Stihl FS 25-4, FS 65-4 User Manual

FS 25-4, 65-4
Contents

1. Safety Precautions 1

2. Introduction 2

3. Specifications 3

3.1 Engine 3

3.2 Fuel System 3

3.3 Ignition System 4

3.4 Gearhead 4

3.5 Special Accessories 4

3.5.1 For User 4

3.6 Dimensions and
Wear Limits 5

3.7 Tightening Torques 6

4. Troubleshooting 7

5. Clutch 10

5.1 Removing and
Disassembling 10

5.2 Assembling and
Installing 10

6. Engine 11

6.1 Checking/Adjusting
Valve Clearance 11

6.2 Checking Oil Level/
Changing Oil 13

6.3 Exhaust Muffler 14

6.4 Rocker Arms/Pushrods 14

6.5 Cam Followers 15

6.6 Cam Gear 15

6.7 Diaphragm 15

6.8 Crankcase,
Lower Half 16

6.9 Crankshaft 17

6.10 Valves/Valve Springs 18

6.11 Crankcase/Cylinder 18

9. Throttle Control 26

9.1 Throttle Trigger/Interlock
Lever (Bike Handle) 26

9.2 Contact Springs/Detent
Spring in Control Handle
(Bike Handle) 27

9.3 Throttle Trigger/Interlock
Lever (Loop Handle) 27

9.4 Slide Control
(Loop Handle) 28

9.5 Throttle Cable 29

9.5.1 Replacing 29

9.5.2 Adjusting 30

10. AV System 30

11. Fuel System 32

11.1 Air Filter 32

11.2 Carburetor 32

11.2.1 Leakage Test 32

11.2.2 Removing/Installing 33

11.2.3 Adjusting Idle Speed 34

11.3 Tank Vent 35

11.4 Pickup Body 35

11.5 Fuel Hoses 36

11.6 Fuel Tank 37

12. Shaft 38

12.1 Bike Handle 38

12.2 Loop Handle 38

12.3 Drive Shaft/
Flexible Liner 39

12.4 Drive Tube 40

13. Cutting Tool Drive 41

13.1 Gearhead 41

13.2 Clutch Drum 41

1. Safety Precautions

If the brushcutter is startedup
in the course of repairs or mainten­ance work, observe all local and
country-specific safety regulations
as well as the safety precautions
and warnings in the owner’s
manual.

Gasoline is an extremely flamma­ble fuel and can be explosive in
certain conditions.

Improper handling may result in
burns or other serious injuries.

Warning! Do not smoke or bring
any fire or flame near the fuel. All
work with fuel must be performed
outdoors only. Spilled fuel must be
wiped up immediately.

Wash hands thoroughly after
every contact with waste oil.

Do not pour waste oil down the
drain or allow it to soak into the
ground.

Collect waste oil and take it to an
official disposal site for environ­ment-friendly disposal.

7. Ignition System 19

14. Special Servicing

Tools and Aids 43

7.1 Ignition Module 19

7.1.1 Ignition Timing 20

7.1.2 Removing/Installing 20

14.1 Special Servicing Tools 43

14.2 Servicing Aids 44

7.2 Flywheel 21

15. Maintenance chart 45

8. Rewind Starter 22

8.1 General 22

8.2 Rope Rotor 22

8.3 Starter Rope 23

8.4 Starter Grip 23

8.5 Rewind Spring 24

8.5.1 Replacing 24

8.5.2 Tensioning 24

8.6 Starter Cup/Fanwheel 25

© 1998, Andreas Stihl AG & Co, Waiblingen

FS 25-4, 65-4 1

2. Introduction

Thisservicemanualcontainsdetai­leddescriptionsofalltherepair
andservicingproceduresspecific
tothispowertoolseries.
Thereareseparatehandbooksfor
servicingproceduresforstand­ardizedpartsandassembliesthat
areinstalledinseveralSTIHLpo­wertoolmodels.Referenceis
madetothesehandbooksinthe
appropriatechaptersinthis
manual.

Servicingthecarburetorisdescri­bedintheseparate"Carburetors"
manual.

