Stihl MS 311, MS 391, MS 191 Service Manual

STIH)

STIHL MS 311, 391

2009-06

Contents

1. Introduction and
Safety Precautions 3

1.1 Introduction 3

1.2 Safety Precautions 4

2. Specifications 5

2.1 Engine 5

2.2 Fuel System 5

2.3 Ignition System 5

2.4 Chain Lubrication 5

2.5 Tightening Torques 6

3. Troubleshooting 8

3.1 Clutch 8

3.2 Chain Drive,
Chain Brake,
Chain Tensioner 9

3.3 Chain Lubrication 10

3.4 Rewind Starter 11

3.5 Ignition System 12

3.6 Carburetor 13

3.7 Engine 16

4. Clutch 17

4.1 Clutch Drum 19

5. Chain Brake 20

6. Engine 27

6.1 Muffler 27

6.2 Leakage Test 28

6.2.1 Preparations 28

6.2.2 Vacuum Test 29

6.2.3 Pressure Test 29

6.3 Oil Seals 30

6.4 Shroud 31

6.5 Cylinder / Crankshaft 32

6.6 Bearings / Crankshaft 35

6.7 Piston 36

6.8 Piston Rings 38

6.9 Decompression Valve 38

7. Ignition System 39

7.1 Ignition Timing 39

7.2 Preseparator 39

7.3 Ignition Module 39

7.4 Testing the
Ignition Module 41

7.5 Spark Plug Boot /
Ignition Lead 42

7.6 Flywheel 43

7.7 Short Circuit Wire 44

7.7.1 Testing 44

7.7.2 Removing and
Installing 44

7.7.3 Ground Wire 46

7.7.4 Contact Spring 46

7.8 Ignition System
Troubleshooting 48

9. Servicing the
AV System 57

9.1 Buffer on Oil Tank 57

9.2 AV Spring on Oil Tank 57

9.3 AV Spring on
Fuel Tank 57

9.4 AV Spring on
Front Handle 58

9.5 Stop Buffer at
Clutch Side 59

9.6 Annular Buffer at
Ignition Side 59

9.7 Handlebar 59

10. Control Levers 61

10.1 Master Control Lever 61

10.2 Throttle Trigger/Interlock
Lever 61

10.2.1 Choke Rod 63

10.2.2 Throttle Rod 64

11. Chain Lubrication 65

11.1 Pickup Body 65

11.2 Oil Suction Hose 65

11.3 Oil Pump 66

11.4 Valve 67

RA_542_00_01_01

5.1 Checking Operation 20

5.2 Brake Band 20

5.3 Brake Lever 22

5.4 Cam Lever 24

5.5 Pins 25

5.6 Chain Tensioner 25

5.7 Bar Mounting Studs 26

8. Rewind Starter 51

8.1 General 51

8.2 Fan housing 51

8.2.1 Segment 51

8.3 Pawls 51

8.4 Rope Rotor 52

8.5 Starter Rope / Grip 53

8.6 Tensioning the
Rewind Spring 54

8.7 Replacing the
Rewind Spring 55

q

© ANDREAS STIHL AG & Co. KG, 2009

1MS 311, MS 391

Contents

12. Fuel System 68

12.1 Air Filter 68

12.2 Baffle 68

12.3 Filter Base 68

12.4 Air Guide Shroud 69

12.5 Carburetor 71

12.5.1 Leakage Test 71

12.6 Servicing the
Carburetor 72

12.6.1 Metering Diaphragm 72

12.6.2 Inlet Needle 73

12.6.3 Pump Diaphragm 74

12.6.4 Air Valve 76

12.6.5 Levers on
Throttle Shaft 76

12.6.6 Adjusting Screws 77

12.7 Carburetor
Adjustment 79

12.7.1 Basic Setting 79

12.7.2 Standard Setting 80

12.8 Carburetor Carrier 81

12.9 Intake Manifold 82

12.10 Tank Vent 83

12.10.1 Testing 83

12.10.2 Removing and
Installing 84

12.11 Fuel Intake 84

12.11.1 Pickup Body 84

12.11.2 Fuel Hose 85

12.11.3 Fuel Suction Hose 86

12.11.4 Tank Housing 87

13. Special Servicing
Tools 88

14. Servicing Aids 90

2 MS 311, MS 391

1. Introduction and Safety Precautions

1.1 Introduction

This service manual contains
detailed descriptions of all the repair
and servicing procedures specific to
this power tool.

