixture (too much fuel or

not enough air)

• Wet, black deposits on the insulator and electrodes indicate the presence of oil in the combustion cham-

ber.

• Heat damaged (melted electrodes / cracked insulator / metal transfer deposits) may indicate detonation.

• A spark plug that is wet with fuel indicates that fuel is present in the combustion chamber, but it is not

being ignited.

NOTE: The F6RTC plug is the only plug that is EPA certi-

fied for the MTD engine.

78/83/90 Series Horizontal Shaft Engines

Air filter (summer engines)

Generally air filters come in two different types, a pleated-paper element or foam. A combination of the two are

used on the MTD engine. See Figure 1.3.

1. The main function of the air filter is to trap air borne

particles before they enter the engine . Dirt ingestion can cause serious internal engine damage.

NOTE: Snow engines do not have air filters

because the snow will plug up the filter . Gen erally the air is clean enough that there is minimal risk of dirt ingestion when the ground is covered with snow.

2. Air filters used on the MTD engine are designed to

prevent particles larger than 3-5 micron from pass ing through into the engine.

3. The filter should be checked on a regular basis pos-

sibly several times in a season.

4. Typically an air filter should be changed before every season.

5. If a foam air pre-cleaner is dirty, but not in bad of condition it can be cleaned and reused. The paper pleate d fil-

ters can be shaken or lightly tapped to free the debris from the filter. NOTE: Never use compressed air on a paper air filter . Compressed air will remove the tiny fibers that are used

to catch the dirt in the air. Without these fibers the filter is useless.

Paper-pleated element

Foam pre-filter

Figure 1.3

6. Foam pre-filters can be washed in warm soapy water.

NOTE: When drying a foam filter either squeeze it in side of a paper towel or let it air dry DO NOT wring it

because the filter will tear.

7. Before installing any foam filter, after it has been washed, it needs to be free of moisture.

NOTE: Always check with factory specification prior to servicing/replacing any engine components. NOTE: Do not oil the foam pre-filter. The paper filer will absorb the oil and it will become plugged.

Introduction

Oil type and capacity

MTD engines use oil with a SF/SG API rating or better is the recommended oil for this engine. The winter engines use SAE 5W-30 oil and the summer engines use SAE 10W-30 oil. The oil capacity for all of the 78/83/90 series engines is 37 fl.oz (1.1 liters).

• Check the oil level frequently and change the oil more frequently in severe operating conditions such as exceptionally deep snow falls.

• Synthetic oil is a suitable alternative, but it does not extend service interva ls.

NOTE: MTD recommends the use of petroleum oil during the break in period to ensure the piston rings cor-

rectly break in.

• Synthetic vs. Petroleum based oil: To simply look at synthetic oil and to compare it with Petroleum based oil there is very little difference. However, when you look at the two thro ugh a microscope it is easy to see the difference. Synthetic is made up of smaller molecules. This allows the oil to get into areas that petroleum based oil cannot.

• No oil additives or viscosity modifiers are recommended. The performance of a good oil meeting the API specifications will not be improved by oil additives.

NOTE: Some oil additives may cause severe and non warrantable engine damage, constituting a lubrication

failure.

NOTE: If the oil is noticeably thin, or smells of gasoline, a carburetor repair may be needed before the engine

can be run safely.

1/4 turn

Figure 1.4

Threaded

NOTE: There are two types of dip sticks that can be found

on MTD engines; a threaded dip stick that was used on older engines and a quarter turn dip stick that is used on engines currently being produced. See Figure 1.4.

78/83/90 Series Horizontal Shaft Engines

To check the oil with a threaded dip stick:

1. Twist and remove the dip stick from the engine.

2. Clean the oil off of the tip of the dipstick.

3. Re-insert the dipstick without threading it in to get the oil level reading.

4. The oil level is determined by the lowest point on the dipstick that is completely covered with oil.

To check the oil with a 1/4 turn dip stick:

1. Twist and remove the dip stick from the engine.

2. Clean the oil off of the tip of the dipstick.

See Figure 1.5.

Do not thread dipstick in to check the oil level

Figure 1.5

Fully seat the dip stick before reading it

3. Re-insert the dipstick and turn it until it is fully seated to get the oil level reading.

4. The oil level is determined by the lowest point on the dipstick that is completely covered with oil.

Changing the oil

NOTE: If the engine has been running, allow the

engine to cool before doing any mainte nance work.

NOTE: The oil should be changed after the first 5 hours of operation and every 50 hours there after.

1. Place a suitable drain pan under the drain plug to collect the oil.

2. Drain the oil by removing the drain plug located at the end of the extension pipe threaded into the base of the engine, using a 10mm wrench. See Figure 1.7.

3. When all of the oil has drained out, reinstall the drain plug. Tigh te n the d rain p l ug to a torq ue of 1 06124 in-lbs (12-14 Nm).

See Figure 1.6.

Figure 1.6

Oil drain

4. Fill the engine with 37 fl.oz (1.1 liters) of fresh, clean oil that is the appropriate weight for the application.

5. Safely dispose of the used oil according to the local laws and regulations.

Figure 1.7

Introduction

Fuel

Gasolines currently on the market are not pur e gasoline. Today’s fuels have alcohol and other additives in them to reduce emissions. The fuel make up can vary seasonally and geographically.

Fuel with alcohol added to it is sometimes referred to as “oxygenated fuel”. The extra oxygen carried by the ethanol increases the oxidation of the fuel. This speeds up the process that causes the fuel to go bad.

Excessive alcohol in fuel creates a lot of problems for gasoline engines. One of the biggest problems is that alcohol attracts and holds water. This corrodes the metal components of the fuel system, especially the carburetor. Alcohol also does not produce as much heat as gasoline when burnt. This results in less power for the en gine.

A 10% alcohol mix (E10) is acceptable for MTD engines. Anything higher than that will result in performance issues.

NOTE: E85 and E20 fuels are not to be used in any MTD engines. NOTE: Use clean, fresh fuel with a pump octane rating of 87 or greater.

• Stale or out-of-date fuel is the leading cause of hard starting issues.

• In areas that have high amounts of alcohol in their fuel, high oct ane fuel may imp rove engine performance and startability.

Fuel filters

fuel filter

Nipple

Figure 1.8

Dirty fuel can clog the carburetor and introduce abrasive materials into the engine. To help prevent that, MTD engines are equipped with a fuel filter . The fuel filter is p art of the fuel tank nipple.

See Figure 1.8.

78/83/90 Series Horizontal Shaft Engines

To replace the fuel filter:

To avoid personal injury or property

! WARN IN G! WARN IN G

flammable and the vapors are explosive. Serious personal injury can occur when gasoline is spilled on yourself and/or your clothes which can ignite. Wash your skin and change clothes immediately.

1. Siphon the fuel out of the fuel tank.

2. Remove the front fuel tank shroud using a 10 mm wrench.

3. Remove the fuel line from the fuel tank nipple: 3a. Squeeze the tabs on the fuel line clamp with a

pair of pliers while sliding the clamp down the fuel line away from the nipple.

1.10.

damage, use extreme care in handling gasoline. Gasoline is extremely

See Figure 1.9.

See Figure

Front fuel tank shroud

Remove these

screws

Figure 1.9

Fuel filter

3b. Gently work the fuel line off of the nipple.

Residual fuel in the fuel tank will

! WARN IN G! WARN IN G

mended to help prevent gasoline from splashing into your eyes.

4. Remove the fuel tank nipple using a 17 mm wrench.

5. Install a new filter by following the above steps in reverse order.

NOTE: Apply a small amount of releasable thread locking compound such as Loctite® 242 (blue ) an d tigh ten

come out when the fuel line is removed. Safety goggles are recom-

the filter by hand and then an additional 3/4 to 1 full turn to compress the gasket.

Fuel line clamp

Figure 1.10

Introduction

Valve lash

V alve lash is the clearance between the top of the valve stem and the rocker arm. The valve lash should be checked after the first 25 hours of use and ever y 100 hour s a f ter th at. Valve lash can be checked and adjusted using the following steps:

Muffle

Spark plug socket

Valve cover

Figure 1.11

Breather

hose

spring clamp

1. If the engine has been run, allow it to cool thoroughly. Position the equipment for easy access to the cylin der head.

2. Disconnect the high-tension lead from the spark plug and ground it well away from the spark plug hole.

3. Remove the spark plug using a 13/16” or 21mm wrench. A flexible coupling or “wobbly” extension may help.

4. Disconnect the breather hose from the valve cover. See Figure 1.12.

5. Remove the five bolts that secure the valve cover using a 10mm wrench. Remove the valve cover from the engine.

NOTE: If care is taken not to damage the valve cover gas-

ket, it can be re-used.

See Figure 1.11.

Figure 1.12

78/83/90 Series Horizontal Shaft Engines

6. Slowly pull the starter rope until air can be heard coming out of the spark plug hole.

7. Confirm that the piston is at Top-Dead-Center on the compression stroke.

• The compression stroke can be distinguished

from the overlap stroke by the presence of air pressure at the spark plug hole and the fact that neither of the valves should move significantly on the compression stroke.

• There is an automatic compression release

mechanism that “bumps” the exhaust valve as the piston rises on the compression stroke. At TDC, the exhaust valve should be fully closed.

8. Check valve lash between each valve stem and rocker arm using a feeler gauge.

9. Intake valve lash (carburetor side) should be 0.004”-

0.006” (0.10 - 0.15mm).

See Figure 1.13.

See Figure 1.14.

Probe to confirm piston

is at top of travel

Valves closed

(push rods slack)

Push rods relaxed

Figure 1.13

.006” feeler

gauge

10. Exhaust valve lash (muffler side) should be 0.006-

0.008” (0.15 - 0.20mm).

11. Use a 10mm wrench to loosen the jam nut, and a 14mm wrench to adjust the rocker arm fulcrum nut. See Figure 1.15.

• Tighten the rocker arm fulcrum nut to close-up

the clearance between the end of the valve stem and the contact point on the rocker arm.

• Loosen the rocker arm fulcrum nut to open-up

the clearance between the end of the valve stem and the contact point on the rocker arm.

12. Hold the fulcrum nut with a 14mm wrench, tighten the jam nut to a torque of 80 - 106 in-lb. (9-12 Nm) using a 10mm wrench.

See Figure 1.15.

Figure 1.14

.008” feeler

gauge

Figure 1.15

Introduction

13. Double-check the clearance after tightening the jam nut, to confirm that it did not shift. Re-adjust if necessary.

14. Rotate the engine through several compression cycles:

• Observe the movement of the valve gear.

• Return the piston to TDC compression stroke and re-check the valve lash to confirm consistent movement of the valve gear, including the slight bump to the exhaust valve from the automatic compression release.

15. Clean-up any oil around the valve cover opening, clean the valve cover, replace the valve cover gasket if necessary.

16. Install the valve cover, tightening the valve cover screws to a torque of 62 - 80 in-lbs (7-9 Nm). IMPORTANT: Over tightening the valve cover will cause it to leak.

17. Install the spark plug.

Cleaning the engine

1. To maintain a proper operating temperature and to keep the equipment looking good; all debris should be removed from the engine.

2. It is recommended to use compressed air to blow all of the debris off of the engine. NOTE: A pressure washer may be used to clean outdoor power equipment but only after the unit has been

allowed to properly cool.

NOTE: Mice and other critters tend to build nests inside the engine shrouds while the snow blower is stored

during the long off season.

78/83/90 Series Horizontal Shaft Engines

General torque specifications

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Frequently used specifications

Intake valve lash

Exhaust valve lash

Spark plug gap

Module air gap

Introduction

Minimum Maximum

in. mm in. mm

78 0.004 0.10 0.006 0.15 83 0.004 0.10 0.006 0.15

90 0.004 0.10 0.006 0.15

78 0.006 0.15 0.008 0.20 83 0.006 0.15 0.008 0.20

90 0.006 0.15 0.008 0.20

78 0.026 0.65 0.030 0.75 83 0.026 0.65 0.030 0.75

90 0.026 0.65 0.030 0.75

78 0.016 0.40 0.024 0.60 83 0.016 0.40 0.024 0.60

90 0.016 0.40 0.024 0.60

Displacement

78 16.9 cid (277 cc) 83 21.8 cid (357 cc)

90 25.6 cid (420 cc)

Governed engine RPM

78 3500 + 100 83 3500 + 100

90 3500 + 100

Oil capacity

78 37 oz 1.1 L 83 37 oz 1.1 L

90 37 oz 1.1 L

Fuel tank capacity

78 1.3 gal 5.0 L 83 1.3 gal 5.0 L

90 1.3 gal 5.0 L

78/83/90 Series Horizontal Shaft Engines

BASIC TROUBLESHOOTING

CHAPTER 2: BASIC TROUBLESHOOTING

Definitions

Troubleshooting - The act of gathering information by preforming tests and direct observations. Diagnosis

shooting.

