Cub Cadet MTD P90 User Manual
Professional Shop Manual
P90 Series Vertical Shaft 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.
© Copyright 2011 MTD Products Inc. All Rights Reserved
Table of Contents
Chapter 1: Introduction
Professional Service Manual Intent . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Safety. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Fasteners . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Assembly instructions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
Model and serial number . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Maintenance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Spark plugs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Air filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7
Oil type and capacity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8
Changing the oil . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9
Oil filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Oil pre-screen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Fuel system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Servicing the fuel system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Fuel filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Valve lash . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Exhaust system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Cleaning the engine . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
General torque specifications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14
Chapter 2: Basic Troubleshooting
Definitions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Steps to troubleshooting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Define the problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15
Identify factors that could cause the problem . . . . . . . . . . . . . . . . . . . . . . . . . . 17
Repairing the problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 21
Prime test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Leak-down test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 22
Compression test . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24
PCV testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25
Chapter 3: Air Intake System
Air filter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
Blower/air filter housing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 29
Carburetor and Insulator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 30
I
Chapter 4: The Fuel System and Governor
Inspecting the fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 33
Test fuel for alcohol . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
Choke/throttle cable adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 35
Carburetors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Inspecting the carburetor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 36
Disassembly and rebuilding the carburetor . . . . . . . . . . . . . . . . . . . . . . . . . . . . 37
Engine speed adjustment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41
Afterfire solenoid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42
Testing the afterfire solenoid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44
Governor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Governor arm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46
Governor shaft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Governor cup and the governor gear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48
Chapter 5: Lubrication
Oil type and quantity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49
Oil dipstick . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Dip stick tube removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 50
Lubrication system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Testing the oil Pump . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
Positive crankcase ventilation valve . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 53
Chapter 6: Starter and Charging Systems
Starter removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
Bench testing the electric starter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 56
Rebuilding the starter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 57
Charging system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 68
Charging system testing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
Stator . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 71
Rotor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
Chapter 7: Ignition System
Troubleshooting the ignition system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 73
The module . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 74
Module removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 75
Installing the module and setting the air gap . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
Flywheel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
Spark plug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Cleaning the spark plug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Inspection of the spark plug . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
Spark plug removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
II
Chapter 8: Exhaust
Spark arrestor (if equipped) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
Muffler removal/replacement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
Catalytic converter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
Chapter 9: Cylinder Head
Cylinder head removal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
Cylinder head installation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 86
Valves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 87
Push rod guide plate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 90
Chapter 10: Crankshaft, piston and Connecting Rod
Crankshaft inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 94
Piston Inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
Connecting rod inspection . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Cylinder inspection. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97
Balance Shaft . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
Reassembly . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
Engine specifications chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 101
Engine torque values chart . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
Chapter 11: Failure Analysis
Abrasive Ingestion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 105
Insufficient lubrication . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
Engine Overspeed . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
Overheated . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 111
Mechanical Breakage/ Wear . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
Detonation/preignition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 112
III
IV
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 MTD P90 series vertical 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 RNING! WA RNING
! 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 procedures 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 Support.
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.
1
P90 Series Vertical 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
2
• 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 summarized 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 procedures. This avoids repeating
the same procedure two or three times in the manual.
3
P90 Series Vertical Shaft Engines
MTD Vertical Engine Model Designators
• Starter/Alternator
1=Recoi l star t
s
• 2=Elect ric star t (12V)
• 3=E. start/alt. 18W
• 4=E. start/alt. 3A/5A
• 5= AutoChoke / Recoil
• 6= AutoChoke /Elec tric
Start
• 7= AutoChoke /Elec tric
Start/Alt
P= Vertica l (1 cyl.)
Q= Vertical (2 cyl.)