You should make use of the
illustrated parts lists while carrying
out repair work. They show the
installed positions of the individual
components and assemblies.

Refer to the latest edition of the
relevant parts list to check the part
numbers of any replacement parts.

A fault on the machine may have
several causes. To help locate the
fault, consult the chapter on
"Troubleshooting" and the
"STIHL Service Training System"
for all assemblies.

Refer to the “Technical Information”
bulletins for engineering changes
which have been introduced since
publication of this service manual.
Technical information bulletins also
supplement the parts list until a
revised edition is issued.

The special tools mentioned in the
descriptions are listed in the chapter
on "Special Servicing Tools" in this
manual. Use the part numbers to
identify the tools in the
"STIHL Special Tools" manual. The
manual lists all special servicing
tools currently available from
STIHL.

Symbols are included in the text and
pictures for greater clarity.
The meanings are as follows:

In the descriptions:

N Action to be taken as shown in

the illustration above the text

– Action to be taken that is not

shown in the illustration above
the text

In the illustrations:

A Pointer

a Direction of movement

@ 4.2 =Reference to another

chapter, i.e. chapter 4.2
in this example.

1

3

2

219RA000 TG

Servicing and repairs are made
considerably easier if the machine
is mounted to assembly stand (3)
5910 890 3100. To do this, secure
the mounting plate (2)
5910 850 1650 to the assembly
stand with two screws (1) and
washers.

The screws must not project since
they, depending on the machine,
may damage housings when the
machine is clamped in position.

The above operation is not
necessary with the new assembly
stand 5910 890 3101 since the
mounting plate is already fitted.

Service manuals and all technical
information bulletins are intended
exclusively for the use of properly
equipped repair shops. They must
not be passed to third parties.

3MS 311, MS 391

Engage the bar mounting studs in
the outer bores in the mounting
plate and secure the machine in
position with the nuts (arrows).

Preparations for servicing

Remove the chain sprocket cover,
saw chain and guide bar before
carrying out repairs or mounting the
machine to the assembly stand.

Always use original STIHL
replacement parts.
They can be identified by the STIHL
part number, the { logo
and the STIHL parts symbol K
This symbol may appear alone on
small parts.

Storing and disposing
of oils and fuels

Collect fuel or lubricating oil in a
clean container and dispose of it
properly in accordance with local
environmental regulations.

1.2 Safety Precautions

If the machine is started up in the
course of repairs or maintenance
work, observe all local and country­specific safety regulations as well
as the safety precautions and
warnings in the instruction manual.

Gasoline is an extremely flammable

216RA000 TG

fuel and can be explosive in certain
conditions.

Always wear suitable protective
gloves for operations in which
components are heated for
assembly or disassembly.

Improper handling may result in
burns or other serious injuries.

Do not smoke or bring any fire,
flame or other source of heat near
the fuel. All work with fuel must be
performed outdoors only. Spilled
fuel must be wiped away
immediately.

Always perform leakage test after
working on the fuel system and the
engine.

Always replace damaged parts.
Check disassembled parts for wear
or damage before re-installing –
replace as necessary.

Run the machine only with the
shroud mounted in position – there
is otherwise a risk of injury from the
fanwheel and a risk of engine
damage due to overheating.