Introduction

Diagnosing an engine is an art form that is built upon several factors. First and most importantly is a good understanding of how the engine works. The second is skills that have been honed by experience. Finally the use of visual observations and a structured, systematic approach to troubleshooting a problem.

The first part of this chapter will outline the steps of troubleshooting an engine so a technician can form a proper diagnosis. The second half of this chapter will describe specific procedures and tests to perform while troubleshooting.

Steps to troubleshooting

Define the problem

The first step in troubleshooting is to define the problem:

- Developing and testing theories of what the problem is, based on the information gathered in trouble-

The first two rules in troubleshooting is to cause no further harm to the engine and prevent

! CA UTION! CAUTION

NOTE: The steps and the order of the steps that follow are a suggested approach to troubleshooting the MTD

• Crankshaft will not turn.

injuries. Always make sure to check the oil for level and condition before starting an engin e. Also check attachments for damage and make sure they are firmly mounted.

engine. The technician does not necessarily have to follow them as described in this chapter

A. Hard to pull rope, steady pressure B. Rope jerks back C. Rope will not pull at all

• Crankshaft turns, no start

• Starts, runs poorly

A. Starts, then dies B. Runs with low power output C. Makes unusual smoke when running

I. Black smoke, usually heavy II. White smoke, usually heavy III. Blue smoke. usually light

D. Makes unusual sounds when running

I. Knock II. Click III. Chirp

78/83/90 Series Horizontal Shaft Engines

IV. Unusual exhaust tone

There are tools that the technician can use in order to define the problem, such as:

1. Interview the customer. 1a. Get a good description of their complaint. 1b. If it is an intermittent problem, verify what conditions aggravate the problem as best as possible. 1c. Get an accurate service history of the equipment. 1d. Find out how the customer uses and stores the equipment.

2. Direct observation: 2a. Do not automatically accept that the customer is correct with their description of the problem. Try to

duplicate the problem.

2b. Check the general condition of the equipment (visually).

I. Cleanliness of the equipment will indicate the level of care the equipment has received. II. Make sure the engine and attachments are securely fastened. III. The tune-up factors.

NOTE: Most hard starting and poor running conditions can be solved by performing a tune-up.

a. Check the condition and amount of oil in the crankcase. b. Check the level and condition of the fuel. c. Check the ignition and “read” the spark plug. d. Look for obvious signs of physical damage, exhaust system blockage or cooling system block-

age.

3. Broken starter rope. 3a. Usually means the engine was hard to start. 3b. Makes it impossible to confirm any running or hard starting symptoms by direct observation. 3c. Some inference can be made from checking other factors of the general condition of the equipment.

Identify factors that could cause the problem

This is the second step in the troubleshooting process.

1. Crankshaft will not turn.

A. Hard to pull rope, steady pressure. This usually indicates a mechanical bind of some sort. The likely

suspects are: I. A slightly bent crankshaft. In some cases the drag may increase and decrease as the crankshaft

rotates. This produces a pulsing feeling that is different than a jerk back. II. A parasitic load from a drive belt that is not releasing or an implement that is jammed. III. An internal drag from a scored or seized piston.

BASIC TROUBLESHOOTING

D. Rope jerks back. This usually indicates that the piston is stopping before top dead center on the com-

pression stroke and is being driven back down by compression or combustion. The likely suspects are: I. Compression that is unusually high.

a. valve lash. b. a partial hydraulic lock.

III. Ignition timing is advanced.

a. Improper air gap. b. Sheared or missing flywheel key. c. The wrong flywheel or module is installed on the engine.

IV. Insufficient inertia to over-come normal compression .

a. Loose implement. b. A light flywheel used on a heavy flywheel application.

C. Rope will not pull at all. This is usually either a quick fix or a catastrophic failure. The likely suspects

are: I. A broken starter recoil (easy fix). II. Complete hydraulic lock (easy fix). III. External binding/jammed implement (easy fix). IV. Bent crankshaft (unrepairable) V. Internal binding, crankshaft, connecting rod or piston (unrepairable)

2. Crankshaft turns, no start. 2a. Most gasoline engine diagnosis involves isolating problems in the four critical factors an engine needs to

run properly:

I. Ignition- sufficient spark to start combustion in the cylinder, occurring at the right time. II. Compression- enough pressure in the cylinder to convert combustion into kinetic motion. It also

needs sufficient sealing to generate the vacuum needed to draw in and atomize the next intake charge.

III. Fuel- correct type and grade of fr esh gasoline; in sufficient quantity, atomized (tiny droplets) and in

correct fuel/air proportions.

IV. Flow- if all of the above conditions are met but the flow of air is constricted on the inlet or exhaust

side, it will cause the engine to run poorly or not at all. This also includes ensuring the valves are timed to open at the proper time.

2a. Isolate the ignition system and compression from the fuel system by preforming a prime test.

I. Burns prime and dies. This would indicate a fuel system issue. II. Does not burn prime. Not a fuel system issue. Check for an ignition, compression or flow problem.

2c. Compression or ignition problem

I. Check the engine stop and safety switch. II. Test the ignition system using a proper tester. III. Replace the spark plug with a new one or a known go od one. IV. Check compression or leak down. V. Check valve lash.

78/83/90 Series Horizontal Shaft Engines

VI. Check valve timing/actuation. VII. Check exhaust.

3. Starts, runs poorly 3a. Starts, then dies

I. Run the engine with a spark tester in-line between the sp ark plug wire and the sp ark plug or use an

oscilloscope and see if the spark goes away at the same time the engine dies.

II. Check choke operation.

a. Black smoke? b. Wet plug?

III. Prime test immediately after engine dies. If it restarts; this may indicate a problem with fuel flow to

the carburetor. Check the gas cap, fuel line, fuel filter, and the float in the carburetor.

3b. Runs with low power output.

I. Look for unusual exhaust color (smoke). II. Unusually hot muffler (may glow red).

a. Retarded ignition b. Exhaust valve opening early (lash too tight)

III. Mechanical bind

a. A slightly bent crankshaft. In some cases the drag may incr ease and decrease a s the crankshaf t

rotates. This produces a pulsing feeling that is different than a jerk back. b. Parasitic external load. A bind in the equipment the engine is powering. c. Internal drag from a scored piston or similar damage.

IV. Low governor setting or stuck governor.

a. Check RPMs using a tachometer. b. RPMs should not droop under moderate to heavy loads.

V. Low compression

a. Check valve lash b. Check compression c. Check leak down to identify the source of the compression loss.

VI. Flow blockage

a. Exhaust blockage, usually accompa nied by an unusual exhaust sound.

• Just as a throttle on the carburetor controls the engine RPMs by limiting the amount of air an engine can breathe in, an exhaust blockage will limit engine performance by constricting the other end of the system.

• The muffler itself my be blocked.

• The exhaust valve may not be opening fully, possibly because of extremely loose valve lash settings.

• The exhaust valve seat may have come loose in the cylinder head. This may cause a loss of compression, a flow blockage or it may randomly alternate between the two.

NOTE: The cause of an exhaust valve coming loose is usually over heating.

BASIC TROUBLESHOOTING

b. Intake blockage

• An intake blockage up-stream of the carburetor will cause a rich fuel/air mixture and constrict the amount of air that the engine can draw in, limiting performance.

• The intake valve not fully opening. A possible cause of this is loose valve lash.

V. Makes unusual smoke when running

a. Black smoke, usually heavy, usually indicates a rich air fuel mixture

• Not enough air: air flow blockage or a partially closed choke.

• Too much fuel: carburetor float or float valve stuck or metering / emulsion issues with the carburetor.

b. White smoke, usually heavy

• Oil in muffler, usually the result of improper tipping. The engine will “fog” for a minute or so, then clear-up on its own.

• Massive oil dilution with gasoline. It may be caused by improper tipping. It can also be caused by leaky carburetor float valve, if there is a down-hill path from the carburetor to the intake port. Check oil for gasoline smell, repair carburetor.

c. Blue smoke, usually light.

PCV system

• May be blocked or unplugged.

• May be over-come by massive over-filling or oil dilution with gasoline.

• Will cause oil to exit the engine via any low-resistance paths.

Piston rings

• Confirm with leak-down test.

• Smoke will be more pronounced under load.

• Repair may not make economic sense.

Valve guides (and intake valve stem seal).

• Smoke will be more pronounced on over-run.

VI. Makes unusual noise when running

a. Knock

• Check for loose mounting of engine or driven implement

• Rotate crankshaft back-and-forth to check for loose connecting rod.

b. Click

• Clicks and pops on engine shut-down: Compression release coming into play as the engine RPMs cross the activation threshold. This will have no ill effects on engine performance.

• Half-engine speed clatter: loose valve lash.

• Half-engine speed clatter, slightly heavier: wrist-pin.

• Rhythmic heavy-light engine speed click: piston slap

c. Spark-knock

• Advanced ignition timing

78/83/90 Series Horizontal Shaft Engines

• Low octane fuel

• Over-heating engine (check for blocked cooling air flow)

• Carbon build-up in cylinder: glowing carbon chunks pre-igniting air fuel mix.

d. Chirp

• Compression, blowing-by the fire-ring of a damaged head gasket will sometimes produce a chirping noise.

• Confirm with a compression test and leak-down test.

e. Unusual exhaust tone

Splashy or blatty

• Splashy idle usually indicates a slight rich condition.

• May indicate an exhaust blockage, usually slightly muffled.

Backfire

• On over-run: unburned fuel igniting past exhaust valve. Mixtu re not b urnin g completely in combustion chamber. It may be too rich or it may be spark-plug or ignition problem.

• Occasional, under load: engine momentarily runs lean, usually will cycle with float bowl level or governor pull-in, sometimes sounds like a slight stumble. Ethanol content exceeding 10% will make the engine run artificially lean.

Skip

• Usually ignition related.

• Run the engine with a spark tester in-line between the spark plug wire and the spark plug or use an oscilloscope and see if the spark goes away at the same time the engine dies.

4. Engine over-speed A. Continual over-speed

• Binding or damaged external governor linkage or carburetor throttle.

• Mis-adjusted governor arm.

• Internal governor failure.

B. Momentary over-speed

• Intermittent bind (very unusual).

• Interference: This is fairly common when debris can fall on the governor linkage duri ng normal operations.

5. Engine RPMs surge (hunting) A. Over-governed condition- Return spring replaced with wron g part or hooked into wrong hole.

NOTE: This is an extremely rare condition, usually created by tampering.

B. Lean Air-fuel mixture condition- When AFR (Air Fuel Ratio) is significantly below stoichiometric ratio

(14.7:1) engine RPMs sink until they reach a po int tha t can be su pp or te d by the availa b l e fuel . Th is causes a momentary surge in power until the available fuel is consumed, then the RPMs fall again, repeating the cycle.

• Too much air: look for an air leak in the intake tract

• Not enough fuel: look for fuel supply or carburetor problems

BASIC TROUBLESHOOTING

Repairing the problem

The third step in the troubleshooting process is to repair the problem. This step consists of:

A. Form a diagnosis by using all of the information gathered fro m the tro ubl eshooting th at was per forme d. B. Physically perform the repair.

The fourth, and hopefully final, step in the troubleshooting process is the follow through. This step consists of:

A. Thoroughly test the repaired equipment: confirmin g that the initial diagnosis was correct. If it was

wrong, start the troubleshooting process over again.

NOTE: Sometimes the engine will have multiple problems at the same time. By performing one repair, other

issues may show up that are unrelated to the first repair.

B. Delivery to customer: We are not just repairing equipment, we are repairing customers.

• Inoculate against recurring problem with education, e.g.: if the problem was caused by stale fuel, make sure the customer is aware that fuel go es bad ove r tim e.