1 P 6 1 M U A
Major Revision
Change
Compliance
U United States (50 State)
HEurope
C California
0 (Zero) 49 State
G U.S .(49 ) and Europe
W U.S.( 50) an d Europe
Bore Dia. (mm)
End Product
B Mower ( long shaft M0)
C Chi pper/Sh redd er M Mower ( long shaft/no shrou d)
D Mower (long shaft M 1) N Mower (short shaft M0)
E Mower ( short shaft M1) P Mower ( long shaft M0)
F Mower (short shaft/no shrou d) Q Mower (short shaft M0)
R Mower (long shaft/ no shroud) T Till er
L Log split ter W Worl d Til ler
Model number
MTD Engine Serial Numbers
1P65FH/0510271A0023
MonthYear
Producing Line# and Shift#:
1A=L ine 1, 1
1B=L ine 1, 2
2A=L ine 2, 1
2B=L ine 2, 2
3A=L ine 3, 1
3B=L ine 3, 2
4A=L ine 4, 1
4B=Line 4, 2nd Shift
Date
st
nd
st
nd
st
nd
st
Shift
Shift
Shift
Shift
Shift
Shift
Shift
Engine
number
4
Introduction
Model and serial number
The model and serial number can be found on a white sticker with a bar code. The sticker is located between the
dipstick and the muffler. See Figure 1.1.
Model /serial number
Oil screen
Muffler
Dipstick
Figure 1.1
NOTE: The serial number will always start with the model number.
Maintenance
The recommended maintenance intervals listed in this manual are a guideline. They are ad justable for local con-
ditions.
Maintenance items Interval
Oil Change* 50 hrs
Oil filter 200 hours
Oil pre-screen Annually
Clean the air filter 100 hrs
Replace the air filter 200 hrs
Spark plugs 100 hrs
Fuel filter 100 hrs
Clean the engine 100 hours
* First oil change at 5 hours.
5
P90 Series Vertical Shaft Engines
Spark plugs
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 fai lures 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
(part # 951-10292) gapped to 0.024” - 0 .031” (0.60 -
0.80 mm).
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 before operating failure (no start) occurs.
3. Cleaning the spark plug:
See Figure 1.2.
Figure 1.2
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 con d ition of th e en gine .
• Light tan colored deposits on insulator and electrodes is normal.
• Dry, black deposits on the insulator and electrod es 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.
6
Air filter
Paper-pleated element
Introduction
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.
2. Air filters used on the MTD engine are designed to
prevent particles larger than 3-5 micron from p assing
through into the engine.
Foam pre-filter
Figure 1.3
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.
6. Foam pre-filters can be washed in warm soapy water.
NOTE: When drying a foam filter either squeeze it inside 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.
3. The filter should b e checked on a regular basis p ossibly several times in a season.
4. Typically an air filter should be ch anged befo re every
season.
5. If a foam air pr e-clean er is di rty, but not in bad of condition, it can be cleaned and reused. The paper
pleated filters can be shaken or lightly tapped to free
the debris from the filter.
7
P90 Series Vertical Shaft Engines
Oil type and capacity
The recommended oil for MTD engines is an SAE 10W-30 oil with an SM API rating or better . The oil cap acity for
all of the P90 series engines is 57 fl.oz (1.7 liters).
• Check the oil level daily and change the oil more frequently in severe operating conditions such as high
ambient temperature, dusty conditions, or high load use in exceptionally thick grass.
• Synthetic oil is a suitable alternative, but it does not extend service intervals.
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 micro scope 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.
To check the oil:
1. Twist and remove the dipstick 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 highest point on
the dipstick that is completely covered with oil.
See Figure 1.4.
Fully seat the
dip stick before
reading it
Figure 1.4
8
Changing the oil
Introduction
The oil change interval is every 100 hrs.
NOTE: The first oil change should be preformed at 8
hours.
NOTE: The oil filter should be replaced when the oil is
changed.
To change the oil:
1. Remove the cap from the oil drain. See Figure 1.5.
Figure 1.5
Figure 1.6
Oil drain
1/2” hose
2. Remove the dipstick.
3. Slide a piece of 1/2” hose onto the drain.
See Figure 1.6.
