Stihl Carburetor Manual

7/30/2019 Stihl Carburetor Manual

1/65

1. Safety Precautions 2

2. Introduction 2

3. Assignment 3

3.1 Designation 8

4. Purpose ofCarburetor 8

5. Construction/Function 9

5.1 Fuel Pump 95.2 Operating Conditions 11

5.2.1 Full Load 115.2.2 Idle 125.2.3 Carburetor with

Fixed Idle Jetand Primary MixtureControl 14

5.2.4 Carburetor withBalanced Idle System(Fixed Idle Jet andAir Bypass System) 15

5.2.5 Part Load 165.3 Problematic

Operating Conditions 175.3.1 Starting 175.3.2 Temperatures/

Humidity 195.3.3 Acceleration 195.3.4 Dirty Air Filter/

Compensator 205.4 Special Features 21

6. TroubleshootingChart 23

7. Servicing 25

7.1 Leakage Test 257.2 Cleaning 257.3 Pump Diaphragm 267.4 Metering Diaphragm 267.5 Inlet Needle 27

7.6 Main Jet 287.7 Full-Load Fixed Jet/

Part-Load Fixed Jet 297.8 Control Valve 297.9 Manual Fuel Pump 307.10 Check Valve 307.11 Carburetor Body 317.12 Throttle Shaft 327.13 Choke Shaft 347.14 Accelerator Pump 357.15 Lever Mechanism

on Carburetors withAutomatic Choke 36

7.15.1 Throttle Shaft/Choke Shaft 38

7.16 HL-324A / HL-327D /LA-S8A Carburetors 38

7.16.1 Throttle Shaft 437.16.2 Choke Shaft 447.17 WYL-63 / WYL-73

Rotary ValveCarburetors 45

7.18 Standard Setting onWalbro Carburetors 50

7.18.1 WT-283B, WT-323,WT-325A, WT-326C,

WT-360, WT-396,WT-403A, WT-493,WT-498A, WT-503Fixed Jet Carburetors 50

7.18.2 HD-13A, HD-16, HD-17,HD-18A, HD-21A, HD-22,

WY-69, WT-329, WT-330,WT-367, WT-393,WT-489, WT-492,WJ-69 Carburetorswith Limiter Caps 51

7.19 Standard Setting onZama Carburetors 54

7.19.1 C1Q-S30C, C1Q-S37A,C1Q-S41, C1Q-S42,C1Q-S34B, C1Q-S36BFixed Jet Carburetors 54

7.19.2 C1Q-SK7, C3A-S39A

Carburetors withLimiter Caps 55

7.20 Standard Setting onTillotson Carburetors 59

7.20.1 Fixed Jet Carburetors:HS-275A, HS-279B 59

7.20.2 HL-366A, HS-280A,HS-281A, HT-12ACarburetors withLimiter Caps 60

7.21 Flow Chart 64

8. Special ServicingTools and Aids 65

8.1 Special Servicing Tools 658.2 Servicing Aids 65

1999 Andreas Stihl AG & Co., Waiblingen

CarburetorsContents

Carburetors 1


2/65

If the chainsaw or power toolis started up in the course ofrepairs or maintenance work,observe all local and country-specific safety regulations aswell as the safety precautionsand warnings in the owners

manual.

Gasoline is an extremely flamm-able fuel and can be explosivein certain conditions.

Improper handling may result inburns or other serious injuries.

Warning! Do not smoke or bringany fire or flame near the fuel. Allwork with fuel must be performedoutdoors only. Spilled fuel must be

wiped away immediately.

Warning! To avoid the risk ofaccidents and personl injury, takeextreme care when performingadjustments without the cuttingtool guard or deflector.

This service manual contains des-criptions of repair and servicingprocedures as well as functionaldiagrams of most carburetorsused in STIHL gasoline powertools.

While carrying out repair workyou should make use of the latestillustrated parts list of the powertool concerned. It shows the instal-led positions and the assembly se-quence of individual componentsand assemblies.Parts lists on microfilm andCD-ROM are always more upto date than printed lists.

A fault on the carburetor mayhave several causes. Consult the

troubleshooting chart - see 6.

Refer to the "Technical Informa-tion" bulletins for engineeringchanges which have been intro-duced since publication of thisservice manual. Technical informa-tion bulletins also supplement theparts list until an update is issued.

Service manuals and all technicalinformation bulletins describingengineering changes are intended

exclusively for the use of STIHLservicing dealers. They must notbe passed to third parties.

Symbols are used in the text andillustrations for greater clarity.