• Make sure the customer understands th e repair , preventing “superstitious” come-backs.

78/83/90 Series Horizontal Shaft Engines

Prime test

To perform a prime test:

1. Prime the engine through the carburetor throat using a squirt bottle, filled with clean fresh gasoline.

2. Make sure the throttle is in the run position and the safety key if fully inserted.

3. Attempt to start the engine.

4. If the engine starts and runs long enough to burn the prime, the problem is effectively isolated to the fuel system. proceed to Chapter 4: The Fuel System and Governor.

5. Check ignition system as described in Chapter 7: Ignition System.

6. If the ignition system is working, check the compression or perform a leak down test.

Leak-down test

A leak-down test is the preferred method to test the engine’s ability to compress the charge. It will also show where pressure is leaking from.

To perform a leak-down test:

NOTE: A leak down test pressurizes the combustion chamber with an external air source and will allow the

technician to listen for air “leaking” at the valves, piston rings and the head gasket.

NOTE: These are general instructions. Read and follow the instructions that came with the tester before

attempting to perform this test.

• If possible, run the engine for 3-5 minutes to warm up the engine.

• Remove the spark plug and air filter.

• Find top dead center of the compression stroke.

! CA UTION! CAUTION

1. Find top dead center by following the steps described in the valve lash section of Chapter 1: Introduction

2. Thread the leak down tester adapter into the spark plug hole.

3. Connect tester to compressed air.

If the engine is not centered at top dead center, the engine will rotate when compressed air is introduce to the combustion chamber.

See Figure 2.1.

4. Adjust the regulator knob until the needle on the gauge is in the yellow or set area of the gauge.

5. Connect the tester to the adapter. NOTE: If the engine rotates it was not at top dead

center.

6. Check the reading on the gauge.

Leak-down tester adapter

Figure 2.1

7. Compare the results to the following chart.

Leak-down Testing Results

Symptom Possible cause

BASIC TROUBLESHOOTING

Air escaping from the breather

Air escaping from the exhaust

Air escaping from the carburetor

Gauge reading low

Gauge reading moderate

Gauge reading high

Worn cylinder or piston rings. Possible blown head gasket

Leaking exhaust valve

Leaking intake valve

Cylinder and piston rings are in good condition

There is some wear in the engine, but it is still usable

excessive wear of cylinder and/ or piston rings. Engine should be short blocked or it could be a blown head gasket.

78/83/90 Series Horizontal Shaft Engines

Compression test

To perform a compression test:

NOTE: Compression should be in the range of 40-75 PSI (2.8-5.2 Bar).

• Disconnect the high-tension lead from the spark plug and ground it well away from the spark plug hole.

• Remove the spark plug using a 13/16” or 21mm wrench. A flexible coupling or “wobbly” extension may

help.

• Pull the starter rope several times to purge any fuel or oil from the combustion chamber. NOTE: Air compresses readily, liquid does not. Liquid in the combustion chamber will result in an artificially

high compression reading.

1. Install a compression gauge in the spar k pl ug hole.

2. Confirm that the gauge is “zeroed”, then pull the starter rope repeatedly, until the needle on the gauge stops rising.

3. Interpreting compression readings.

See Figure 2.2.

Compression Readings

Readings in

psi

<20 (1.4 Bar)

Most likely a stuck valve or too tight of a valve lash, provided the starter rope pulls with normal effort.

Compression gauge

Reading ~

Figure 2.2

Possible causes

40 PSI

20 - 40 (1.4-2.8 Bar)

40 - 75 (2.8-5.2 Bar)

>75 (>5.2 Bar)

Valve seat damage or piston ring and/or cylinder wear.

Normal readings

Excessive valve lash, a partial hydraulic lock, a bad cam or a bad automatic compression relief.

BASIC TROUBLESHOOTING

PCV testing

The PCV (Positive Crankcase Ventilation) valve is located in the valve cover and allows the crankcase pre ssure to escape.

Leakage and blockage are the two failure modes for a PCV system. Either mode will cause crankcase pressure to build-up, though the effects of a blocked PCV are generally more dramatic. Increased case pressure will result in oil entering the combustion chamber.

crankcase air chamber

Figure 2.3

• Oil will be forced by the rings and valve guides, being burnt in the combustion chamber.

• The cause of this oil burning can be mistaken for a worn-out engine, if proper diagno sis (compression, leakdown, and case pressure) is not performed.

4. Experimentation by MTD’s Training and Education Department has revealed the following characteristics of MTD engines:

1. The PCV chamber is vented to the heat box through a molded rubber hose. The rubber hose directs crank case fumes to the heat box assembly. See Figure 2.3.

2. When functioning properly, the PCV valve works with the inherent pumping action of the piston in the bore to expel pressure from the crankcase.

NOTE: Normally, small engines run with slightly negative

case pressure. This case pressure can be mea sured using a slack-tube water manometer, or an electronic version of the same tool. Less than -3” to -4” (-7.6 - 10.2cm) of water is a typical reading at idle.

3. An engine that fails to purge extra case pressure in a controlled manner will build case pressure. The pres sure will find it’s own way out of the engine in undesirable ways.

• A leaky PCV system will not build-up substantial case pressure.

• A leaky PCV system will allow the engine to ingest contaminants through the system, accelerating engine wear.

• A blocked PCV system will allow crankcase pressure to build very rapidly . Noticeable oil fumes will be evident in the exhaust within several minutes of normal operation.

78/83/90 Series Horizontal Shaft Engines

AIR INTAKE SYSTEMS

CHAPTER 3: AIR INTAKE SYSTEMS

MTD builds horizontal crank engines for snow blowers and chore performers. The differences between snow engines and chore engines are the muffler and the air intake system. Therefore the air intake system for the snow and chore engines will be discussed separately, as will the mufflers in a later chapter.

Heat box (snow engines)

One of the big differences between snow engines and all other small engines is that the air intake of the snow engine does not have an air filter. This is because air filters tend to freeze, cutting off air flow to the engine. The snow engine however does have a heat box to preheat the intake air.

The bottom of the heat box is open to draw in a large volume of cold air. The top of the heat box has a small opening were the choke rod connects to the ca rburetor . Th is draws in just enough warm air from the top of the engine to heat the cold air to the desired temperature as it enters the carburetor.

To remove/replace the heat box:

1. Pull off the choke and throttle knobs.

2. Remove the control panel by taking off the five

Remove these

screws

screws secures it using a 10 mm wrench. See Figure 3.1.

Remove the breather hose

Figure 3.1

Remove these

nuts

Figure 3.2

3. Disconnect the wire from the ignition switch and the primer line from the primer button.

4. Slide the breather hose out of the heat box. See Figure 3.2.

5. Remove the two carburetor nuts using a 10 mm wrench.

NOTE: When installing, tighten the carburetor nuts to a

6. Remove the choke selector assembly.

7. Slide the heat box off of the carburetor studs.

8. Install the heat box by following the previo us steps in reverse order.

NOTE: There is no gasket between the heat box and the

See Figure 3.2.

torque of 80 - 106 in-lbs. (9-12 Nm)

carburetor.

78/83/90 Series Horizontal Shaft Engines

Air filter (summer engines)

Generally air filters come in two different types, a pleated-paper element or foam. A combination of the two are

used on the MTD engine.

• Air filters used on the MTD engine are designed to prevent particles larger than 3-5 micron from passing through into the engine.

• The filter should be checked on a regular basis possibly several times in a season.

NOTE: Never use compressed air on a paper air fil-

ter. Compressed air will remove the tiny fibers that are used to catch the dirt in the air. Without these fibers the filter is useless.

NOTE: Refer to Chapter 1: Introduction for the

maintenance interval and cleaning instruc tions for the air filter.

To replace an air filter:

1. Wipe down the air filter housing to prevent any debris from getting into the engine.

2. Unthread the two wing nuts in the air filter cover. See Figure 3.4.

Paper-pleated element

Foam pre-filter

Figure 3.3

Wings nuts

NOTE: The wing nuts are part of the filter housing

and do not come off.

3. Swing the housing away from the fuel tank. See Figure 3.5.

4. Remove the air filter assembly.

Figure 3.4

Air filter

housing

Air filter assembly

Figure 3.5

Riser

Air filter base

Figure 3.6

AIR INTAKE SYSTEMS

5. Install the air filter by following the previous steps in reverse order.

NOTE: When installing the air filter, the hole in the bottom

of the paper element must fit over the riser in the air filter base.

See Figure 3.6.

78/83/90 Series Horizontal Shaft Engines

Air filter base and control panel (summer engines)

To remove/replace the air filter base and control panel:

1. Remove the air filter following the steps described in the previous section.

2. Remove the three screws that hold the air filter base to the intake elbow.

See Figure 3.7.

3. Pull off the throttle lever knob. See Figure 3.8.

4. Remove the two carburetor nuts using a 10 mm wrench.

5. Remove the bolt from the front of the control panel using a 10 mm wrench.

6. Align the throttle lever, choke lever and the fuel shut-off lever in the middle of their slots.

7. Remove the bolt that holds the rear of the control panel to the throttle bracket.

See Figure 3.9.

Air filter base

Control panel

Carburetor nuts

Screws

Figure 3.7

Throttle lever

Choke lever

Bolt

Fuel shut-off

Figure 3.8

Throttle bracket

Bolt

8. Disconnect the breather hose.

Breather hose

Figure 3.9

Breather hose

AIR INTAKE SYSTEMS

NOTE: All summer engines built on or after January 1,

2011 will have a hose from the evaporative emis sions (EVAP) system att ached to the control panel above the breather hose.

9. Pull the control panel straight off the front of th e engine.

10. Install the control panel by following the previous steps in reverse order.

See Figure 3.10.

EVAP hose

Figure 3.10

NOTE: tighten the carburetor nuts to a torque of 80 - 106

in lbs (9 - 12 Nm).

11. Test run the engine before returning it to service.

78/83/90 Series Horizontal Shaft Engines

Carburetor and Insulator

1. Siphon the fuel out of the fuel tank.

To avoid personal injury or prop-

! WAR N IN G! WAR N IN G

extremely flammable and the vapors are explosive. Keep away from sources of ignition.

Serious personal injury can occur when gasoline is spilled on yourself and/or your clothes which can ignite. Wash your skin and change clothes immediately

NOTE: Dispose of drained fuel in a safe and

responsible manner.

2. Remove the fuel tank front bezel using a 10 mm wrench.

NOTE: The bezel may get stuck on the cup for the

See Figure 3.11.

recoil starter. If it does, the recoil starter is easily removed with an 8 mm wrench.

erty damage, use extreme care in handling gasoline. Gasoline is

Remove these screws

Figure 3.11

3. Disconnect the fuel line from the fuel tank. See Figure 3.12.

Residual fuel in the fuel tank will

! WAR N IN G! WAR N IN G

help prevent gasoline from splashing into your eyes.

come out when the fuel line is removed. Wear safety goggles to

Remove the fuel line

from the fuel tank

Figure 3.12

Figure 3.13

Throttle linkage

Throttle spring

AIR INTAKE SYSTEMS

4. Remove the Heat box (snow engines) or the control panel (summer engines) by following the steps described in appropriate section of this chapter.

5. Disconnect the throttle linkage and spring. See Figure 3.13.

6. Slide the carburetor off of the mounting studs.

7. Remove the carburetor.

NOTE: The barb on the carburetor inlet is very sharp. If

The fuel line is pulled off of it, the line will be dam aged and must be replaced.

NOTE: The carburetors are not inter -changeable from one

engine model to another. To help prevent carbure tor mix-ups, the engine model number is stamped on the carburetor by the fuel nipple. See Figure 3.14.

Engine model number

Ignition wires

Figure 3.14

Figure 3.15

Spark plug wire

Clip

8. Unhook the ignition wires from the clip molded into the insulator plate.

See Figure 3.15.

78/83/90 Series Horizontal Shaft Engines

NOTE: An insulator block separates the carburetor

from the cylinder head. There is a gasket on each side of the insulator.

NOTE: The gaskets are different, and there is an

orientation to the insulator.

See Figure 3.16.

• The bowl vent channel in the insulator faces the carburetor, with the exit toward the bottom.