The chapter on tightening torques
lists all machine components that
have to be tightened to a specific
torque or coated with threadlocking
adhesive. The specifications must
be maintained when tightening
down screws, nuts and other
fasteners in all the procedures
described in this service manual.

Fuel system – hose barb
connectors

Pull off or push on fuel hoses in line
with the connector, preferably by
hand, to ensure the tightness of the
fuel system.

Avoid damaging the hose barb
– do not use sharp-edged pliers,
screwdrivers, etc.
Do not cut open fuel hoses with a
knife or similar tool.

Do not re-use fuel hoses after
removal. Always install new hoses
– fuel hoses can be overstretched
during removal.

Install new fuel hoses either dry or
with the aid of STIHL press fluid,
b 14.

Other press fluids are not approved
and may result in damage to the fuel
hoses.

Coat the ends of the hoses and the
connectors with STIHL press fluid
and then push the new hoses on to
the hose barbs, b 14.

4 MS 311, MS 391

2. Specifications

2.1 Engine

MS 311 MS 391

Displacement: 59 cm

3

64.1 cm

3

Bore: 47 mm 49 mm
Stroke: 34 mm 34 mm
Engine power to ISO 7293: 3 kW (4 bhp)

at 9,500 rpm

3.2 kW (4.4 bhp)
at 9,500 rpm

Maximum permissible engine speed
with guide bar and chain: 13,000 rpm 13,000 rpm

Idle speed: 2,800 rpm 2,800 rpm
Clutch: Centrifugal clutch without

linings

Centrifugal clutch without

linings
Clutch engages at: 3,500 rpm 3,500 rpm
Engine housing leakage test
at gauge pressure: 0.5 bar
under vacuum: 0.5 bar

2.2 Fuel System

Carburetor leakage test at

0.8 bar

gauge pressure:
Operation of tank vent at

0.5 bar

gauge pressure:
Fuel: as specified in instruction

manual

2.3 Ignition System

2.4 Chain Lubrication

Air gap between ignition
module and fanwheel: 0.20...0.30 mm

Spark plug (resistor type): NGK BPMR 7 A

BOSCH WSR6F

Electrode gap: 0.5 mm

Speed-controlled oil pump with reciprocating piston and
manual flow control

Oil delivery rate:
Ematic oil pump

Oil delivery rate:
Adjustable oil pump

11.5 (+2.5) cm

at 10,000 rpm

6.0 (+/-2.0)...17.5 (+3.0 / -4.5)

3

cm

/min at 10,000 rpm

3

/min

5MS 311, MS 391

2.5 Tightening Torques

DG and P (Plastoform) screws are used in polymer and light metal components. These screws form a
permanent thread when they are installed for the first time. They can be removed and installed as often as
necessary without impairing the strength of the screwed assembly, providing the specified tightening torque is
observed.
For this reason it is essential to use a torque wrench.

Fastener Thread size For component Torque

Remarks

Nm

Screw P 4x14 Chain tensioner cover/engine housing 1.5
Screw P 4x12 Brake band/engine housing 2.0
Collar screw M 10 Collar stud for bar / engine pan 30.0 1)
Collar screw D 8x24 Collar stud for bar / engine housing 16.0
Screw P 5x16 Cover, chain brake / engine housing 4.0