Asthedesignconceptofmodels
FS25-4andFS65-4isalmost
identical,thedescriptionsand
servicingproceduresinthis
manualgenerallyapplytoboth
machines.Differencesare
describedindetail.

Youshouldmakeuseofthe
illustratedpartslistswhilecarrying
outrepairwork.Theyshowthe
installedpositionsoftheindividual
componentsandassemblies.

Refertothelatesteditionofthe
relevantpartslisttocheckthepart
numbersofanyreplacementparts
needed.
PartslistsonmicroficheandCD­ROMarealwaysmoreuptodate
thanprintedlists.

Thespecialservicingtoolsmen­tionedinthedescriptionsare
listedinthelastchapterofthisma­nual.
Usethepartnumberstoidentify
thetoolsinthe"STIHLSpecial
Tools"manual.
Themanuallistsallspecial
servicingtoolscurrentlyavailable
fromSTIHL.

Symbolsareincludedinthetext
andpicturesforgreaterclarity.
Themeaningsareasfollows:

Inthedescriptions:

•

=Actiontobetakenas

shownintheillustration
(abovethetext)

-=Actiontobetakenthat
isnotshowninthe
illustration
(abovethetext)

Intheillustrations:

=Pointer
=Directionofmovement

Servicemanualsandalltechnical
informationbulletinsdescribing
engineeringchangesareintended
exclusivelyfortheuseofSTIHL
servicingdealers.Theymustnot
bepassedtothirdparties.

Securetheclamptotheassembly
standwithtwowashersandtwo
M8nuts.

Thecompletemachinecanthen
beswivelledtothebestposition
fortheongoingrepairandthislea­vesbothhandsfree.

AlwaysuseoriginalSTIHL
replacementparts.

Theycanbeidentifiedbythe
STIHLpartnumber,

the

STIHL

andthe
STIHLpartssymbol
Thesymbolmayappearaloneon
smallparts.

logo

(

Afaultonthepowertoolmayhave
severalcauses.Apartfromthe
chapter"Troubleshooting"inthis
manual,alsoconsultthetroubles­hootingchartsforallassembliesin
the"StandardRepairs,Troubles­hooting"handbook.

Refertothe"TechnicalInforma­tion"bulletinsforengineering
changeswhichhavebeenintro­ducedsincepublicationofthis
servicemanual.Technicalinforma­tionbulletinsalsosupplementthe
partslistuntilarevisededitionis
issued.

2 FS25-4,65-4

Servicingandrepairsaremade
considerablyeasierifthemachine
ismountedonassemblystand(2)
59108903100withtheaidof
clamp(1)59108908800.

3. Specifications

3.1 Engine

Singlecylinderfour-strokeoverheadvalveengine

FS25-4 FS65-4

Type: GX22 GX31
Displacement: 22cm

3

31cm

3

Bore: 33mm 39mm
Stroke: 26mm 26mm
PoweroutputtoISO8893: 0.6kW(0.8bhp) 0.85kW(1.2bhp)

at7,000rpm at7,000rpm
Max.enginespeed: Maximumrpmisdeterminedbymainjetandisnotadjustable
Idlespeed: 3,100±200rpm
Compressionpressure: 29-59kPa(2.9-5.9bar)

at1,200rpm
Pistonpindiameter: 8mm
Rewindstarter: Pawlengagement
Pawls: Singlepawlsystem
Reservepullonroperotor: min.1/2turn
Starterropediameter: 3mm
Clutch: Centrifugalclutchwithpress-fittedlinings
Clutchengagesat: 4,200±200rpm
Valveclearance
Inlet: 0.12±0.02mm
Exhaust: 0.15±0.02mm
Oilcapacity: 0.1l(100cm

3

)

Oilgrade: SAE15W-40

3.2 FuelSystem

Carburetor: Diaphragmcarburetor
Carburetorleakagetest
atgaugepressure: 8kPa(0.8bar)
Functionoftankvent

atgaugepressure: ≤3kPa(0.3bar)
undervacuum: ≤0.5kPa(0.05bar)

Fueltankcapacity: 0.45l(450cm

3

) 0.65l(650cm3)
Fuel: Regularbrandnamegasoline
Octanerating: min.90RON
Airfilter: Oilimpregnatedfoamfilter