• There is a small hole in the insulator to carburetor gasket. The hole should be aligned to allow passage of air through the bowl vent channel to the throttle side bowl vent in the carburetor body.

9. Install the insulator and carburetor by following the above steps in reverse order.

NOTE: Tighten the carburetor mounting nuts to a

torque of 80 - 106 in-lbs (9-12 Nm).

10. Test run the engin e befo re returning to service.

Insulator block

Gaskets

Figure 3.16

FUEL SYSTEM AND GOVERNOR

CHAPTER 4: THE FUEL SYSTEM AND GOVERNOR

The function of the fuel system is to store fuel, mix the fuel with air in the correct ratio and deliver it to the intake

port. The fuel system consists of the following components:

• Fuel tank

• Fuel lines

• Fuel filter

• Carburetor NOTE: When working on the fuel systems, look at the whole system. A problem will rarely be isolated to one

component.

Inspecting the fuel

NOTE: Fuel is the maintenance item most often overlooked by consume rs. A lot of f uel systems prob lems are

caused by gas that is out of date or fuel with too much alcohol in it. When inspecting the fuel:

• Look for water.

• Look for dirt.

• Look for discoloration.

• Sniff carefully to see if it smells like varnish or kerosene.

• Save the fuel to show to customer.

• Look for oil in the fuel.

• Test the fuel for alcohol content if there is a reason to suspect it.

NOTE: Save a sample of the fuel collected to show the customer. NOTE: Customers pouring engine oil into the fuel tank seems to be a growing problem.

78/83/90 Series Horizontal Shaft Engines

Test fuel for alcohol

Fuels currently on the market contain a wide array of additives. Some of these additives oxygenate the fuel. Oxygenated fuel reduces emissions, and is required in some parts of the United States. Fuel make-up varies seasonally and geographically. Ethanol is the primary additive used to oxygenate fuel.

Ethanol in fuel creates a lot of problems for gasoline engines. The biggest problem is that alcohol attracts and holds water. Th is corrod es the met a l components of the fuel system, especially the carburetor. Alcohol also does not produce as much heat as gasoline when burnt. This results in less power for the engine.

A 10% ethanol (E10) mix is acceptable for MTD engines. Anything higher than that will result in performance issues.

NOTE: E20 and E85 fuels are not to be used in any MTD engines.

There are several alcohol test kit available commercially. See Figure 4.1.

Generally these kits involve mixing a measured amount of water and gas together and seeing were the boundary layer is.

The test kit should come with a chart to compare the boundary layer height to alcohol percentage.

See Figure 4.2.

Figure 4.1

Figure 4.2

FUEL SYSTEM AND GOVERNOR

Fuel tank vent

The fuel tank vent performs the important task of allowing air into the fuel tank. As fuel is being used by the engine, the fuel level in the tank drops. The dropping fuel level then create s a vacuum in the tank. If the fuel tank could not draw air through the vent, the vacuum would prevent the fuel from getting to the carburetor. The vent is located in the fuel cap.

See Figure 4.3.

Fuel cap

Vent

Figure 4.3

NOTE: All summer engines built on or after January 1, 2011 will have an evaporative emissions (EVAP) sys-

tem that the fuel tank vents through, not the fuel cap. Refer to the evaporative emissions section of this chapter to understand how this system works and how to test it.

To test the cap vent:

1. Remove the fuel cap.

2. Clean off the vent.

3. Blow air into the vent hole. The air should blow throw the vent with little back pressure.

4. Suck air through the vent hole. Air should freely enter through the vent.

• Replace the cap if the vent builds pressure or restricts air movement.

• A cap that maintains pressure will cause the engine to run rich as the fuel in the tank heats and expands, forcing it’s way past the float valve in the carburetor.

• A cap that maintains vacuum will cause the engine to run lean as the fuel is depleted and no air comes in to replace it.

• The two conditions may both be present, but the symptoms vary with fuel, fuel level, and operating conditions.

• A bad fuel cap vent usually presents as a “Runs and quits” scenario.

78/83/90 Series Horizontal Shaft Engines

The fuel filter

The fuel filter is located in the fuel tank nipple. It can be remove d and cl eaned with a can of ca rb cleaner or it can

be replaced.

See Figure 4.4.

NOTE: If cleaning a filter, back-flush it by spraying the carb cleaner through the barb end and out of the screen. Also make sure the fuel tank is clean.

nset:

Fuel filter/barb

Filter

Figure 4.4

NOTE: To replace the fuel filter follow the steps described in Chap ter 1: Introduction.

Inspect the fuel lines

• Are they cracked?

• Are they clogged?

• Are they brittle? NOTE: If the answer to any of the above is yes, replace the fuel lines. When replacing fuel lines, low perme-

able fuel line must be used in order to meet EPA and CARB standards.

NOTE: The nipple has a sharp edge that will damage the inner lining of the fuel line. Replace the fuel line

every time it is removed from the carburetor fuel nipple.

! WARN IN G! WARN IN G

Gasoline is extremely flammable and the vapors are explosive. Avoid all sources of heat when working on the fuel system.

• Drain the fuel tank before starting work to prevent spillage.

• Dispose of drained fuel in a safe and responsible manner.

The fuel tank

Remove these screws

Figure 4.5

FUEL SYSTEM AND GOVERNOR

To remove the fuel tank:

1. Drain the tank.

2. Remove the four screws that secure the fuel tank using a 12 mm wrench.

3. Lift up on the fuel tank and disconnect the fuel line. See Figure 4.6.

See Figure 4.5.

Disconnect the fuel line

Figure 4.6

Roll over valve

Figure 4.7

NOTE: On engines equipped with an EVAP system, dis-

connect the hose from the roll over valve. See Figure 4.7.

4. Install the fuel tank by following the above steps in reverse order.

78/83/90 Series Horizontal Shaft Engines

Evaporative (EVAP) emissions system

Gasoline is made from the graduated distillation of crude oil. It consists of a multitude of individual hydrocarbons

and has a boiling range of 86 - 410 makes gasoline an ideal fuel for spark ignited, internal combustion engines. However, the hydrocarbons are not good for the environment. To reduce or eliminate the release of fuel vapors into the atmosphere, an evaporative (EVAP) emissions system is used.

Effective January 1, 2011; the EPA has mandated that all summer en gines with 225cc’ s of displacement or more

must be equipped with an evaporative emissions system. The EVAP system used by MTD consists of:

°F (30-210°C)

. The large quantity of hydrocarbons and the low boiling range

• A charcoal canister

• The fuel tank and cap

• A roll over valve vent

• The control panel.

• Vacuum lines

Roll over valve

Charcoal canister

Figure 4.8

Reference

1. Dr. Ullmann, J, Fuels, Automotive Handbook, seventh edition. Bosch, Robert distributed by SAE Society of Automotive Engineers, 2007. 320.

Unvented

Tethered

Figure 4.9

Vapor

FUEL SYSTEM AND GOVERNOR

This system operates as follows:

1. The gasoline evaporate and lets off vapors.

2. The vapors exit the fuel tank through the roll over valve vent.

NOTE: The fuel cap used with the charcoal canister sys-

tem is not vented. If a vented cap is used, the EVAP system will not work.

NOTE: The new EPA standard also requires the fuel caps

to be tethered to the fuel tank.

3. The vapors are routed through the charcoal canister. See Figure 4.10.

4. The activated charcoal inside the canister absorbs the hydrocarbons allowing the air to pass through and out to the atmosphere.

See Figure 4.9.

Figure 4.10

Figure 4.11

Charcoal canister

5. The second line on the charcoal canister goes to the control panel. When the engine is running, vapors are drawn out of the charcoal canister by the air being drawn into the carburetor. This re-charges the canister of the next down period.

See Figure 4.11.

78/83/90 Series Horizontal Shaft Engines

Troubleshooting the EVAP System

There are four failure modes for the EVAP system:

• A leak in the system - will allow dirt ingestion.

• A blockage in the system (between the canister and the fuel tank) - acts a plugged vent.

• A blockage in the system (between the canister and the control panel) - The canister can not purge its vapors.

• Charcoal canister poisoned (raw fuel in the canister) - prevents the canister from absorbing fuel vapors.

NOTE: The only failure mode that will affect the operation of the engine in a way the operator will notice is a

blockage in the system.

To test the system for a blockage:

1. Remove the fuel cap.

2. Clamp off the hose that runs from the canister to the control panel.

See Figure 4.12.

3. Attach a vacuum pump to the vent port of the charcoal canister. See Figure 4.12.

4. Apply a vacuum to the canister. NOTE: When applying the vacuum, squeeze the

pump slowly. Too much vacuum can close the roll over valve giving a false reading.

5. If the canister does not hold a vacuum, the canister and the roll over valve are working properly.

NOTE: If the canister does hold a vacuum, test the

roll over valve to determine if the roll over valve or the canister is at fault.

Figure 4.12

Charcoal canister

Figure 4.13

FUEL SYSTEM AND GOVERNOR

Testing the roll over valve

The roll over valve vent has two functions. The first function is to vent the t ank and the second func tion is to close

off the vent if the tank is inverted.

Hose to roll over valve

Figure 4.14

Test the roll over valve by:

1. Disconnect the hose that runs from the charcoal canister to the roll over valve.

2. Connect a vacuum pump to the hose.

3. Slowly apply a vacuum to the roll over valve.

NOTE: If the roll over valve does not hold a vacuum,

replace the canister.

4. If the roll over valve held a vacuum, disconnect the hose from the roll over valve.

5. Attach the vacuum pump directly to the roll over valve.

6. Slowly apply a vacuum to the roll over valve.

NOTE: If the roll over valve does not hold a vacuum, the

hose has blocked. Replace the hose.

See Figure 4.14.

78/83/90 Series Horizontal Shaft Engines

Charcoal canister

To remove/replace the charcoal canister:

1. Disconnect the hoses from the canister. See Figure 4.15.

Hoses

Figure 4.15

2. Remove the screw from the canister mounting strap using a 10 mm wrench.

To Install the canister:

3. Insert the tab of the mounting strap into the slot in the fuel tank mounting bracket.

4. Install the mounting screw.

5. Attach the hoses.

See Figure 4.16.

See Figure 4.17.

Screw

Figure 4.16

Slot

6. Test run the engine in a safe are a be fo re retu r ning to service.

Tab

Figure 4.17

Roll over valve

Grommet

Roll over valve assembly

Figure 4.18

FUEL SYSTEM AND GOVERNOR

To remove the roll over valve:

1. Gently pry the grommet from in between the roll over valve and the fuel tank.

NOTE: Leaving the hose connected to the roll over valve

will help prevent it from falling inside the tank once the grommet is removed.

2. Lift the valve out of the fuel tank.

3. Disconnect the hose.

To install the roll over valve:

1. Inspect the rubber grommet, replace if damaged.

2. Slide the rubber grommet over the roll over valve.

See Figure 4.18.

Figure 4.19

3. Press the roll over valve assembly into the fuel tank opening.

NOTE: Soapy water can be used as a lubricant.

4. Install the hose from the canister.

5. Test run the engine in a safe area before returning to service.

78/83/90 Series Horizontal Shaft Engines

Choke

78/83/90 series MTD engines are equipped with a choke. The snow engines are also equipped with a primer,

both of which must be used to start the engine.

NOTE: The choke should be opened after the engine starts. This can be a source of starting issues with cus-

tomers who are not familiar with manual chokes.

NOTE: On the summer engines, the entire choke

mechanism is part of the carburetor. See Figure 4.20.

Choke lever

Choke plate

Choke mechanism (snow engines)

The choke is operated by a knob on the engine shroud. If the choke plate fails to close fully, the engine will be dif

ficult or impossible to start when cold.

See Figure 4.21.

Figure 4.20

Engine shroud

Choke knob

Figure 4.21

Choke rod

Figure 4.22

Heat box

FUEL SYSTEM AND GOVERNOR

The choke rod is part of the heat box assembly mounted on the front of the carburetor.

NOTE: The choke rod can be bent slightly to facilitate

adjustment.

See Figure 4.22.

Choke

rod

Choke

plate

Figure 4.23

To adjust the choke rod:

1. Remove the choke knob and the engine shroud by following the steps described in Chapter 3: Air Intake Systems.

2. Rotate the choke knob shaft to verify full choke movement.

3. If the choke plate does not open fully or close fully, adjust the choke linkage.

NOTE: When adjusting the choke linkage, make small

bends and recheck the movement of the choke plate. Repeat this step until full movement is achieved.