M 10x1 Decompression valve 14.0
Screw P 5x16 Handlebar/plug, AV spring 3.0
Screw P 6x26.5 Handlebar / tank housing, right (polymer) 7.0
Screw P 6x26.5 Handlebar / tank housing, bottom (polymer) 7.0
Screw P 6x25 Hand guard / fan housing 8.0
Screw M 6x30 Shroud / engine housing 10.0
Screw P 6x38 Chain catcher / bearing plug 6.0
Screw P 5x16 Spiked bumper / engine housing 4.0
Screw M 4x12 Manifold/cylinder 4.0 3)
Screw P 6x21.5 Bearing plug / engine housing 6.0
Screw M 5x16 Bearing plug/cylinder 10.0 4)
Screw P 5x20 Fan housing / engine housing 4.0
Screw P 4x12 Air baffle / engine housing 2.0
Carrier M 12x1 LH Carrier / crankshaft 50.0
Screw M 6x25 Engine housing / cylinder, stage 1 4.0 3)
Screw M 6x25 Engine housing / cylinder, stage 2 12.0 3)
Screw D 4x18 Oil pump 4.0 3)
Screw M 5x16 Muffler / cylinder 10.0 1) 3)
Screw M 8x1 Flywheel/crankshaft 33.0 5)
Nut M 5 Carburetor/collar stud 3.5
Screw P 4x14 Pre-separator / engine housing 2.0
Screw P 4x12 Cover plate/fan housing 2.0
Spark plug M 14x1.25 Spark plug 25.0
Screw D 4x18 Ignition module / engine pan 4.0 3)

6 MS 311, MS 391

Remarks:

1) Loctite 242 or 243, medium strength

2) Loctite 648, high strength

3) Screws with antifricton coated binding head

4) Screws with micro-encapsulated binding head

5) Degrease crankshaft/flywheel and mount oil-free

Use the following procedure when refitting a DG or P screw in an existing thread:

Place the screw in the hole and rotate it counterclockwise until it drops down slightly.
Tighten the screw clockwise to the specified torque.

Coat micro-encapsulated screws with medium-strength threadlocking adhesive before re-installing.

This procedure ensures that the screw engages properly in the existing thread and does not form a new thread
and weaken the assembly.

Power screwdriver setting for polymer: DG and P screws max. 500 rpm
Do not use an impact wrench for releasing or tightening screws.

Do not mix up screws with and without binding heads.

7MS 311, MS 391

3. Troubleshooting

3.1 Clutch

Condition Cause Remedy

Saw chain stops under load at full

Clutch shoes badly worn Install new clutch

throttle

Clutch drum badly worn Install new clutch drum

Saw chain rotates at idle speed Engine idle speed too high Readjust with idle speed screw

LA (counterclockwise)

Clutch springs stretched or fatigued Replace the clutch springs or install

new clutch

Clutch spring hooks broken Replace the clutch springs or install

new clutch

Loud noises Clutch springs stretched or fatigued Replace the clutch springs or install

new clutch

Needle cage damaged Fit new needle cage

Clutch shoe retainer broken Install new retainer or clutch

Clutch shoes and carrier worn Install new clutch

8 MS 311, MS 391

3.2 Chain Drive, Chain Brake, Chain Tensioner

Condition Cause Remedy

Chain sprocket wears rapidly Chain not properly tensioned Tension chain as specified

Wrong chain pitch Fit chain of correct pitch

Insufficient chain lubrication Check chain lubrication

Chain sprocket worn Fit new chain sprocket

Saw chain stops under load at full

Clutch shoes badly worn Install new clutch

throttle

Clutch drum badly worn Install new clutch drum

Brake band blocked Check freedom of movement and

operation of brake band

Saw chain rotates at idle speed Engine idle speed too high Readjust with idle speed screw

LA (counterclockwise)

Clutch springs stretched or fatigued Replace the clutch springs or install

new clutch

Clutch spring hooks broken Replace the clutch springs

Saw chain does not stop

Brake spring stretched or broken Fit new brake spring
immediately when brake is
activated

Brake band stretched or worn Fit new brake band

Clutch drum worn Install new clutch drum

9MS 311, MS 391

3.3 Chain Lubrication

In the event of trouble with the
chain lubrication system, check and

.
rectify other sources of faults
before disassembling the oil pump.