4. Route the other end of the hose into an approved oil
drain pan.
5. Turn the oil drain a quarter turn counter-clockwise to
unlock it, then pull out 3/8” (9.5 mm) to open the
valve.
6. After all of the oil has been drained, close the oil drain
by pushing it in and turning it back a quarter turn.
7. Remove the drain hose.
8. Place the cap back on the oil drain.
9. Fill engine with 57 oz (1.7 L) of SAE 10W-30 oil with
a SM API rating or better.
NOTE: Refer to the oil chart to determine the proper
weight of oil to use.
10. Check the dip stick to verify that the oil is at the
proper level before returning to service.
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9
P90 Series Vertical Shaft Engines
Oil filter
To replace the oil filter:
1. Drain the oil by following the steps described in the
previous section of this chapter.
2. Clean the area around the oil filter
3. Remove the oil filter by turning it counter-clockwise,
as seen from the left side of the engine.
See Figure 1.7.
4. Place a light coating of oil on the O-ring of the new
filter.
5. Pre-fill the new filter with fresh, clean oil.
6. Thread the new filter on to the engine. Hand tighten
only .
7. Fill engine with 57 oz (1.7 L) of SAE 10W-30 oil with
a SM API rating or better.
8. Test run the engine and check for leaks before
returning the engine to service.
Oil pre-screen
To clean the pre-screen:
1. Drain the oil by following the procedures described
in the previous section of this chapter.
2. Remove the pre-screen plug using a 15/16” wrench.
NOTE: There is a spring that will come out with the
plug.
See Figure 1.8.
3. Remove the screen.
4. Inspect the o-ring for signs of damage or wear.
Replace the o-ring if any are found.
5. Clean the screen in parts cleaning solution.
6. Rinse the screen in warm water.
Oil filter
Figure 1.7
Pre-screen
Spring
Plug
7. Dry the screen using compressed air.
8. Install the screen.
9. Install the plug and spring.
10. Fill engine with 57 oz (1.7 L) of SAE 10W-30 oil with a SM API rating or better.
11. Test run the engine and check for leaks before returning the engine to service.
10
Figure 1.8
Introduction
Fuel system
What you should know about fuel.
Most of the fuel presently available in North America is oxygenated to some extent. This is commonly done
through the addition of ethanol. Most engines offered for sale on outdoor power equipment in the North American
markets are designed to tolerate no more than 10% ethanol by volume
Ethanol is hygroscopic, meaning it absorbs water. If left exposed to air, it will draw water out of the air.
Ethanol is an oxygenator, which means that it will oxidize (corrode) metal that it comes into contact with. Exposure to air causes fuel to go bad quickly, leaving gum and varnish deposits.
Fuel used in Cub Cadet outdoor power equipment should be no more than 30 days old. Because it may already
have been stored at the refinery or gas station for a week or more, fuel should be purchased in small quantities and
stored in safety approved gas cans with the caps closed.
For storage, all fuel should be run out of the tank and engine. Anti-oxidation additives will help keep the fuel
fresher.
Servicing the fuel system
Inspect the fuel system every time the engine is operated. If dirty fuel is found in the fuel tank or fuel that does not
smell “right”, drain the fuel tank and replace the fuel filter. Dispose of bad fuel in a safe and legal manner.
Refer to the units service manual for the procedures to drain the fuel tank.
! CAUTION! CAUTION
Fuel filter
2. Squeeze the tabs on the fuel line clamps and slide them away from the filter.
3. Carefully slide the fuel lines off of the filter. If there are pieces of rubber on the barbs of the fuel filter, replace
the affected fuel line.
IMPORTANT: The P90 series engines uses low permeation fuel line to meet EPA guidelines. When replacing
4. Install the new filter by following the above steps in reverse order.
5. Test run the engine and check for leaks before returning to service.
Gasoline and its vapors are extremely flammable. Use common sense when working around
the fuel system. Avoid sparks, open flames or heat sources that can ignite the fuel vapors.