4. Reassemble by following step 1 in reverse order.

5. Test run the engine before returning to service.

See Figure 4.23.

78/83/90 Series Horizontal Shaft Engines

Primers (snow engines)

MTD engines use a dry bulb primer. This means that there is no fuel in the primer bulb. The primer works by pushing air into the float chamber of the carburetor when the primer bulb is depressed. This will force fuel to be sprayed out of the main nozzle into the throat of th e ca rb ur et or.

To test the primer:

1. Remove the engine shroud by following the steps described in Chapter 3: Air Intake Systems.

NOTE: Do not disconnect the primer hose.

2. Press the primer bulb while looking down the carburetor throat. If there is fuel squirting into the carburetor

throat, the primer is working properly. If not, replace the primer and hose. NOTE: The primer and primer hose come as an assembly so there is no need to determine which part is bad.

To replace the primer:

3. If the primer is bad, disconnect hose from the carbu-

retor.

4. Remove the hose camp at the rear of the primer

bulb.

See Figure 4.24.

Clamp

5. The primer is held to the shroud by a pair of split,

barbed posts. Squeeze the posts to release the barbs.

See Figure 4.24.

NOTE: The primer bulb and hose will slide out as an assembly.

6. Install the new primer by following the above steps

in reverse order.

Barbed posts

Figure 4.24

FUEL SYSTEM AND GOVERNOR

Carburetors

If diagnosis indicates a fuel problem, inspect the carburetor. This is important even if problems are identified

elsewhere in the fuel system.

IMPORTANT: the fuel must be tested for alcohol content before diagnosing anything else on the engine. NOTE: It is important to perform a compression or leak down te st befor e conde mnin g a car bur etor. An engine

can have a borderline compression reading and not create enough of a vacuum to draw in a sufficient fuel/air charge.

NOTE: To determine if border-line compression is the problem; remove the spark plug. Squirt a little bit of oil

into the combustion chamber to seal the rings. Reinst all the spark plug. If the engine st arts and runs ok, then that was the problem. If it does not start, move on to the carburetor.

Inspecting the carburetor

1. Remove the float bowl and check for dirt and/or varnish.

2. Inspect the needle valve and needle valve seat for dirt and/or damage.

3. Inspect the gaskets and O-rings for damage.

4. Inspect the vents and orifices, verify that they are free of debris. NOTE: If a little cleaning and new gaskets will fix the carburetor, do it. If the carburetor requires extensive

cleaning; it is better to replace the carburetor.

IMPORTANT: Never try to mechanically clean orifices. That will damage them and ruin the carburetor. NOTE: The jet markings (if present) may be used for identification purposes, but the technician should not

attempt to infer orifice sizes from the identification numbers.

NOTE: Installing the wrong main jet, or a carburetor with the wrong main jet will produce performance and

emissions issues.

78/83/90 Series Horizontal Shaft Engines

Disassembly and rebuilding of the carburetor

1. Clamp-off the fuel line to prevent fuel spillage, then disconnect the fuel line from the carburetor.

2. Disconnect the primer hose.

3. Remove the carburetor by following the steps described in Chapter 3: Air Intake and Filter. NOTE: An insulator separates the carburetor from the cylinder head.

• A bowl vent port is in a recessed passage on the end of the carburetor that faces the insulator.

• A second passage in the insulator supplements the passage on the carburetor.

• Gaskets separate the insulator from the cylinder head and the carburetor from the insulator.

• A port in the carburetor to insulator gasket ties the bowl vent passages together.

4. Check the vent passages. See Figure 4.25.

5. Check the gaskets and the insulator block.

6. Remove the bowl bolt using a 10mm wrench. See Figure 4.27.

NOTE: From this point an assessment can b e made

about the viability of rebuilding the carbure tor.

• If extensive corrosion is evident, replace the

carburetor.

• If varnish build-up is too extensive to clean,

replace the carburetor.

7. When inverted, the float should rest in a level position. See Figure 4.26.

Bowl vent port

Bowl vent

channel

Figure 4.25

Float

insulator block

gasket

Float pin

Float valve

Fuel inlet

Figure 4.26

FUEL SYSTEM AND GOVERNOR

Float bowl

Drain bolt

Flat fiber

gasket

Figure 4.27

Bowl bolt

with recess in

head for O-ring

Gasket seal

Float

Float valve

8. Remove the pin that the float hinges on to remove the float.

NOTE: The float is not adjustable. Spring tension against

the float valve begins to build from the horizontal position, putting progressively more pressure between the tip of the valve and the seat. See Figure 4.28.

NOTE: Because the float valve is crucial to the functioning

of the carburetor, and the viton tip of the valve is subject to wear, technicians should replace the valve and spring any time the carburetor is disas sembled for cleaning.

• A square cross-section gasket seals the bowl to

the body of the carburetor.

Figure 4.28

Main jet

Bowl gasket

Figure 4.29

Compression

spring

9. Remove the main jet using a narrow-shank straight blade screwdriver.

NOTE: Fuel enters the central column through a port

about 1/2” (1cm) from the bottom, to help prevent the ingress of any residue in the bottom of the bowl.

NOTE: The orifice in the main jet meters fuel into the cen-

tral column.

NOTE: Air from the main jet emulsion port enters the cen-

tral column near the top, then gets bubbled through the emulsion tube into the mete re d fu el flow to promote atomization.

See Figure 4.29.

78/83/90 Series Horizontal Shaft Engines

NOTE: The main jet secures the emulsion tube in the

central column of the carburetor. See Figure 4.30.

10. The throttle stop screw has a large pliable lip around the head of the screw. That lip secures a metering plug for the pilot and transition ports. Remove the screw to reach the plug.

See Figure 4.31.

Emulsion air port: pilot jet

Emulsion air port: main jet

Welch plug

Emulsion tube

Main jet

Figure 4.30

Throttle stop

screw

11. Carefully pry out the metering plug and spacer using a small screwdriver.

See Figure 4.32.

shot plug in feed bore

Fuel port to

central column

Fuel feed leg on central column for pilot and transition

Figure 4.31

Throttle stop

screw

Spacer

Metering

plug

Figure 4.32

End view

FUEL SYSTEM AND GOVERNOR

12. Examine the metering plug: See Figure 4.33.

O-rings

Air passage

Fuel metering orifice

Figure 4.33

Transition ports Pilot port

Pilot screw (before head is removed)

• Fuel, drawn from the central column via the long fuel feed leg, is metered by the brass orific e in th e tip of the metering plug.

• Air, drawn from the emulsion air port, is metered by the size of the brass orifice at the entrance to the port.

• The fuel and air that feed the pilot and transition ports are mixed at the metering plug.

• The metering plug creates a small venturi. The pressure drop of the air passing throu gh the me ter ing plug draws the fuel into the passage to the pilot and transition ports, in an emulsified mixture.

NOTE: The pilot screw regulates how much of this pre-

mixed fuel/air emulsion is allowed to enter the throat of the carburetor , to atomize down-stream of the throttle plate. On current production units, it is set at the factory and the screw head is removed. See Figure 4.34.

NOTE: The transition ports are fixed. They are drilled into

the throat of the carburetor, down-stream of the venturi. They lie behind the brass welch plug near the pilot screw.

Figure 4.34

Adjustment screw

Figure 4.35

13. Soak the Carburetor body in a suitable solvent until clean.

NOTE: Ultrasonic cleaning using a suitable water/deter-

gent mixture will clean carburetors safely and effectively.

14. Rinse it thoroughly.

15. Dry the carburetor body using compressed air.

16. Reassemble the carburetor and install it by following steps 1-8 in reverse order.

17. Start the engine and check the idle RPM using a tachometer.

18. Check the top no load speed of the engine.

NOTE: The top no-load speed of the engine is 3500

RPM’s

19. The top no-load speed is easily adjusted by tightening/loosing the speed adjustment screw. Tighten the screw to decrease speed and loosen it to increase speed.

+ 100.

See Figure 4.35.

78/83/90 Series Horizontal Shaft Engines

Fuel shut-off valve

To prevent the needle valve from un-seating and flooding the engine while being towed, engines designed to go

onto log splitters have a fuel shut-off valve built into the carburetor. To service the shut-off valve:

1. Remove the carburetor by following the procedures described in Chapter 3: Air Intake Systems.

2. Remove the choke lever pivot screw. See Figure 4.36.

3. Lift the choke lever off of the carburetor.

4. Remove the shut-off valve cover using a #2 phillips screw driver.

5. Remove the wave washer. See Figure 4.37.

See Figure 4.36.

Fuel shut-off valve

Figure 4.36

Choke lever

pivot screw

Cover

Wave washer

Figure 4.37

Fuel shut-off lever

Figure 4.38

Fuel passage

FUEL SYSTEM AND GOVERNOR

6.Remove the fuel shut-off lever. See Figure 4.38.

NOTE: There is a passage cast into the shut-off lever.

When this passage lines up with the two posts in the carburetor, fuel will flow to the needle valve.

Figure 4.39

Rubber Gasket

Figure 4.40

7. Remove the rubber gasket. See Figure 4.40.

8. Re-assemble the fuel shut-off valve by following the previous steps in reverse order.

9. Test run the engine before returning it to service.

78/83/90 Series Horizontal Shaft Engines

Governor

The engine speed is controlled by a balance between the force applied by a spring (pulling the throttle open) and a flyweight mechanism within the engine applying force to the governor arm (pushing the throttle closed). See Figure 4.41.

NOTE: While the mechanism is simple and robust,

it is important to pay attention when working on parts near the governor. Binding caused by interference with mis-routed lines or cables may make the governor unrespon sive.

NOTE: When a governed engine “hunts”, it is gen-

erally an indication of a lean fuel/air mixture, rather than a problem with the governor.

Spring tension

Governor action

Figure 4.41

Governor arm

To remove the governor arm from the governor shaft:

1. Remove the fuel tank by following the steps

described in the Fuel Tank section of this chapter. NOTE: Mark or note which holes the spring and

linkage was in to ensure they go back in the same holes.

2. Unhook the governor spring.

3. Loosen the nut and clamp bolt. See Figure 4.42.

4. Carefully spread open the seam on the arm.

5. Carefully slide the Governor arm off of the governor

shaft.

6. Unhook the governor linage and throttle return

spring.

To install the governor arm:

1. Rotate the governor shaft clockwise until it stops.

2. Slide the arm onto the shaft. The flat on the top of the shaft should be roughly parallel to the arm.

See Figure 4.42. NOTE: There is a hairpin clip that keeps the gove rnor shaft from sliding into the engine. It may be necessary to

hold the shaft while sliding the arm on to prevent the hairpin clip from “popping off” and allowing the governor shaft to fall into the engine.

Loosen nut

Spread here

Figure 4.42

3. Tighten the nut on the clamp bolt to secure the arm.

4. Attach the governor linkage and spring.

5. Adjust the governor to maintain top no-load speed as described in a previous section of this chapter.

Governor shaft

FUEL SYSTEM AND GOVERNOR

To remove or replace the governor shaft:

1. Remove the engine from the equipment that it powers.

2. Remove the governor arm by following the previously described steps.

3. Remove the flywheel by following the steps described in Chapter 7: Ignition Systems.

4. Remove the crank case cover and crankshaft from the engine by following the steps described in Chap ter 10: Cam, Crankshaft and Piston.

5. Remove the hairpin clip from the governor shaft. See Figure 4.43.

Remove

hairpin clip

Figure 4.43

6. Slide the governor arm out of the engine block from the inside of the engine.

7. Check the movement of the fly-weights and cap on the governor gear.

8. Install the shaft by following the above steps in reverse order.

9. Install the engine on the equipment it powers.

10. Test run the engine and adjust the top no load engine rpms by following the steps described in the carbure tor section of this chapter.

78/83/90 Series Horizontal Shaft Engines

Governor cup and the governor gear

The Governor gear and cup are located inside the crankcase cover. The flyweig hts and the governor cup are inter locked on this family of engines. The governor gear and cup are serviced as a complete assembly.

1. Remove the engine from the equipment that it pow-

ers.