Condition Cause Remedy

Chain receives no oil Oil tank empty Fill up with oil and check setting of

oil pump if necessary

Oil inlet hole in guide bar is blocked Clean oil inlet hole

Intake hose or pickup body clogged
or intake hose ruptured

Fit new intake hose and pickup
body

Valve in oil tank blocked Clean or replace valve

Teeth on worm worn Install new worm

Worm drive spring broken Install new worm

Oil pump damaged or worn Install new oil pump

Machine losing chain oil Oil pump body damaged Install new oil pump

Oil pump damaged or worn Install new oil pump

Oil intake hose connection

Install new oil intake hose

damaged

Oil pump delivers insufficient oil Oil pump worn Install new oil pump

Worm drive spring not engaged in
clutch drum

Oil pump delivery rate set too low Adjust oil pump

10 MS 311, MS 391

Engage drive spring in notch in
clutch drum

(only on version with adjustable oil
pump)

3.4 Rewind Starter

Condition Cause Remedy

Starter rope broken Rope pulled out too vigorously as

Fit new starter rope
far as stop or over edge,
i.e. not vertically

Normal wear Fit new starter rope

Starter rope does not rewind Very dirty or corroded Clean or replace rewind spring

Insufficient spring tension Check rewind spring and increase

tension

Rewind spring broken Fit new rewind spring

Starter rope cannot be pulled out
far enough

Starter rope can be pulled out
almost without

Spring overtensioned Check rewind spring and reduce

tension

Guide peg on pawl or pawl itself is

Fit new pawl
worn

resistance (crankshaft does not
turn)

Starter rope is difficult to pull or
rewinds very slowly

Spring clip on pawl fatigued Fit new spring clip

Starter mechanism is very dirty Thoroughly clean complete starter

mechanism

Lubricating oil on rewind spring
becomes viscous at very low
outside temperatures (spring
windings stick together)

Coat rewind spring with a small

amount of standard solvent-based

degreasant (containing no

chlorinated or halogenated

hydrocarbons), then pull rope

carefully several times until normal

action is restored

Decompression valve is not open Open, check and replace

decompression valve if necessary

11MS 311, MS 391

3.5 Ignition System

Exercise extreme caution while
carrying out maintenance and
repair work on the ignition system.
The high voltages which occur can
cause serious or fatal accidents.

Condition Cause Remedy

Engine runs roughly, misfires,
temporary loss of power

Spark plug boot is loose Press boot firmly onto spark plug

and fit new spring if necessary

Spark plug sooted, smeared with oil Clean the spark plug or replace if

necessary.
If sooting keeps recurring, check air
filter

Ignition lead loose in ignition

Secure ignition lead properly

module

Fuel/oil mixture

Use correct mixture of fuel and oil

– too much oil

Incorrect air gap between ignition

Set air gap correctly

module and flywheel

Flywheel cracked or

Install new flywheel
damaged or pole shoes have
turned blue

Ignition timing out of adjustment,
flywheel warped, machined key in
flywheel sheared

Install new flywheel, check

crankshaft stub for damage and

replace crankshaft if necessary

Weak magnetization in flywheel Install new flywheel

Irregular spark Check operation of switch shaft/

Engine housing damaged (cracks) Install new engine housing

12 MS 311, MS 391

contact springs and ignition

module.

Faulty insulation or break in ignition

lead or short circuit wire. Check

ignition lead/ignition module and

replace ignition module if

necessary.

Check operation of spark plug.

Clean the spark plug or replace if

necessary.

3.6 Carburetor

Condition Cause Remedy

Carburetor floods; engine stalls Inlet needle not sealing

– foreign matter in valve seat or

Remove and clean the inlet needle,
clean the carburetor

cone

Inlet needle worn Replace the inlet needle

Inlet control lever sticking on
spindle

Helical spring not located on nipple
of inlet control lever

Perforated disc on diaphragm is

Check the inlet control lever and
replace if necessary.