A dirty fuel filter can result in a lean run condition. The
fuel filter should be replaced every 100 hours.
To replace the fuel filter:
NOTE: The part number for the fuel filter is BS - 29 8090S.
It is a 150 micron (red) filter. Use of a filter with a
lower micron rating will cause fuel starvation
issues.
1. Clamp off the fuel lines to prevent fuel from leaking
Fuel filter
Figure 1.9
the fuel lines, they must be replaced with the same type of low permeation fuel line.
when the lines are disconnected.
NOTE: Take care that the fuel lines are not damaged
when clamping them off. Never insert a screw or
anything else into the fuel line to prevent fuel from
coming out. This will damage the inside of the fuel
line.
NOTE: There are commercially available fuel line clamp-
ing tools that will not damage the fuel lines.
See Figure 1.9.
11
P90 Series Vertical Shaft Engines
Valve lash
Valve 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 checke d and adju sted using
the following steps:.
1. If the engine has been run, allow it to cool thor-
oughly. Position the mower for easy access to the
cylinder 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. Remove the four bolts that secure the valve cover
using a 10mm wrench, and remove the valve cover
from the engine.
NOTE: If care is used not to damage the valve
5. Confirm that the piston is at Top-Dead-Center on
the compression stroke.
See Figure 1.10.
cover gasket, it can be re-used.
See Figure 1.11.
High tension lead
Muffler
Figure 1.10
Probe to confirm piston
is at top of travel
Valve cover
• The compression stroke ca n be distin g uis he d
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.
12
Valves closed
(push rods slack)
Figure 1.11
.005” feeler
gauge
Figure 1.12
Introduction
6. Check valve lash between each valve stem and
rocker arm using a feeler gauge.
7. Intake valve lash (top valve) should be 0.004” -
0.006” (0.10 - 0.15mm).
8. Exhaust valve lash (bottom valve) shou ld be 0 .006” -
0.008” (0.15 - 0.20mm).
See Figure 1.12.
See Figure 1.13.
9. Use a 10mm wrench to loosen the jam nut, and a
14mm wrench to adjust the rocker arm fulcrum nut.
See Figure 1.13.
• 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.
.007” feeler
gauge
Figure 1.13
11. Double-check the clearance after tightening the jam nut, to confirm that it did not shift. Re-adjust if necessary.
12. Rotate the engine through several compression cycles:
• Obse rv e th e mo ve m en t of th e valv e ge ar.
• 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.
13. Clean-up any oil around the valve cover opening, clean the valve cover, replace the valve cover gasket if necessary.
14. Install the valve cover, tightening the valve cover screws to a torque of 62 - 80 in-lbs (7 - 9 Nm).
• 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.
10. 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.
IMPORTANT: Over tightening the valve cover will cause it to leak.
15. Install the spark plug.
16. Test run the engine before returning it to service.
13
P90 Series Vertical Shaft Engines
Exhaust system
The exhaust system is a frequently overlooked component of an engine. It is important to make sure the muf f ler is in
good condition and free of blockage.
NOTE: A blocked muffler will result in poor performance. If a muffler is completely blocked, the engine may not
start.
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.