The meanings are as follows:

In the descriptions:

= Action to be taken asshown in the illustration(above the text)

- = Action to be taken thatis not shown in theillustration(above the text)

In the illustrations:

= Pointer

= Direction of movement

Always use original STIHLreplacement parts.They can be identified by theSTIHL part number,

the lgo

and the STIHL parts symbol(

The symbol may appear alone onsmall parts.

1. Safety 2. IntroductionPrecautions

2 Carburetors


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3. Assignment

Model Carburetor Idle systemPartload

Fullload

Setting Equipment

depen

den

t

indepen

den

t

fixe

didlejet

pre

lim

inarym

ixturecon

tro

l

ba

lance

d

withbypassa

irs

crew

(LD)

part-l

oa

djet

full

-loa

dfixe

djet

full

-loa

dsem

i-fix

edjet

H

un

lim

ite

da

dju

stmen

t

Lun

lim

ite

da

djus

tmen

t

H,

Lw

ithlim

itercaps

cho

kes

hu

tter

in

carb

manua

lfue

lpum

p

Compensa

tor

009 ... 012 C1S-S1 x x x

009, 010, 011 WT-29A x x x

009, 011 C1S-S1C 2) x x x

009, 011 1) WT-323 x x x x

017 WT-325A x x x x

017 WT-325 x x x x019T C1Q-S37A x x x x x x

020 WT-15A x x x

020 C1Q-S32 x x x

020T C1Q-S16A x x x x x x

020T 1) WT-326C x x x x x

020T WT-326 x x x x

021 1) WT-503 x x x x x

021 WT-286A x x x x x

021, 023 1) WT-283A x x x x x

021, 023, 025 C1Q-S11 x x x x

023 WT-215 x x x x023 WT-283B x x x x x

023 WT-286A x x x x x

023C WT-286A x x x x x

023C WT-498 x x x x x x

023CZ 1) WT-498A x x x x x x

023L WT-360 x x x x x

023L 1) WT-396 x x x x x

024 WT-194 x x x

024 WT-110 x x x

024, 026 WT-194 x x x

025 WT-215 x x x x025 WT-286A x x x x x

025 WT-396A x x x x

025 WT-498A x x x x x x

025 WT-283B x x x x x

025 WT-313 x x x x x

026 WT-426 x x x x x

026 1) WT-493 x x x x

026 1) WT-403A x x x x x

026W/Pro WT-427 x x x x x

028 WT-16 x x x

Carburetors 3


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Fullload

Setting Equipment

depen

den

t

indepen

den

t

fixe

didlejet

pre

lim

inarym

ixturecon

tro

l

ba

lance

d

withbypassa

irscrew

(LD)

part-l

oa

djet

full

-loa

dfixe

djet

full

-loa

dsem

i-fix

edjet

H

un

lim

ite

da

dju

stmen

t

Lun

lim

ite

da

djus

tmen

t

H,

Lw

ithlim

iter

caps

cho

kes

hu

tter

in

carb

manua

lfue

lpum

p

Compensa

tor

029 HD-19B x x x x x x x

029 1) HD-18A x x x x x x

029, 039 HD-5 x x x x x x

034 C3A-S38A x x x x x x

034 C3A-S38 x x x x x x

034, 036 C3A-S19 x x x x036 C3A-S4C x x x x

036 C3A-S31D x x x x x x x

036 1) C3A-S39A x x x x x x

036W C3A-S27C x x x x x x x

038 HK-42 x x x x

038 Bing 48 x x x x

039 HD-19B x x x x x x x

039 1) HD-21A x x x x x x

044 HD-10 x x x x x x x

044 HD-11 x x x x x x x

044 HD-15 x x x x x x x044 C3M-S20 x x x x x x

044 C3M-S5G x x x x x

044 1) HD-17 x x x x x x

044 (SA) C3M-S21/S12 x x x x x x

046 HD-14 x x x x x x x

046 1) HD-16 x x x x x x

046 BR HD-24 x x x x x x x

051, 050, 076 HS-60D x x x x

056 HS-118B x x x x

056 WJ-4B x x x x

064 WJ-48 x x x x x x x064 WJ-51 x x x x x x x

064 WJ-52 x x x x x x x

064 WJ-65 x x x x x x x

064, 066 WJ-35/41 x x x x x x x

066 WJ-66 x x x x x x x

066 WJ-67 x x x x x x x

066 1) WJ-69 x x x x x x

066 BR WJ-76 x x x x x x x

066 MW WJ-66 x x x x x x x

070 LB-S9 x x x x

4 Carburetors


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Fullload

Setting Equipment

depen

den

t

indepen

den

t

fixe

didlejet

pre

lim

inarym

ixturecon

tro

l

ba

lance

d

withbypassa

irscrew

(LD)