Remove the inlet control lever and
refit it correctly

Fit a new metering diaphragm
deformed and presses constantly
against the inlet control lever

Metered diaphragm deformed Fit a new metering diaphragm

Poor acceleration Setting of low speed screw too lean Check basic carburetor setting,

correct if necessary

Setting of high speed screw too
lean

Check basic carburetor setting,

correct if necessary

Inlet needle sticking to valve seat Remove inlet needle, clean and

refit

Diaphragm gasket leaking Fit new diaphragm gasket

Metering diaphragm damaged or

Fit a new metering diaphragm
shrunk

Tank vent faulty Replace tank vent

Leak on fuel hose from tank to
carburetor

Seal connections or install new fuel

hose

13MS 311, MS 391

Condition Cause Remedy

Engine will not idle, idle speed too
high

Throttle shutter opened too wide by
idle speed screw LA

Oil seals/engine housing leaking Seal or replace oil seals/engine

Throttle shutter does not close Replace throttle shutter and shaft

Air valve does not close Replace end cover with air valve.

Engine stops while idling Idle jet bores

or ports blocked

Low speed screw too rich or
too lean

Setting of idle speed screw LA
incorrect – throttle shutter
completely closed

Tank vent faulty Replace tank vent

Reset idle speed screw LA
correctly

housing

Clean the carburetor

Reset low speed screw L correctly

Reset idle speed screw LA
correctly

Leak on fuel hose from tank to
carburetor

Seal connections or install new fuel
hose

Saw chain rotates at idle speed Engine idle speed too high Readjust with idle speed screw

LA (counterclockwise)

Clutch springs stretched or fatigued Replace the clutch springs or install

new clutch

Clutch spring hooks broken Replace the clutch springs

14 MS 311, MS 391

Condition Cause Remedy

Engine speed drops quickly under
load – low power

Air filter dirty Clean air filter or replace if

necessary

Throttle shutter not opened fully Check throttle cable and rod

Tank vent faulty Replace tank vent

Fuel pickup body dirty Install new pickup body

Fuel strainer dirty Clean fuel strainer in carburetor,

replace if necessary

Leak on fuel hose from tank to
carburetor

Setting of high speed screw H too
rich

Seal connections or install new fuel

hose

Check basic carburetor setting,

correct if necessary

Main jet bores or ports blocked Clean the carburetor

Pump diaphragm damaged or

Fit new pump diaphragm
fatigued

Fit key if necessary and secure

flywheel properly or install new

flywheel

or replace if necessary.

Engine running extremely rich, has
no power and a very low maximum

Ignition timing wrong, flywheel out
of adjustment, key in flywheel has
sheared off

Air valve does not open Check the carburetor and service

speed

Engine running too rich, loss of
power and maximum speed too low

Air valve does not open fully in full
throttle position

Check the carburetor and service

or replace if necessary.

Erratic idle – too lean Air valve does not close completely Check the end cover with air valve

and replace if necessary

Intake manifold faulty Install new intake manifold

15MS 311, MS 391

3.7 Engine

Always check and, if necessary,
repair the following parts before
looking for faults on the engine:

- Air filter

- Fuel system

- Carburetor

- Ignition system

Condition Cause Remedy

Engine does not start easily, stalls
at idle speed, but operates normally
at full throttle

Engine does not deliver full power
or runs erratically

Oil seals in engine housing

Replace the oil seals

damaged

Engine housing leaking/

Seal or replace the engine housing

damaged (cracks)

Piston rings worn or broken Fit new piston rings

Muffler / spark arresting screen
carbonized

Clean the muffler (inlet and
exhaust), replace spark arresting
screen, replace muffler if necessary

Air filter dirty Replace air filter

Fuel/impulse hose severely kinked
or damaged

Fit new hoses or position them free
from kinks

Decompression valve is not closed Close, check and replace

decompression valve if necessary

Engine overheating Insufficient cylinder cooling. Air

inlets in fan housing blocked or
cooling fins on cylinder very dirty

16 MS 311, MS 391

Thoroughly clean all cooling air
openings and the cylinder fins

4. Clutch

– Troubleshooting, b 3.1

– Remove the shroud, b 6.4.

