Stihl FS 25-4 User manual

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
7. Ignition System 19
7.1 Ignition Module 19
7.1.1 Ignition Timing 20
7.1.2 Removing/Installing 20
7.2 Flywheel 21
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
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
14. Special Servicing
Tools and Aids 43
14.1 Special Servicing Tools 43
14.2 Servicing Aids 44
15. Maintenance chart 45
1. Safety
Precautions
If the brushcutter is started up
in the course of repairs or mainten-
ance 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 flamma-
ble 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 environ-
ment-friendly disposal.
© 1998, Andreas Stihl AG & Co, Waiblingen
FS 25-4, 65-4
Contents
FS 25-4, 65-4 1

Thisservicemanualcontainsdetai-
leddescriptionsofalltherepair
andservicingproceduresspecific
tothispowertoolseries.
Thereareseparatehandbooksfor
servicingproceduresforstand-
ardizedpartsandassembliesthat
areinstalledinseveralSTIHLpo-
wertoolmodels.Referenceis
madetothesehandbooksinthe
appropriatechaptersinthis
manual.
Servicingthecarburetorisdescri-
bedintheseparate"Carburetors"
manual.
Asthedesignconceptofmodels
FS25-4andFS65-4isalmost
identical,thedescriptionsand
servicingproceduresinthis
manualgenerallyapplytoboth
machines.Differencesare
describedindetail.
Youshouldmakeuseofthe
illustratedpartslistswhilecarrying
outrepairwork.Theyshowthe
installedpositionsoftheindividual
componentsandassemblies.
Refertothelatesteditionofthe
relevantpartslisttocheckthepart
numbersofanyreplacementparts
needed.
PartslistsonmicroficheandCD-
ROMarealwaysmoreuptodate
thanprintedlists.
Afaultonthepowertoolmayhave
severalcauses.Apartfromthe
chapter"Troubleshooting"inthis
manual,alsoconsultthetroubles-
hootingchartsforallassembliesin
the"StandardRepairs,Troubles-
hooting"handbook.
Refertothe"TechnicalInforma-
tion"bulletinsforengineering
changeswhichhavebeenintro-
ducedsincepublicationofthis
servicemanual.Technicalinforma-
tionbulletinsalsosupplementthe
partslistuntilarevisededitionis
issued.
Thespecialservicingtoolsmen-
tionedinthedescriptionsare
listedinthelastchapterofthisma-
nual.
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.
Servicingandrepairsaremade
considerablyeasierifthemachine
ismountedonassemblystand(2)
59108903100withtheaidof
clamp(1)59108908800.
Securetheclamptotheassembly
standwithtwowashersandtwo
M8nuts.
Thecompletemachinecanthen
beswivelledtothebestposition
fortheongoingrepairandthislea-
vesbothhandsfree.
AlwaysuseoriginalSTIHL
replacementparts.
Theycanbeidentifiedbythe
STIHLpartnumber,
the
STIHL logo
andthe
STIHLpartssymbol
(
Thesymbolmayappearaloneon
smallparts.
2. Introduction
2FS25-4,65-4

3.1 Engine
Singlecylinderfour-strokeoverheadvalveengine
FS25-4 FS65-4
Type: GX22 GX31
Displacement: 22cm331cm3
Bore:33mm39mm
Stroke:26mm26mm
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(100cm3)
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(450cm3)0.65l(650cm3)
Fuel: Regularbrandnamegasoline
Octanerating: min.90RON
Airfilter: Oilimpregnatedfoamfilter
3.Specifications
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
4FS 25-4, 65-4

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
3.6 Dimensions and Wear Limits
FS 25-4, 65-4 5

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
3.7 Tightening Torques
6FS 25-4, 65-4

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
Check spark plug
for spark.
Powerful spark
Check compression.
Adequate compression
Install spark plug and start
engine according to
instructions.
No fuel
Fuel does not flow
to carburetor
Dry
No spark or only a
weak spark
Compressioninorder
Compression too low
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
– 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.
4. Troubleshooting
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 kin-
ked, pinched or cracked. Check
carburetor for looseness or air
entering through spacer flange.
Normal
Check fuel filter.
Not dirty
Check correct function of
carburetor.
Normal
Check compression.
Normal
Start engine according to
instructions.
Dirty
Abnormal
Abnormal
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.
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.
8FS 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 kin-
ked, pinched or cracked. Check
carburetor for looseness or air
entering through spacer flange.
Normal
Check correct function of
carburetor.
Normal
Start engine according
to instructions.
Dirty
Abnormal
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.
Replace carburetor.
FS 25-4, 65-4 9

Troubleshootingchart-see
"StandardRepairs,Trouble-
shooting"handbook.
-Removetheshroud-see6.1.
-Removecontrolhandlefrombike
handle-see12.1.
-Onloophandlemachine,dis-
connectthrottlecablefrom
carburetor-see9.5.1.
•Takeoutthescrews.
-Pulloffclutchhousingwithdrive
tube.
•Inspectclutchdrum.There
shouldbenoscoresorsigns
ofexcessivewear.
Important:Iftherearesignsof
seriouswearontheinside
diameter,fitanewclutchdrum-
see13.2.
-Blockpistonwithlockingscrew-
see7.2.
•Takeoutthecollarscrews(1).
•Removetheclutchshoes(2).
•Twisttheclutchshoesand
detachthespring.
Important:Clutchshoesmust
alwaysbereplacedinpairs.
•Attachspringtoclutchshoes.
•Fitthewashers.
•Fitclutchshoessothatthe
marksfaceoutwards.
""=FS25-4
"R"=FS65-4
5. Clutch
5.1 RemovingandDisassembling 5.2 AssemblingandInstalling
10 FS25-4,65-4