part-l

oa

djet

full

-loa

dfixe

djet

full

-loa

dsem

i-fix

edjet

H

un

lim

ite

da

dju

stmen

t

Lun

lim

ite

da

djus

tmen

t

H,

Lw

ithlim

iter

caps

cho

kes

hu

tter

in

carb

manua

lfue

lpum

p

Compensa

tor

070 HL-324A x x x x

070, 090 HL-324A x x x x

08 HL-166C x x x x

084 HT-7A x x x x x x x

088 1) HT-12A x x x x x x

08S LA-S8A x x x x08S IVH8 x x x x

090 LB-S9 x x x x

BG 72 WT-253 x x x x

BG 72 1) WT-330 x x x

BG 75 C1Q-S30C x x x x x

BR 106 C1Q-SK6 x x x x x x

BR 320 1) HD-13A x x x

BR 320 HD-4B x x x x

BR 320 L WT-230 x x x x

BR 320 L WT-230B x x x x

BR 320 L 1) WT-489 x x xBR 320 L WT-331 x x x

BR 320, 400 1) HD-7 x x x

BR 400 1) HD-13A x x x

BR 400 HD-4B x x x x

BR/SR 320, 400 HD-4 x x x x

BT 106, 108 C1Q-SK5 x x x x x x

BT 360 HL-327E x x x x

FC 44 1) WT-327 x x x

FC 72 WT-227F x x x x

FC 72 1) WT-329 x x x

FR 106, 108 C1Q-SK5 x x x x x xFR 108 C1Q-SK5 x x x x x x

FR 108 1) C1Q-SK7 x x x x x

FS 106, 108 C1Q-SK5 x x x x x x

FS 120, 300 C1Q-S35B x x x x x x x x

FS 120, 300 C1Q-S36D x x x x x x x x

FS 160 C1S-S3D x x x x

FS 160, 180, 220 C1S-S3C x x x x

FS 200, 350 C1Q-S36D x x x x x x x x


FS 25-4 WYL-63/-73 x x x x x

Carburetors 5


6/65


Fullload

Setting Equipment

depen

den

t

indepen

den

t

fixe

didlejet

pre

lim

inarym

ixturecon

tro

l

ba

lance

d

withbypassa

irscrew

(LD)

part-l

oa

djet

full

-loa

dfixe

djet

full

-loa

dsem

i-fix

edjet

H

un

lim

ite

da

dju

stmen

t

Lun

lim

ite

da

djus

tmen

t

H,

Lw

ithlim

iter

caps

cho

kes

hu

tter

in

carb

manua

lfue

lpum

p

Compensa

tor


FS 280 WT-223 x x x x

FS 36, 40, 44 WT-160B x x x x x

FS 36, 40, 44 1) WT-492A x x x x x

FS 36, 40, 44 WT-327 x x x x

FS 360, 420 HD-3 x x x xFS 400 C1Q-S34C x x x x x x x x

FS 450 C1Q-S34C x x x x x x x x

FS 500 HD-22 x x x x x

FS 500 HD-23 x x x x x

FS 550 1) HD-22 x x x x x

FS 550 HD-23 x x x x x x

FS 65-4 WYL-63 x x x x

FS 72, 74, 76 WT-227F x x x x

FS 72, 74, 76 1) WT-393 x x x

FS 75, 80, 85 C1Q-S41A x x x x x

FS 75, 80, 85 WT-447 x x x xFS 81, 86 2) WT-112 x x x x

FS 86 2) WT-45A x x x x

FS 88, FR 88 1) WT-367 x x x

FS 88, FR 88 2) WT-45A x x x x

FS/FR 108 1) C1Q-SK7 x x x x x

HS 72, 74, 76 WT-264 x x x x x

HS 72, 74, 76 1) WT-329 x x x

HS 75, 80, 85 C1Q-S42A x x x x x

TS 350 HL-292 x x x x

TS 350 HL-292G x x x x

TS 3501)