– Remove the clutch drum, b 4.1

1

: Pull boot (1) off the spark plug.

– Unscrew the spark plug.

: The locking strip (1)

0000 893 5903 must butt against
the cylinder wall (arrow) as
shown.

4903RA001 TG

1

1

4903RA106 TG

– Pull the clutch shoes off the

carrier.

: Remove the retainers (1).

1

4903RA005 TG

1

: Push the locking strip (1)

0000 893 5903 into the spark
plug hole, wide end first, so that
"OBEN-TOP" faces up.

: Apply wrench to hexagon (arrow)

and unscrew the clutch (1).

Note that the clutch has a left-hand
thread.

4903RA002 TG

Disassembling

21

: Use hook (2) 5910 890 2800 to

remove the clutch springs (1).

4903RA003 TG4903RA004 TG

– Clean all parts, b 14

– Replace any damaged parts.

If the clutch is noticeably worn,
replace all three clutch shoes at the
same time – not individual shoes –
since runout may otherwise affect
correct operation of the clutch.

4903RA006 TG

17MS 311, MS 391

1

2

: Fit the retainers (1).

1

2

: Fit the clutch shoes (2) over the

arms (1).

1

4903RA007 TG

Attach the springs (1) on the side
with the raised hexagon (arrow).

: Attach one end of each spring (1)

to the clutch shoes.

: Use the hook (2) 5910 890 2800

to attach the other ends of the
springs and press them firmly into
the clutch shoes.

4903RA008 TG

1

4903RA010 TG

Make sure the washer (1) is in
place.

Installed position is correct when
TOP (arrow) faces outwards.

4903RA012 TG

4903RA013 TG

: Clamp the clutch in a

vise (arrow).

: Position the clutch on the

crankshaft stub so that the raised
hexagon (arrow) faces outwards.

4903RA011 TG

: Check the clutch – all springs

(arrows) must be properly
attached.

4903RA009 TG

18 MS 311, MS 391

4.1 Clutch Drum

– Remove and install the clutch

1

drum, see instruction manual.

: Screw the clutch (1) on to the

crankshaft stub and tighten down
the hexagon (arrow) firmly – left­hand thread.

– Remove the locking strip from the

cylinder.

– Reassemble all other parts in the

reverse sequence.

1

4903RA014 TG

4903RA015 TG

: Pull off the needle cage (1).

– Clean the needle cage and

crankshaft stub, b 14

– Lubricate the needle cage and

crankshaft stub, b 14

1

100%

80%

!

– Inspect the clutch drum (1) for

signs of wear.

If there are signs of serious wear on
the inside diameter of the clutch
drum (1), check the remaining wall
thickness. If it is less than about
80% of the original thickness, install
a new clutch drum.

– Install the clutch drum.

165RA029 TG

19MS 311, MS 391

5. Chain Brake

5.1 Checking Operation

The chain brake is one of the most
important safety devices on the
machine. Its efficiency is measured
in terms of the chain braking time,
i.e. the time that elapses between
activating the brake and the saw
chain coming to a complete
standstill.

Contamination (with chain oil,
chips, fine particles of abrasion,
etc.) and smoothing of the friction
surfaces of the brake band and
clutch drum impair the coefficient of
friction, which prolongs the braking
time. A fatigued or stretched brake
spring has the same negative effect.

– Starting the engine

– With the chain brake

activated (locked), open the
throttle wide for a brief
period (max. 3 seconds) – the
chain must not rotate.

5.2 Brake Band

– Remove the clutch drum, b 4.1

– Troubleshooting, b 3.2

1

: Pull off the bumper strip (1).

1

1

4903RA018 TG

– Engage the chain brake.

The brake band is now tensioned.

: Remove the screw (1) from the

underside of the machine.

4903RA016 TG

1

– With the chain brake released,

open the throttle wide and
activate the brake manually – the
chain must come to an abrupt
stop.