2. Remove the crank case cover from the engine by

following the steps described in Chapter 10: Cam, Crankshaft and Piston.

3. Position the crank case cover over an open vise so

that the governor gear can pass through its jaws.

4. Drive out the governor pin using a pin punch.

See Figure 4.44.

Pin punch

Governor pin

NOTE: There is a washer between the governor

cup and the crank case cover. this washer will fall out when the governor comes out.

5. Install the governor gear and cup assembly by plac-

ing the washer on the inside of the crank case cover.

See Figure 4.45.

6. Apply a small amount of releasable thread locking

compound such as Loctite® 242 (blue) to the gover nor pin.

7. Position the governor gear and cup assembly with

the pin in the hole in the crank case cover.

Figure 4.44

Washer

Figure 4.45

8. By gently striking the governor cup, drive the gover-

nor pin into the crank case cover until it is flush with the out side of the cover.

9. Install the crank case cover by following the proce-

dures described in Chapter 10: Cam, Crankshaft and Piston.

10. Install the engine onto the equipment it came off of. 1 1. Test run the engine in a safe area before returning it

to service.

Governor cup

Figure 4.46

Governor adjustment

Rotate shaft

Push on arm

Figure 4.47

Loosen nut

FUEL SYSTEM AND GOVERNOR

To adjust the governor:

1. Remove the fuel tan k by following the procedures described in the fuel tank section of this chapter.

2. Loosen the governor arm nut but do not remove the nut completely.

3. Pry open the governor arm crimp with a flat head screwdriver.

4. Using pliers, grab the flat section at the top of the governor shaft and rotate it in a counter clockwise direction as far as it can go.

5. Push the governor arm to the right (the spring should pull it in this direction).

NOTE: Rotating the shaft counter clockwise will ensure

the governor cup is pressed all the way down on the governor gear flyweights. Pushing on the gov ernor arm to the right ensures the throttle is wide open against the throttle stop.

See Figure 4.47.

6. Re-tighten the governor arm nut to crimp the governor arm onto the governor shaft.

7. Install the fuel tank.

8. Test run the engine in a safe area. Set the engine RPM’s to 3500 + 100.

78/83/90 Series Horizontal Shaft Engines

CHAPTER 5: LUBRICATION

Oil type and quantity

MTD engines use oil with a SF/SG API rating or better is the recommended oil for this engine.

• The winter engines use SAE 5W-30 oil.

• The summer engines use SAE 10W-30 oil.

• The oil capacity for all of the 78/83/90 series engines is 37 fl.oz (1.1 liters).

Lubrication

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• If the oil is noticeably thin, or smells of gasoline, a carburetor repair may be needed before the engine can be run safely.

• Check the oil level frequently and change the oil more frequently in severe operating conditions such as exceptionally deep snow falls.

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• Synthetic oil is a suitable alternative, but it does not extend service interva ls.

NOTE: MTD recommends the use of petroleum oil during the break in period to ensure the piston rings correctly break in.

Synthetic vs. Petroleum based oil: To simply look at synthetic oil and to comp are it with Petroleum based oil there is very little difference. However, when you look at the two through a microscope it is easy to see the difference . Syn thetic is made up of smaller molecules. This allows the oil to get into areas that petroleum based oil cannot.

78/83/90 Series Horizontal Shaft Engines

Oil dip stick

NOTE: There are two types of dip sticks that can

be found on MTD engines; a threaded dip stick that was used on older engines and a quarter turn dip stick that is used on engines currently being produced.

See Figure 5.1.

To check the oil with a threaded dip stick:

1. Twist and remove the dip stick from the engine.

2. Clean the oil off of the tip of the dipstick.

3. Re-insert the dipstick without threading it in to get the oil level reading.

4. The oil level is determined by the lowest point on the dipstick that is completely covered with oil.

See Figure 5.2.

1/4 turn

Do not thread dipstick in to check the oil level

Threaded

Figure 5.1

Figure 5.2

To check the oil with a 1/4 turn dip stick:

1. Twist and remove the dip stick from the engine.

2. Clean the oil off of the tip of the dipstick.

3. Re-insert the dipstick and turn it until it is fully seated to get the oil level reading.

4. The oil level is determined by the lowest point on the dipstick that is completely covered with oil.

See Figure 5.3.

Fully seat the dip stick before reading it

Figure 5.3

Dip stick tube removal

Remove these

screws

Lubrication

To remove/replace the dip stick tube: NOTE: The procedure to remove the dip stick tube is the

same for both styles of dip stick.

NOTE: A threaded dip stick and tube assembly is inter-

changeable with a quarter turn dip stick and tube assembly.

1. Remove the upper screw that secures the dip stick tube to the fuel tank support bracket using a 10 mm wrench.

2. Remove the screw at the bottom of the dip stick tube using a 10 mm wrench.

3. Pull the dip stick tube out of the engine block.

See Figure 5.4.

Figure 5.4

4. Inspect the O-rings on the dip stick and the dip stick tube. Replace if damaged.

NOTE: Lubricate the O-ringfor installation.

5. Install by following the above steps in reverse order.

78/83/90 Series Horizontal Shaft Engines

Lubrication system

MTD uses a splash lube system for its horizontal shaft engines. The connecting rod has a dipper on it that “splashes” oil around the inside of the engine. See Figure 5.5.

NOTE: The cam and the balance shaft were

removed for a better view of the lubrication system.

Dipper

Figure 5.5

The splashing action will create an oil mist that reaches the cylinder head. There are two oil passages that run along the engine cylinder. The one on the top side of the engine is the oil supply passage. The oil mist will flow through this passage to the cylinder head.

The second oil passage runs along the bottom side of the cylinder. This is the oil return passage. As the name implies, it allows the oil collecting in the cylinder head to return to the sump. The return passage is the tiny hole that is in between the two tappet passages.

NOTE: Because these engines use splash lubrica-

tion, the type of oil and the oil level is critical for proper operation of the engine. If the oil level is too low , the dipper on the con necting rod cannot splash the oil into the engine. If the oil level is too high, the oil will not change into a mist to reach the upper side of the engine.

See Figure 5.6.

Oil supply passage

Oil return

Tappet

passages

Figure 5.6

Positive crankcase ventilation valve

Lubrication

Figure 5.7

Remove these

screws

Breather hose

Clamp

To remove the valve cover and PCV valve: NOTE: The PCV valve is located inside the valve cover.

The function and test procedures for the PCV valve is covered in Chapter 2: Basic Troubleshoot ing.

1. Disconnect and ground the spark plug wire.

2. Squeeze the spring clamp that secures the breather hose to the valve cover nipple and slide it back. Then remove the breather hose from the valve cover nip ple. See Figure 5.7.

3. Remove the five screws that hold the valve cover to the cylinder head using a 10mm wrench. See Figure 5.8.

NOTE: The PCV valve is not serviceable. If it is faulty, the

valve cover must be replace d .

4. Reassemble the PCV and valve cover by following the above steps in reverse order.

Figure 5.8

NOTE: Tighten the cover bolts to a torque of 62 - 79.7 in-

lbs. (7-9 Nm).

5. Inspect the PCV tubing for cracks, brittleness or signs of leaking. Replace the PCV tube if any are found.

6. Test run the engine before returning to service.

78/83/90 Series Horizontal Shaft Engines

Starter and Charging System

CHAPTER 6: STARTER AND CHARGING SYSTEMS

There are two starter systems available for the MTD engine. The first one is a recoil starter and the second is a 110 volt electric starter. All of the 78/83/90 series MTD engines equipped with an electric starter, are also equipped with a recoil starter .

Recoil Starter Removal

To remove recoil assembly from the engine:

Remove these screws

Figure 6.1

1. Remove the three nuts that secure the recoil assembly to the engine using a 8mm wrench. See Figure 6.1.

2. Install the starter by following the above step in reverse order. Tighten the screws to a torque of 53 71 in-lbs (6-8 Nm).

78/83/90 Series Horizontal Shaft Engines

Starter Cup

The starter cup is a steel cup that is bolted to the flywheel.

1. Inspect the inside of the starter cup. See Figure 6.2. NOTE: If the starter was failing to engage the fly-

wheel, and the edges of the slots inside the cup are burred or damaged, replace the starter cup.

NOTE: If the starter cup is replaced, the complete

starter should be replaced as well, to prevent a repeat failure.

2. Block the piston to prevent the crank shaft from turning by:

2a. Remove the spark plug.

Starter cup

Inspect slots

2b. Insert approximately 3.5 feet of starter rope in

the spark plug hole.

NOTE: Leave part of the rope sticking out of the

engine so that the rope can be removed later.

3. Remove the starter cup by removing the flywheel nut.

4. Install a starter cup:

• Place the starter cup on the flywheel, with the

three bosses on the bottom of the starter cup into the dimples in the flywheel

• Align the pin on the flywheel fan with the hole in

the starter cup. See Figure 6.3.

5. Install the flywheel nut and tighten it to a torque of 47 - 52 ft-lbs (64 - 70 Nm).

Flywheel dimples

Alignment pin

Alignment hole

Figure 6.2

Starter cup

bosses

Figure 6.3

Starter and Charging System

Starter Rope

The most common failure mode for most recoil assemblies is a broken rope.

NOTE: If the spring was not damaged when the recoil sprung back, it is possible to simply remove the rem-

nants of the old rope and install a new rope.

Rope inserted from the inside out

1. Remove the starter by following the steps described earlier in this chapter.

2. Remove the old starter rope by prying out the starter cord knot and pulling the rope out with it.

3. Cut a piece of #6 recoil rope 75” (1.9 meters) long.

4. Heat fuse the ends of the starter rope, and tie a double half-hitch in one end.

5. The rope may be easily installed from the outside-in. Pull the rope tight to seat the knot firmly in the recess in the back of the pulley.

See Figure 6.4.

Figure 6.4

Figure 6.5

pulley blocked

NOTE: It may be necessary to wind the pu lley clockwise to

line up the hole in the pulley to the hole in the starter housing. If so, use a punch or screwdriver to block the pulley, preventing it from rewinding. See Figure 6.4.

6. Wind the spring 6 - 7 turns and block it with a punch or screwdriver to keep it from rewinding.

7. Install the handle and handle insert on the loose end of the rope, again using a double half-hitch. See Figure 6.5.

8. Remove the blocking tool and let the rope rewind into the starter at a controlled rate.

78/83/90 Series Horizontal Shaft Engines

9. Give the starter a couple of test pulls to verify the right amount of tension on the starter rope and check the action of the pawls.

NOTE: If starter rope tension needs to be adjusted,

hook the rope into the notch in the pulley and wind the pulley a couple of turns to add tension-.

See Figure 6.6.

Rope-return tension may be increased by winding

the rope and pulley counter clockwise.

10. Install the starter and tighten the starter nuts to a torque of 53 - 71 in-lbs (6-8 Nm).

notch in pulley

Figure 6.6

Starter pulley and recoil spring

Figure 6.7

Pressure plate

Starter and Charging System

If damage is suspected, the recoil may be disassembled by:

NOTE: The recoil spring is nested within the starter pulley

and both parts are sold as a single part number.

Eye protection should be worn if

! CAUTION! CAUTION

1. Remove the starter by following the steps described earlier in this chapter.

2. Relieve the spring tension by: 2a. Pull some slack in the rope inside of the starter 2b. Hook the rope into the notch in the starter pul-

ley.

2c. Wind the pulley clockwise until all tension is

removed.

3. Remove the shoulder screw and pressure plate using a 10 mm wrench.

the starter pulley is to be removed.

See Figure 6.7.

Torsion springs

Pawls

Figure 6.8

NOTE: Beneath the pressure plate is a compression

spring and two starter pawls that are held in the disengaged position by two torsion springs.

4. Inspect the pawls and torsion springs for wear and damage.

See Figure 6.8.

78/83/90 Series Horizontal Shaft Engines

5. Carefully lift the spring and pulley out of the recoil housing.

! CAUTION! CAUTION

Eye protection should be worn while removing the starter pulley.

NOTE: If the spring is undamaged, but has been

NOTE: Evaluate the damage, including parts prices

6. To re-assemble, apply a small amount of lithiumbased chassis grease to the surface of the recoil housing that contacts the spring.

See Figure 6.9.