FS25-4,65-4 3

3.3 Ignition System Type: Electronic magneto

ignition (breakerless)

with integral trigger unit
Ignition timing: 27 degrees B.T.D.C.
Air gap: 0.35 - 0.5 mm
Spark plug (suppressed): NGK CR5HSB,

Denso U16FSR-UB
Electrode gap: 0.6 - 0.7 mm

3.4 Gearhead Type: Spiral-toothed bevel gear drive

Gear ratio: 1:1.4
Bearings: Deep groove ball bearings
Lubrication: STIHL gear lubricant

0781 120 1117, 1118
(capacity 7 g)

3.5 Special Accessories

3.5.1 For User Full harness

Safety glasses
Transport guard for
Metal cutting tools 4119 717 3005
STIHL gear lubricant (80 g tube) 0781 120 1109
STIHL gear lubricant (80 g tube) 0781 120 1117

4 FS 25-4, 65-4

3.6 Dimen sionsandWear Limits

Description Standard Wear limit

Cylinder inside diameter 33.00 mm * 33.10 mm *

39.00 mm ** 39.10 mm **

Piston outside diameter 32.99 mm * 32.90 mm *

38.99 mm ** 38.90 mm **
Piston pin outside diameter 8.00 mm 7.95 mm
Cam lift 22.79 mm 22.49 mm

(inlet and exhaust)
Free length of valve springs 23.40 mm 22.50 mm
Valve stem outside diameter

Inlet: 3.985 mm 3.900 mm
Exhaust: 3.950 mm 3.880 mm

Valve guide inside diameter 4.000 mm 4.060 mm
Clearance between valve stem and guide

Inlet: 0.015 - 0.048 mm 0.098 mm
Exhaust: 0.050 - 0.083 mm 0.120 mm

Valve head diameter
Inlet: 12.5 mm *

13.5mm**
Exhaust: 12.0 mm

Clutch lining: 2.0 mm 1.0 mm

* FS 25-4
** FS 65-4

FS 25-4, 65-4 5

3.7 Tightening Torques

DG and P screws (Plastoform) are used in polymer and lightmetal 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 detrimentally affecting 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 For component Torque Remarks

size Nm

Pan head screw M5x12 Valve cover 4.0
Pan head screw M5x22 Valve cover 4.0
Nut M5 Valve adjusting screw 5.0
Collar nut M8 Flywheel 15.0

M8 Starter cup 8.0
Nut M5 Carburetor 5.0
Pan head screw M6x28 Clutch shoes 10.0 1)
Pan head screw M8x28 Clutch shoes 15.0 2)
Pan head screw M4x16 Cam gear cover 3.0
Pan head screw M5x18 Crankcase 5.0
Pan head screw M5x55 Crankcase cover

1st stage 3.0

2nd stage 7.0
Spline screw IS-DG5x24 Gear housing/drive tube 8.5
Spline screw IS-M5x12 Clamp, control handle/

drive tube (loop handle) 2.0
Spline screw IS-M5x16 AV sleeve/drive tube (clamp screw) 5.5
Spline screw IS-M5x12 Clamp, control handle/drive tube 2.0
Spline screw IS-M5x16 Clamp/drive tube (harness) 4.5
Spline screw IS-M5x30 Control handle (bike handle) 2.0
Spline screw IS-M6x25 Clamp/loop handle 4.5
Screw plug M11x10 Gearhead 8.5
Plastoform screw IS-P4x16 Control handle/handle moldings 1.0
Collar screw IS-P3.5x10.6 Detent spring/slide control

(bike handle) 1.1
Nut M12x1.5 L Cutting tool 25.0
Spline screw IS-M5x16 Deflector/gearhead 4.3
All other screws M3 1.0
and nuts M5 5.0

M6 9.0

Use the following procedure to fit a DG or P screw in an existing thread:
– Place the DG or P screw in the hole and rotate it counterclockwise until it drops down slightly.

– Tighten the screw clockwise to the specified torque.
This procedure ensures that the screw engages properly in the existing thread and does not form a new thread,

which would weaken the assembly.