The braking time is in order if
deceleration of the saw chain (less
than a second) is imperceptible to
the eye.

If the chain brake does not operate
properly, refer to troubleshooting,

b 3.2.

: Take out the screws (arrows).

– Remove the cover (1).

: Pry the brake band (1) out of its

4903RA017 TG

seat (arrow).

– Remove the brake band (1).

Do not overstretch the brake band.

– Pull the hand guard towards the

handlebar to simplify assembly of
the brake band.

4903RA019 TG

20 MS 311, MS 391

2

1

1

: Turn the brake band (1) to one

side and disconnect it from the
brake lever (2).

Install a new brake band if there are
noticeable signs of wear (large
areas on inside diameter and/or
parts of outside diameter – arrows)
and its remaining thickness is less
than 0.6 mm.

4903RA020 TG

: Position the brake band (1) it the

guide (arrow) first.

– Disengage the chain brake.

1

219RA049 TG

: Push the brake band (1) over the

guide lugs (arrows) and into its
seat.

4903RA022 TG4903RA023 TG

: Fit the screw (1) on the underside

of the machine and tighten it
down firmly.

1

: Engage the cover (1) in the slots

(arrows) first and then swing it
into position.

4903RA018 TG

4903RA025 TG

Installing

1

: Hold the brake band (1)

sideways, attach it to the brake
lever (2) and then swing it in the
direction of its seat.

2

: Push the brake band (1) into its

seat (arrow) as far as stop.

4903RA021 TG

1

4903RA024 TG

21MS 311, MS 391

5.3 Brake Lever

1

1

: Insert the screws (arrows) and

tighten them down firmly.

– Fit the bumper strip.

– Install the clutch drum, b 4.1

– Check operation, b 5.1

– Reassemble all other parts in the

reverse sequence.

4903RA026 TG

– Troubleshooting, b 3

– Remove the brake band, b 5.2

– Engage the chain brake.

The brake spring is now relaxed.

: Use the assembly tool

1117 890 0900 to disconnect the
brake spring (1) from the anchor
pin (arrow).

– Remove the brake spring from

the brake lever.

: Remove the E-clip (1).

4903RA027 TG

1

: Lift and turn the strap (1) slightly,

push it towards the hole (arrow)
and remove it.

: Remove the spacer sleeve (2)

from the brake lever.

4903RA029 TG

2

4903RA030 TG

2

1

4903RA119 TG

– Remove the shroud, b 6.4

: Take out the screws (arrows).

– Remove the sleeve (1).

– Lift the hand guard a little and

remove the fan housing.

22 MS 311, MS 391

: Pull the hand guard (1) and brake

lever (2) off the pivot pins
(arrows) together.

– Remove the hand guard and

brake lever.

4903RA031 TG

1 2

1

1

: Take the brake lever (2) out of the

hand guard (1).

– Inspect the pivot pins and replace

if necessary, b 5.5

– Inspect the cam lever and

replace if necessary, b 5.4

Installing

1

4903RA032 TG

: Push the hand guard with brake

lever (1) over the machine until
they are positioned against the
pivot pins (arrows).

: Lift the bearing boss of the hand

guard and the brake lever a little
and position them over the pivot
pins (arrows).

4903RA033 TG

4903RA034 TG

: Fit the spacer sleeve (1).

1

– Fit the strap (1).

4903RA035 TG4903RA036 TG

: Fit the E-clip (2).

4903RA272 TG

2

4903RA037 TG

– Clean all disassembled parts,

b 14

: Hold the brake lever (1) so that

the brake spring attachment point
(arrow) is at the top.

: Push the brake lever (1) into the

hand guard recess and line up
the holes.

1

: Turn the cam lever (1) to one side

until the cam of the hand
guard (arrow) slips passed it.

– Push the hand guard bearing

boss and the brake lever on to
the pivot pins.

23MS 311, MS 391

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