HL-366A x x x xTS 350 LAS6 x x x x

TS 360 HL-327D x x x x

TS 360 HL-327E x x x x

TS 360 LAS7 x x x x

TS 400 HS-274A x x x x x x x

TS 400 HS-274D x x x x x x x

TS 400 1) HS-279B x x x x x x

TS 460 HS-262B x x x x x x x

TS 460 1) HS-275A x x x x x x

TS 460 HS-276D x x x x x x x

6 Carburetors


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Fullload

Setting Equipment

depen

den

t

indepen

den

t

fixe

didlejet

pre

lim

inarym

ixturecon

tro

l

ba

lance

d

withbypassa

irscrew

(LD)

part-l

oa

djet

full

-loa

dfixe

djet

full

-loa

dsem

i-fix

edjet

H

un

lim

ite

da

dju

stmen

t

Lun

lim

ite

da

djus

tmen

t

H,

Lw

ithlim

iter

caps

cho

kes

hu

tter

in

carb

manua

lfue

lpum

p

Compensa

tor

TS 510 HS-277A x x x x x

TS 510 1) HS-280A x x x x x x

TS 510, 760 HS-212B x x x x

TS 760 HS-277A x x x x x

TS 760 1) HS-281A x x x x x x

1) CARB, EPA2) Accelerator pump

Carburetors 7


8/65

The carburetor designation isstamped or printed on the bodyof some models (e.g. HT-7A orWT-223).

On some models the familydesignation (e.g. C3M or HD) isintegrally cast on the body.

Other identification numbers arestamped on the body of suchmodels (e.g. S 20) or printed (e.g.S36C).

The carburetor has to mix fuel andair in an optimum ratio for eachphase of engine operation. It pre-pares the mixture for vaporizationby breaking it down into finedroplets.

In the carburetor, fuel is atomizedbut not completely vaporized.The heat necessary to vaporizethe fuel is taken from the compo-nents the fuel flows through, i.e.primarily the engine. Therefore,complete vaporization does nottake place until the mixture rea-ches the combustion chamber.

The absorption of vaporizationheat by the fuel represents a formof cooling for the components that

give off the heat. One talks aboutinternal cooling.

The ideal fuel-air ratio (Lambda ()= 1) is reached when the composi-tion of the mixture is 14.8 kg air to1 kg fuel. Fuel burns cleanly at thismixture ratio.The mixture is combustiblebetween Lambda 0.7 and 1.25.

The mixture ratio deviates fromthe ideal value depending on the

prevailing operating conditions(temperature, humidity, air pre-ssure) and the operating phase(starting, idle, part load, full load).

When the actual mixture ratiodeviates from the ideal value, themixture is said to be either rich orlean.

A lean mixture contains more air.

As there is a lack of fuel, theengine does not produce its fullpower. Furthermore, the engineoverheats because insufficientheat is dissipated to vaporize thefuel. The lack of fresh gas contain-ing oil results in inadequate lubri-

cation and a risk of seizure.

A rich mixture contains less air.

Combustion is incomplete. Apartfrom other effects, unburned fuelis seen as smoke at the exhaust.

A certain amount of oil is addedto the fuel for lubrication of thebearings and piston.

3.1 Designation 4. Purpose of Carburetor

WT

223

HT 7A

915RA001VA

L H

L H

C3M

VA

915RA002

HD

VA

915RA003

S20

8 Carburetors


9/65

A carburetor consists of a hollowhousing which has a specialinternal contour, i.e. the choketube (venturi) (1). Engine intakeair flows through this venturi. Thesmaller the cross section of theventuri, the greater the air flowspeed and the vacuum created.

Fuel jets open into the venturi atvarious points from which the air-stream draws fuel.

The fuel pump, nozzle or jet sys-tems and metering devices foroptimum preparation of the fuel-airmixture are integrated in ormounted on the carburetor.

The flow of air, and thus enginepower, is regulated by the throttleshutter (2).

In view of the machine applica-tions, the carburetor must operatetrouble-free in all positions. Forthis reason only all-positiondiaphragm carburetors with anintegral fuel pump are used inSTIHL outdoor power tools. Such

carburetors have no float.

The fuel pump operates as acompletely separate unit eventhough it is integrated in thecarburetor housing.

The fuel pump is controlled bythe changes of pressure in thecylinder and crankcase.

Intake process

The impulse chamber (1) is con-nected to the crankcase via theimpulse port (2).

Low pressure is created in thecrankcase by the upward stroke ofthe piston. This causes the pumpdiaphragm (3) to flex. A vacuum isalso created in the pumps fuelchamber (4).

Atmospheric pressure in the tanknow pushes fuel via the intakefitting (5) and inlet valve (6) intothe fuel chamber (4).

The outlet valve (7) is pressedagainst its seat and closes thepassage to the fuel port (8).

Note: The pump inlet and outletvalves are flaps stamped out ofthe pump diaphragm but stillattached to it at one side.

Delivery process

The downward stroke of the pistoncreates an overpressure whichpresses the pump diaphragm (3)

towards the fuel chamber.