The recoil spring is under tension and can release as the pulley is removed.

removed from the pulley, the spring may be re-wound. Hook the end of the spring into the slot in the outer lip of the recess of the pulley and wind the spring into the recess in a counter-clockwise direction.

and local labor rates. In some parts of the country , it makes economic sense to replace the complete assembly, in other areas labor rates favor repair.

Pulley

Spring

Housing

Lithium grease

Figure 6.9

Pawls

NOTE: Use low temperature grease on the snow

engines.

7. Carefully position the pulley and spring in the recoil housing. Rotate the pulley gently counter-clockwise until the spring seats, allowing the pulley to fall into position.

8. Install the torsion springs and p awls so th at the long arm of the spring reaches outside of the pawl, and draws it toward the center of the assembly. See Figure 6.10.

NOTE: The rolled end of the pawl fits in the recess

in the starter pulley. The bent end engages the starter cup. The roll faces inward and the hook faces outward.

NOTE: The extrusions on the pressure plate should fall inside of the pawls as the starter is assembled. NOTE: Drag on the pressure plate, from the friction between the compression spring and the head of the

shoulder screw causes these extrusions to force the pawls outward, engaging the starter cup.

9. Apply a small amount of thread locking compound such as Loctite 242 (blue) to the threads of the shoulder screw, and install the screw. Tighten it to a torque of 71 - 89 in-lb. (8 - 10 Nm).

10. Install the starter rope by following the steps described in the previous section of this chapter.

11. Install the starter and tighten the starter nuts to a torque of 53 - 71 in-lbs (6-8 Nm).

Figure 6.10

Starter and Charging System

Electric starter

The electric starter is only available on the snow engine. It is powered by an extension cord that is plugged

into household 120 volt AC current. The starter and switch assembly are one piece and are not serviceable.

To replace the starter assemb ly:

Remove these screws

Figure 6.11

1. Disconnect the extension cord.

2. Remove the two screws that secures the switch box to the engine.

See Figure 6.11.

Remove these

screws

Figure 6.12

3. Remove the starter by removing the two screws that hold it to the engine block using a 10mm socket and a long extension.

See Figure 6.12.

78/83/90 Series Horizontal Shaft Engines

4. Pull back on the starter approximately 1/2”. Then angle it away from the engine while sliding it out of the engine.

NOTE: Before condemning a starter make sure to

A. Remove the starter from the engine. B. Plug the extension cord into the switch housing. C. Hold the starter down and press the starter but-

• If the starter works on the bench, confirm that

• If the crankshaft does not rotate with normal

NOTE: This includes adjusting the valve lash.

• If the crankshaft rotates with normal force but the starter is unable to turn it, replace the starter.

• If the starter does not work, replace the starter.

4. Install the starter by following the above steps in reverse order. NOTE: Tighten the starter screws to a torque of 195 - 221 in-lbs (22-25 Nm).

See Figure 6.13.

bench test it. To bench test a starter:

ton.

the engine crankshaft rot ates with nor mal fo rce.

Figure 6.13

force, identify and repair the engine problem.

Pull back and angle the starter away while sliding it out

Starter and Charging System

Charging system

Some engines are equipped with a charging system. The charging system used on MTD engines consists of three components, the rotor, stator and the rectifier.

• Alternator rotor: The rotor consists of five magnets on the inside of the flywheel that rotate around a stator that is mounted to the cylinder block. As the

Magnets

Figure 6.14

crankshaft and flywheel rotate, the moving mag nets induce a charge in the stator. See Figure 6.14.

Stator

Figure 6.15

• Alternator stator: The stator consists of copper field windings around an iron core. The stator is attached to the engine block beneath the flywheel. See Figure 6.15.

• Rectifier: A set of four diodes that convert the AC current into DC current. The rectifier is built into the stator and it is not serviceable.

78/83/90 Series Horizontal Shaft Engines

Charging system testing

To test the charging system:

1. Locate the charger harness. It will be by the right handle bar of the snow thrower when the engine is installed.

2. Start the engine and run it at full throttle.

See Figure 6.16.

Charger

harness

Figure 6.16

3. Connect the black (-) lead of a digit al multimeter to a good ground on the engine.

4. Set the multimeter to read AC voltage.

5. Back probe the yellow wire in the charger harness with the red (+) lead of the multimeter. See Figure 6.17.

6. The multimeter should read a voltage of 13 - 18Vac.

7. Set the multimeter read DC voltage.

8. Back probe the red wire of the charger harness. See Figure 6.18.

9. The multimeter should read 17 - 26Vdc.

10. If the results do not match what is listed above, check the magnets on the rotor.

Charger harness

Yellow wire

Figure 6.17

NOTE: If the magnet are still magnetic, replace the

stator.

Red wire

Figure 6.18

Stator

Baffle

Figure 6.19

Grommet

Starter and Charging System

To remove/replace the stator:

1. Remove and ground the spark plug wire.

2. Remove the flywheel by following the steps described in Chapter 7: Ignition System.

3. Remove the baffle that covers the charger harness using a 10mm wrench.

4. Slide the grommet out of the engine block. See Figure 6.19.

5. Remove the two screws that secure the stator with a 10mm wrench and lift the stator off of the engine. See Figure 6.20.

See Figure 6.19.

6. Install the stator by following the above steps in reverse order.

7. Test run the engine in a safe area and retest the voltage output before returning to service.

Remove these

screws

Figure 6.20

Rotor

Rotor failures are extremely rare. To check the rotor:

• Confirm that the magnets are firmly attached to th e flywheel.

• Hold a screwdriver or a similar tool made of ferrous metal within a 1/4” of each magnet.

• If the tool is drawn to the magnet, the rotor is good.

78/83/90 Series Horizontal Shaft Engines

Troubleshooting the ignition system

CHAPTER 7: IGNITION SYSTEM

The purpose of the ignition system is to provide a spark in the combustion chamber at the proper time to efficiently ignite the fuel/air mixture. The steps in troubleshooti ng the ignition system are:

1. Examine the spark plug(s) by following the steps described in the spark plug section of this chapter.

NOTE: It is convenient to check the compre ssion when the

spark plug is removed for examination.

2. Connect a spark tester between the spark plug wire and a good ground point on the engine. See Figure 7.1.

Ignition System

Spark tester

! CAUTION! CAUTION

Figure 7.1

ing out of the spark plug hole will catch on fire.

NOTE: It only takes 1,000 volts to jump a 0.025” a ir gap in o pen atmo sp her e, it takes 10,000 volts to jump the

same gap at 120 psi; therefore, an open air spark test in not valid.

NOTE: The spark should be a minimum of 10 Kv (10,000 volts) at pull over speed.

3. Make sure any remote stop switch is turned to the “run” position.

4. Pull the starter rope. If sparks can be seen in the spark tester, the ignition system is working. NOTE: If there are sparks present in the spark tester, install a known-good spark plug and prime test the

engine. If the engine does not start, check the fly wheel key. If the fly wheel key is intact, the problem is not in the ignition system. Check the engine’s compression.

5. If no sparks or weak sparks are seen in the spark tester, use the electric starter to spin the engine. NOTE: If sparks are now seen in the spark tester, check the module air gap. If no sparks are seen, further test-

ing is required.

6. Test the stop switch by following the steps described in the stop switch section of this chapter.

7. If the stop switch is working properly, inspect the flywheel and flywheel key. If the flywheel and key are OK, replace the module.

Never remove the spark plug and hold it against the cylinder head to test for spark. The fuel/air mix com-

78/83/90 Series Horizontal Shaft Engines

Stop switch

All MTD horizontal engines that are in use in North America have a stop switch built into the throttle lever assembly. MTD engines used on snow blowers have an additional stop (ignition) switch in the engine shroud.

To test the stop switch (throttle):

1. Remove the control panel by following the steps described in Chapter 3: Air Intake Systems.

2. Test the remote (ignition) stop switch first by following the procedures described in the next section of this chapter.

3. Disconnect the lead that runs from the module to the stop switch.

NOTE: The blower housing wa s removed for picture

clarity.

4. Connect one lead of a digital multimeter to the lead going to the stop switch. Connect the other lead of the digital multimeter to a good ground.

See Figure 7.2.

Disconnect switch

Figure 7.2

Throttle lever

Switch lead

5. Set the multimeter to the ohms (Ω) scale.

6. Operate the throttle lever while watching the multimeter.

• When the throttle is all the way to the right (stop), the multimeter should read at or near 0.0Ω, indicat- ing continuity. See Figure 7.3.

• When the throttle is all the way to the left (full throttle), the multimeter should not show continuity. See Figure 7.4.

continuity

Figure 7.3

no continuity

Figure 7.4

Remote (ignition) stop switch (snow engines)

Wires

Remote switch

Figure 7.5

Key inserted

Ignition System

To test the remote stop switch:

1. Remove the engine shroud by following the procedures described in Chapter 3: Air Intake System.

2. Disconnect the two wires from the remote switch. See Figure 7.5.

3. Connect a digital multimeter to the two tabs on the back of the remote switch.

No continuity

Key removed

Continuity

Figure 7.6

4. Set the multimeter to the ohms (Ω) scale.

• With the key fully inserted, the multimeter should

not show continuity.

• With the key removed, the meter should show con-

tinuity. See Figure 7.7.

5. If the test results do not match the results described in step 4, replace the remote switch.

See Figure 7.6.

Figure 7.7

78/83/90 Series Horizontal Shaft Engines

Test for ignition that won’t turn off

MTD engines are turned off by removing the spark from the en gine. This is accomplished by sh orting the primary

windings of the coil to ground.

If the engine does not stop when the key is removed

and/or the throttle is moved to the stop position:

1. Test the stop switch and the remote ignition switch by following the procedures described in the previ ous sections of this chapter.

NOTE: If the switches are working properly, leave the front engine shroud off.

2. Remove the blower housing.

3. Move the throttle to the wide open throttle (rabbit) position.

4. Connect one lead of the multimeter to one of the wires that goes to the remote switch.

NOTE: One wire will go to one of the module screws to provide a ground path. The other wire goes to the primary winding of the coil.

Coil reading

One of the wires

5. Set the multimeter to the ohms (Ω) scale.

6. Connect the other lead of the multimeter to a good ground.

7. Repeat steps 6 - 8 on the other wire. See Figure 7.9.

• The ground wire should have a resista nce read-

• The wire that goes to the primary windings of

See Figure 7.8.

ing of 0.2 Ω or less. If the reading is > 0.2 Ω, check the ground at the module and check the wire for a fault.

the coil should have a resistance reading of approximately 1.0 Ω. If the reading is >1.2 Ω, replace the coil.

Figure 7.8

Ground reading

Ground wire

Blue wire

Figure 7.9

Ignition System

The module

The coil in this ignition system is an inductive discharge magneto, contained in a single module.

• The inductive discharge magneto has a two leg design.

• The magneto is energized by the passing of a magnet mounted in the flywheel.

• Ignition timing is set by the location of the flywheel in relation to the crankshaft. Proper timing is maintained by a steel key.

Normal performance of the coil is to produce at least 10,000 volts at starter-rope pull-through speed. The presence or absence of strong spark, with the stop switch known to be good, is generally enough to identify

the ignition coil as good or bad. Resistance readings may help confirm the sou rce of the failu re , bu t ar e ge ne r ally meaningless because they only measure a small part of the module.

NOTE: No spark or a weak spark may be the result of an improper air gap. The air gap space should be

0.016”-0.024” (0.4-0.6mm).

Simple spark-testers are readily available a nd inexpe nsive. Thexto n Part # 404 is available from a variety of retailers, and similar tools are available from other manufacturers. See Figure 7.10.

Instructions on back of package

Figure 7.10

NOTE: If the complaint is that the engin e quits running when it g ets warm, the ign ition module should be tested

with the engine at normal operating temperature.

• At operating speed, the ignition should produce voltage approaching 12,000.

• At pull-over speed (~

NOTE: The voltage required for a flash-over will vary with spark plug condition and gap. NOTE: Pull-over speed may vary from operator to operator. NOTE: Failure of the magnets in the flywheel is exceedingly rare. To test the magnets, simply hold an item

made of ferrous metal roughly 1/4” (0.635cm) away from the magnets in the flywheel. It shou ld be drawn to the flywheel. A wrench or screwdriver is suitable for this test.