1) FS 25-4

2) FS 65-4
Note: Power screwdriver speed settings for polymer: Plastoform screws max. 600 rpm

DG screws max. 500 rpm

6 FS 25-4, 65-4

4. Troubleshooting

1. Engine difficult to start

Check fuel level in tank.

Adequate fuel in

tank

Operate manual fuel pump
and check that fuel is flowing to
carburetor.

Fuel flows to

carburetor

Remove spark plug and
check for damp or oily
electrodes.

Damp

No fuel

Fuel does not flow

to carburetor

Dry

Fill up with fuel and try starting
again.

– Check fuel line and fuel

filter for blockages.

– Faulty carburetor diaphragm

or float needle valve,
replace carburetor

Replace carburetor

Check spark plug
for spark.

Powerful spark

Check compression.

Adequate compression

Install spark plug and start
engine according to
instructions.

No spark or only a

weak spark

Compressioninorder

Compression too low

– Change spark plug and

repeat the spark test.

– Check ignition lead for

damaged insulation and
leakage current.

– Check ignition coil.

Check combustion chamber
for excessive build-up of
combustion deposits.

– Check, adjust valve

clearance.

– Valves must be in good

condition.

– Check piston, rings and

cylinder for wear.

FS 25-4, 65-4 7

2. Erratic running behavior at low rpm

Inspect air filter element.

Not dirty

Remove spark plug and
check electrodes for coking,
check electrode gap.

Normal

Check to see if fuel line is kin­ked, pinched or cracked. Check
carburetor for looseness or air
entering through spacer flange.

Normal

Dirty

Abnormal

Abnormal

Clean, replace air filter
element.

– Clean spark plug and

adjust electrode gap.

– Replace spark plug.

– If fuel line is kinked

or pinched, fit a
new one.

– Tighten loose carburetor

nuts. Examine spacer
flange for damage and
install properly.

Check fuel filter.

Not dirty

Check correct function of
carburetor.

Normal

Check compression.

Normal

Start engine according to
instructions.

Dirty

Abnormal

Abnormal

Clean, replace filter.

Replace carburetor.

– Check, adjust valve

clearance.

– Check combustion chamber

for excessive build-up of
combustion deposits.

– Check valves for faults.
– Check piston, rings and

cylinder for wear.

8 FS 25-4, 65-4

3. Erratic running behavior at high rpm

Inspect air filter element.

Not dirty

Remove spark plug and
check electrodes for coking,
check electrode gap.

Normal

Check to see if fuel line is kin­ked, pinched or cracked. Check
carburetor for looseness or air
entering through spacer flange.

Normal

Dirty

Abnormal

Abnormal

Clean, replace air filter
element.

– Clean spark plug and

adjust electrode gap.

– Replace spark plug.

– If fuel line is kinked

or pinched, fit a
new one.

– Tighten loose carburetor

nuts. Examine spacer
flange for damage and
install properly.

Check correct function of
carburetor.

Normal

Start engine according
to instructions.

Abnormal

Replace carburetor.

FS 25-4, 65-4 9

5. Clutch

5.1 RemovingandDisassembling 5.2 AssemblingandInstalling

Important:Iftherearesignsof

seriouswearontheinside

diameter,fitanewclutchdrum-

see13.2.

Troubleshootingchart-see
"StandardRepairs,Trouble­shooting"handbook.

-Removetheshroud-see6.1.

-Removecontrolhandlefrombike

handle-see12.1.

-Onloophandlemachine,dis-

connectthrottlecablefrom
carburetor-see9.5.1.

•

Takeoutthescrews.

-Pulloffclutchhousingwithdrive

tube.

-Blockpistonwithlockingscrew-

see7.2.

•

Takeoutthecollarscrews(1).

•

Removetheclutchshoes(2).

•

Attachspringtoclutchshoes.

•

Fitthewashers.

•

Inspectclutchdrum.There
shouldbenoscoresorsigns
ofexcessivewear.

10 FS25-4,65-4

•

Twisttheclutchshoesand
detachthespring.

Important:Clutchshoesmust
alwaysbereplacedinpairs.