General torque specifications
0H]LV
0H]LV
0H]LV
VEOQLHGDU*
VEOQLHGDU*
VEOQLHGDU*
P1
P1
P1
VEOQL
VEOQL
VEOQL
P1
P1
1
1
1
)
)
)
ODFLWLUFQR
ODFLWLUFQR
ODFLWLUFQR
QLVUHQHWVD
QLVUHQHWVD
QLVUHQHWVD
PXQLPXO$
PXQLPXO$
PXQLPXO$
P1
P1
P1
P1
P1
P1
P1
P1
P1
P1
00
00
00
0
0
0
VEOQL
VEOQL
VEOQL
I
I
I
VEOW
VEOW
VEOW
VEOWI
VEOWI
VEOWI
Useful Engine Specifications
Description SAE Metric
Engine displacement 25.6 cubic inch 420 cc
000H]LV
000H]LV
000H]LV
VEOWI
VEOWI
VEOWI
P1P1
P1P1
P1P1
P1
P1
P1
P1
P1
P1
VEOWIVEOQL
VEOWIVEOQL
VEOWIVEOQL
P1
P1
P1
VEOWIVEOQL
VEOWIVEOQL
VEOWIVEOQL
P1
P1
P1
VEOWIVEOQL
VEOWIVEOQL
VEOWIVEOQL
P1
P1
P1
Spark plug gap 0.024” - 0.031” 0.6 - 0.8 mm
Spark plug torque 177 - 221 in lbs 20 - 25 Nm
Ignition module air gap 0.016” - 0.024” 0.4 - 0.6 mm
Intake valve lash 0.004” - 0.006” 0.10 - 0.15 mm
Exhaust valve lash 0.006” - 0.008” 0.15 - 0.20 mm
Oil capacity 57 oz 1.7 L
14
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! CA UTION
NOTE: The steps and the order of the steps that follow are a suggested approach to tro ubleshoo ting 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. Starter not working
B. Engine in a bind (external - attachment jammed)
C. Engine in a bind (internal - engine seized)
• Crankshaft turns, no start
• Starts, runs poorly
A. St arts, then dies
B. Runs with low power output
C. Make s 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
15
P90 Series Vertical 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.
16
Identify factors that could cause the problem
This is the second step in the troubleshooting process.
1. Crankshaft will not turn.
A. Starter not working. This can be an electrical failure or a mechanical failure. The likely suspects are:
I. A dead battery.
II. A bad ground
III. A failure in the electrical circuit.
IV. A failure of the starter itself.
B. Engine in a bind (external - atta ch men t jamme d) . T his usual ly indicates that the unit be ing power ed by
the engine either failed or has something jammed in it, locking up the system.
C. Engine in a bind (internal - engine seized). This is usually either a quick fix or a catastrophic failure. The
likely suspects are:
I. Complete hydraulic lock (easy fix).
II. Bent crankshaft (unrepairable)
III. Internal binding, crankshaft, connecting rod or piston (unrepairable)
BASIC TROUBLESHOOTING
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 st art 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 fresh gasoline; in sufficient quantity, atomized (tiny droplet s) an d 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 good one.
IV. Check compression or leak down.
V. Check valve lash.
VI. Check valve timing/actuation.
VII. Check exhaust.
3. Starts, runs poorly
3a. Starts, then dies
17
P90 Series Vertical Shaft Engines
I. Run the engine with a sp ark tester in-line between the spark plug wir e and the spark plug or use a n
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 increase and decrease as 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 accompanied 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.
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 performanc e.
• T he intake valv e no t fully open i ng . A poss ible cause of this is loose valve lash.
18
BASIC TROUBLESHOOTING
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
• 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
19
P90 Series Vertical Shaft Engines
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. Mixture not burning 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 during normal
operations.
5. Engine RPMs surge (hunting)
A. Over-governed condition- Return spring replaced with wrong 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
20
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 gathere d from the tro ub leshooting th at was p erforme 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: confirming 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.
• M ak e s ure th e cu sto m er und er stands th e re pair, preventing “superstitious” come-backs.
21
P90 Series Vertical 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.
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. If the engine did not start, 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.
Leak down
tester
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.
22
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.
23
P90 Series Vertical Shaft Engines
Compression test
To perform a compression test:
NOTE: Compression should be in the range of 55 - 80 PSI (3.8 - 5.5 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
Figure 2.2
Possible causes
24
20 - 55
(1.4-3.8 Bar)
55 - 80
(3.8-5.5 Bar)
>80
(>5.5 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.
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