600 RPM), voltage should be at least 10,000V.

78/83/90 Series Horizontal Shaft Engines

Module removal

1. Unplug the spark plug.

2. Remove the engine shroud by following the steps procedures in Chapter 3: Air Intake Systems.

3. Remove the front fuel tank shroud using a 10 mm wrench.

4. Remove the blower housing. NOTE: The recoil starter will come off with the

5. Unhook the spark plug wire from the clip in the carburetor insulator. See Figure 7.12.

See Figure 7.11.

blower housing.

Front fuel tank shroud

Disconnect the throttle

switch lead Unhook spark

plug wire

Remove these

screws

Figure 7.11

6. Disconnect the leads that runs from the module to the stop switches.

7. Remove the module using a 10mm wrench.

See Figure 7.11.

Remote stop switch leads

Figure 7.12

Installing the module and setting the air gap

0.020” feeler gauge

Magnet

Ignition System

NOTE: If just setting the air gap, loosen the module

mounting screws first then follow the same steps as described below.

1. Rotate the flywheel so that the magnets are away from where the module is mounted.

2. Install the module. Do not tighten the module down.

3. Place a non-ferrous feeler gauge between the module and the flywheel.

NOTE: The air gap should be 0.016” - 0.024” (0.4-0.6

mm).

4. Rotate the flywheel so that the magnet s align with the legs of the module while holding the feeler gauge in place.

See Figure 7.13.

Figure 7.13

6. Rotate the flywheel to remove the feeler gauge.

7. Install the blower housing and starter.

8. Hook the spark plug wire from the clip in the carburetor insulator.

9. Install the Heat box and intake elbow by following the steps described in Chapter 3: Air Intake Systems.

10. Connect the spark plug wire to the spark plug.

11. Test run the engine before returning to service.

5. Tighten the module mounting screws to a to rque of 80 - 106 in-lbs (9 - 12 Nm).

78/83/90 Series Horizontal Shaft Engines

Flywheel

The flywheel holds the magnets. These magnets induce a field in the module which in turn produces a spark. It also controls the timing of the ignition system by controlling when the magnets are introduced to the module.

A sheared flywheel key will throw off the ignition timing. Sheared keys are uncommon on MTD engines. If one is found, check the crankshaft and flywheel for damage. To Remove and/or inspect the flywheel and key:

1. Remove the blower housing. 1a. Remove the engine shroud by following the steps procedures in Chapter 3: Air Intake Systems 1b. Remove the front fuel tank shroud using a 10 mm wrench . 1c. Remove the five screws securing the blower housing and slide it off of the engine.

2. Block the piston to prevent the crank shaft from turning by: 2a. Remove the spark plug. 2b. Insert approximately 3.5 feet of starter rope in the sp ark plug hole. IMPORTANT: Leave part of the rope sticking out of the engine so that the rope can be removed later.

3. Remove the flywheel nut, starter cup and flywheel fan using a 23mm wrench.

4. Remove the flywheel by applying a sharp blow to the crankshaft using a brass drift punch and a ham mer while gently prying with a pry bar. The flywheel will loosen then lift it off.

NOTE: The magnets on the inside of the magnet

will hold it down, preventing the typical “pop” when the flywheel loosens from the tapper on the crankshaft.

NOTE: Never strike the crankshaft directly with a

hammer. To prevent damage to the crank shaft use a brass drift punch or a piece of wood between the hammer and the crank shaft. See Figure 7.14.

If the flywheel shows any signs of physical damage such as cracks, broken vanes, or damaged

! CAUTION! CAUTION

5. Inspect the key, keyway, and tapered mating surfaces of the flywheel and crankshaft.

NOTE: If the key is damaged, it must be replaced. If there is damage to the crankshaft, the engine must be

NOTE: On installation, confirm that the key is properly seated (the flat of the key parallel with the threaded

IMPORTANT: The tapers in flywheel and on the crankshaft must be clean and dry. The flywheel is held in

6. Install the flywheel nut to a torque of 66-81 ft-lbs (90-110Nm).

7. Adjust the air gap by following the steps described in the previous section of this chapter.

8. Reassemble the engine.

9. Test run the engine before returning to service.

key-way, replace it. A damaged flywheel poses a threat of burst failure. Burst failures are extremely hazardous to surrounding people and property.

short blocked because the crankshaft is not available as a service part.

section of the crankshaft) in the keyway, and that the tapers are fully seated. Key or keyway failure may result from improper seating.

place by the friction between the flywheel and the crankshaft, not the key. The key is only to guide the flywheel to the proper position until it is torqued down.

Brass punch

Figure 7.14

Ignition System

About the spark plug

• The spark plug is a F6RTC, gapped to 0.026” - 0.030” (0.65 - 0.75 mm). NOTE: The F6RTC plug is the only plug that is EPA certified for the MTD engine.

• Wear rate will vary somewhat with severity of use. If the edges of the center electrode are rounded-off, or any other apparent wear / damage occurs, replace the spark plug before operating failure (no start) occurs.

Cleaning the spark plug

• Cleaning the spark plug is not recommended. If th e plug needs to be cleaned, replace it.

• Use of a wire brush may leave metal deposits on the insulator that cause the spark plug to short-out and fail to spark.

• Use of abrasive blast for cleaning may damage the ceramic insulator or leave blast media in the recesses of the spark plug. When the media comes loose during engine operation, severe and non-warrantable engine damage may result.

Inspection of the spark plug

Inspection of the spark plug can provide indications of th e op er a ting con d ition of th e en gin e .

• Light tan colored deposits on insulator and electrode s is nor mal.

• Dry , black deposits on the insulator and electrodes indicate an over-rich fuel / air mixture (too much fuel or not enough air)

• Wet, black deposits on the insulator and electrodes indicate the presence of oil in the combustion chamber.

• Heat damaged (melted electrodes / cracked insulator / metal transfer deposits) may indicate detonation.

• A spark plug that is wet with fuel indicates that fuel is present in the combustion chamber, but it is not being ignited.

Spark plug removal

13/16” spark plug

socket

Muffle

1. Disconnect and ground the spark plug wire.

2. Remove the spark plug using a 13/16” or 21mm wrench. A flexible coupling or “wobbly” extension may help.

3. Gap a new spark plug to 0.026” - 0.030” (0.65 - 0.75 mm).

4. Install the new spark plug and tighten to a torque of 15 - 18 ft - lbs (20 - 25 Nm).

See Figure 7.15.

Valve cover

Figure 7.15

78/83/90 Series Horizontal Shaft Engines

Exhaust

CHAPTER 8: EXHAUST

The exhaust system is a frequently overlooked component of an engine. It is important to make sure the muffler

is in good condition and free of debris and/or insects.

NOTE: A blocked muffler will result in poor performance. If a muffler is completely blocked the engine may not

start.

Summer engines

One of the main differences between the summer and the snow engines is the exhaust system. Because of this

they will be addressed separately.

Spark arrestor (if eq uipped)

A spark arrestor is available as an option on summer engines.

NOTE: Spark arr estors are an option that are required on

all engines used in California and U.S. national parks.

See Figure 8.1. Inset.

The spark arrestor also serves to keep blockages out of the exhaust system.

Figure 8.1

Muffler shield

NOTE: T ypical blockages include insect nest s built during

the dormant season.

The spark arrestor should be checked and/or cleaned every month. The spark arrestor can be inspected by shin ing a flash light into the muffler. See Figure 8.1.

If The spark arrestor needs to be cleaned or replaced:

1. Remove the four screws that retain the muffler shield using a 10mm wrench and lift it off of the engine. See Figure 8.2.

2. Remove the spark arrestor retain ing screw using a #2 phillips screwdriver.

3. Pry the spark arrestor out of the muffler.

4. The spark arrestor can be:

•Replaced

• Cleaned by mechanical means

• Solvent cleaned

See Figure 8.1.

Figure 8.2

• Burned clean using a butane or propane torch.

5. Install the spark arrestor by following steps 1-3 in reverse order.

78/83/90 Series Horizontal Shaft Engines

Muffler removal/replacement

1. Remove the two nuts that hold the manifold pipe to the cylinder head using a 13mm wrench. See Figure 8.3.

2. Lift the muffler off of the engine.

3. Clean all of the gasket material off of the cylinder head and the muffler.

NOTE: The MTD engine uses a graphite exhaust

gasket. It is not reusable and must be replaced every time the muffler nuts are loosened.

NOTE: The graphite exhaust gasket transfers heat

from the cylinder head to the muffler. The heat transfer helps to keep the engine oper ating temperature under control. Do not substitute an exhaust gasket made from another material.

See Figure 8.4.

Nuts

Figure 8.3

Remove all gasket material

Graphite gasket

Figure 8.4

NOTE: If reusing the muffler skip to step #8

4. Disconnect the manifold pipe from the muf f ler usin g a 13mm wrench.

5. Clean all of the gasket material off of the manifold pipe and the muffler.

Manifold pipe

Figure 8.5

Outer heat shield

Exhaust

6. Remove the four screws that secure the outer heat shield to the muffler.

7. Slide the outer heat shield off of the muffler.

NOTE: The inner heat shield is serviced with the muffler.

8. Install the muffler by following the previous steps in reverse order.

NOTE: Install a new gaskets between the muffler and the

manifold pipe also between the manifold pipe and the cylinder head.

NOTE: Tighten the muffler and the manifold nuts to a

torque of 13 - 16 ft-lbs (18 - 22 Nm).

See Figure 8.6.

Figure 8.6

9. Test run the engine before returning to service.

78/83/90 Series Horizontal Shaft Engines

Muffler removal/replacement (snow engines)

To remove/replace the muffler:

1. Remove the control panel by following the procedures described in Chapter 3: Air Intake System.

2. Remove the muffler heat shield by: 2a. Remove the two rear heat shield screws using

a 10 mm wrench.

See Figure 8.7.

Remove these

screws

Figure 8.7

3. Remove the screw that holds the heat shield to the cylinder head using a 10 mm wrench. See Figure 8.8.

4. Work the heat shield off of the engine.

5. Remove the three screws that secure the exhaust pipe shield using a 10 mm wrench.

See Figure 8.9.

Remove these screws

Remove this screw

Figure 8.8

Exhaust pipe shield

Figure 8.9

Cub Cadet MTD 90 User Manual

Specifications and Main Features

Frequently Asked Questions

User Manual

CHAPTER 1: INTRODUCTION

Professional Service Manual Intent

Safety

Fasteners

Assembly instructions

Model and serial number

Maintenance

Air filter (summer engines)

Oil type and capacity

Changing the oil

Fuel

Fuel filters

Valve lash

Cleaning the engine

General torque specifications

Maintenance Chart

Frequently used specifications

CHAPTER 2: BASIC TROUBLESHOOTING

Definitions

Introduction

Steps to troubleshooting

Define the problem

Identify factors that could cause the problem

Repairing the problem

Prime test

Leak-down test

Compression test

PCV testing

CHAPTER 3: AIR INTAKE SYSTEMS

Heat box (snow engines)

Air filter (summer engines)

Air filter base and control panel (summer engines)

Carburetor and Insulator

CHAPTER 4: THE FUEL SYSTEM AND GOVERNOR

Inspecting the fuel

Test fuel for alcohol

Fuel tank vent

The fuel filter

Inspect the fuel lines

The fuel tank

Evaporative (EVAP) emissions system

Troubleshooting the EVAP System

Charcoal canister

Roll over valve

Choke

Primers (snow engines)

Carburetors

Inspecting the carburetor

Disassembly and rebuilding of the carburetor

Fuel shut-off valve

Governor

Governor arm

Governor shaft

Governor cup and the governor gear

Governor adjustment

CHAPTER 5: LUBRICATION

Oil type and quantity

Oil dip stick

Dip stick tube removal

Lubrication system

Positive crankcase ventilation valve

CHAPTER 6: STARTER AND CHARGING SYSTEMS

Recoil Starter Removal

Starter Cup

Starter Rope

Starter pulley and recoil spring

Electric starter

Charging system

Charging system testing

Stator

Rotor

Troubleshooting the ignition system

CHAPTER 7: IGNITION SYSTEM

Stop switch

Remote (ignition) stop switch (snow engines)

Test for ignition that won’t turn off