•

Fitclutchshoessothatthe

marksfaceoutwards.
" "=FS25-4
"R"=FS65-4

•

Insertcollarscrews.

6. Engine 6.2 Checking/Adjusting

6.1 ValveClearance

Note:Checkandadjustvalveclea-

ranceonlywhenengineiscold.

-Removetherewindstarter.

-Removetheairfilter-see11.1.

•

Tightendowncollarscrewsto10
Nm(FS25-4)or15Nm(FS65-

4).

-Removelockingscrewfromcylin­der.

-Installclutchhousingwithdrive
tube.

-Onloophandlemachine,attach
throttlecabletocarburetor.

-Onbikehandlemachine,install
thecontrolhandle.

-Fittheshroud.

-Pulloffthesparkplugboot.

•

Takeignitionleadoutofthe
retainer.

•

Slackenoffnut(1).

•

Pullthrottlecable(2)withshort
circuitwire(3)andwasher(4)
outofretainer.

FS25-4,65-4 11

Note:Iftheexhaustvalveopens

whileyouareturningtheflywheel
toT.D.C.,rotateflywheelcounter­clockwiseuntiltherecessand
projectionlineupagain.

•

Removeshroudfrombossesat
rewindstarterside.

•

Takeoutscrews(1)atclutch
side.

•

Removetheshroud(2).

•

Takeoutthescrews(1).

•

Removethevalvecover(2).

•

Removethegasket.

-Removeclutchhousingwithdri­vetube-see12.4.

•

Insertfeelergauge(1)between
rockerarmandvalvestem.

Inletvalve(2):0.12±0.02mm
Exhaustvalve(3):0.15±0.02mm

Checking

•

Pullventhose(1)offthe
valvecover.

•

Pullignitionlead(2)andshort
circuitwire(3)outofthe
retainers.

12 FS25-4,65-4

•

Rotateflywheeluntilblade
recess(1)linesupwiththe
projection(2)onthecamgear
cover(pistonatT.D.C.).

6.2 CheckingOilLevel/
ChangingOil

Adjusting

•

Slackenoffnut(1).Use2.5mm
Allenkeytorotateadjusting
screw(2).

Toincreasevalveclearance:
Turnscrewcounterclockwise.
Toreducevalveclearance:
Turnscrewclockwise.

-Holdadjustingscrewsteadyand
tightendownnutto5Nm.

Checking

-Putenginedownonalevel
surface.

-Unscrewthedipstickandwipeit
withacleancloth.

-Pushthedipstickintofillerneck
asfarasstopbutdonotscrewit
home.

-Putenginedownonalevel
surface.

-Fillupwithengineoiltoedgeof
fillerneck.

-Screwdipstickhomeandtighten
downfirmly.

-Checkvalveclearanceagain.
Assembleinthereversesequence.

-Usenewgasketforvalvecover.

-Tightendownvalvecover
screws
to4Nm.

•

Removethedipstickandcheck
theoillevel.

•

Ifoillevelislow,topupwith
freshengineoiltoedgeoffiller
neck.

-Screwdipstickhomeandtighten
downfirmly.

Changing

-Unscrewthedipstick.

-Tiptheenginetoonesideto
drainoilintoasuitablecontainer.

Note:Observesafetyprecautions-

see1.

FS25-4,65-4 13

6.3 ExhaustMuffler 6.4 RockerArms/ Pushrod s

-Removetheshroud-see6.1.

•

Takeoutthescrews.

-Removetheguard.

•

Takeoutthescrews.

-Removethemuffler.

-Removethevalvecover-

see6.1.

-SetpistontoT.D.C.(bothrocker
armsrelieved).

•

Pulloutpin(1).

•

Removerockerarms(2).

•

Removethepushrods.

Assembleinthereverse
sequence.

-Pushrodsmustlocateproperly
intheadjustingscrews.

-Adjustvalves-see6.1.

•

Removecoverplate.

Assembleinthereverse
sequence.

Notedifferentlengthsofscrews.

14 FS25-4,65-4

•

Toreplace,slackenoffnut(1)
andtakeouttheadjusting
screw(2).