5. Construction/ 5.1 Fuel PumpFunction

VA

915RA004

4

56

7

8

2 31

56

VA

915RA005

4

7

8

32 1

Carburetors 9


10/65

The pump diaphragm thereforereduces the volume of the fuelchamber (4).

Inlet valve (6) is now pressedagainst its seat and closes thepassage to the intake fitting (5).

The outlet valve (7) is lifted off itsseat and allows a certain amountof fuel to flow into the fuel port (8)which leads to the carburetor inletvalve.

Note: During each complete pumpstroke an amount of fuel is deliver-ed which is equivalent to thedifference in volume between thetwo end positions of the pumpdiaphragm in the fuel chamber.

On machines with a fuel tankmounted lower than the carburetorit is necessary to prime the carbu-retor with fuel after a prolongedout-of-service period by pulling thestarter rope a few times.

This problem can be overcomewith a manual fuel pump (seebelow).

Inlet needle valve

The chamber (1) above themetering diaphragm (2) is filledwith fuel through port (3).

Atmospheric pressure is applied tothe metering diaphragm throughbore (4).If the machine is equipped with acompensator, the pressure prevail-ing at the clean side of the air filteracts on the metering diaphragm

(see chapter 4.3.4).

When air flows through theventuri, fuel is drawn out of themetering diaphragm chamber, i.e.the volume of fuel is reduced. Themetering diaphragm is pushedupwards by atmospheric pressure.

The metering diaphragm (2)applies pressure to the inlet lever(5) and thus lifts the inlet needle(6) off its seat. Fuel now flows

from the pumps fuel chamber tothe metering diaphragm chamberuntil the pressure in the meteringdiaphragm chamber is approxi-mately equal to the pressureapplied to the outside of themetering diaphragm.

The metering diaphragm alwaysallows a quantity of fuel to flowwhich is equal to that which isdrawn out of the jets.

Manual fuel pump

Some carburetors (on power tools)are equipped with an additionalmanual fuel pump for priming thefuel system prior to starting.

Note: On chainsaws the manualfuel pump is a separate compo-nent, i.e. it is not integrated in thecarburetor.

When the pump bulb (1) is de-pressed, the valve (2) opens andallows air or any fuel in the bulbto flow back to the tank via thefitting (3).

A vacuum is created in the meter-ing diaphragm chamber when thepump bulb is released. This cau-ses the metering diaphragm (1) tobe pulled against the inlet controllever (2). The inlet needle (3)opens and fuel then flows from thepump chamber into the meteringdiaphragm chamber.

Excess fuel pumped into themetering diaphragm chamberpasses through the bore (4) to the

valve. The valve tabs open andfuel flows into the pump bulb.When the pump bulb is pressed,fuel is pumped back into the tankvia valve (2).

10 Carburetors


11/65

Check valve

A check valve is installed in themetering diaphragm chamber ofall carburetors equipped with anadditional manual fuel pump.

When the manual fuel pump isoperated, the check valve (1) clo-ses to prevent air being suckedinto the metering diaphragm cham-ber from the venturi and the idle

jet bores. During operation thecheck valve opens and allows fuelto flow to the main jet and the idle

jet bores.

Full load is the operating conditionwith the throttle shutter wide open.

Fuel is drawn from the main jet (1)and the idle jets (2 and 3).

Main jet (valve jet)

The main jet (1) is situated at thenarrowest point of the venturi. It isat this point that air flow speed ishighest and pressure is lowest.

The main jet is equipped with acheck valve (2) to prevent airbeing sucked into the meteringdiaphragm chamber at idle speed.

The amount of fuel drawn in by the

main jet can be adjusted with thehigh speed screw (3).

Full-load semi-fixed jet

At full load, a defined quantity offuel (approx. 80% at H = 1) is al-ways pumped from the metering di-aphragm chamber (3) via the full-load semi-fixed jet (1) and throughthe main jet (4) to the venturi, irre-

spective of the setting of the highspeed screw (2). This means thatthe change in the composition ofthe mixture for a given adjustmentof the high speed screw is far lessthan on a carburetor that has nofull-load semi-fixed jet.

The semi-fixed jet may be installedas a separate component or beintegrated as a nozzle bore in themain jet. There is no direct connec-tion between the separate semi-

fixed jet and the venturi. It is con-nected to the main jet via apassage (5).

Full-load fixed jet

On carburetors with a full-loadfixed jet (1) all the fuel required forfull-load operation flows via this

jet.

5.2 Operating Conditions5.2.1 Full Load

VA

915RA009

Carburetors 11


12/65

There is no high speed screw. Thefull-load setting cannot be altered.

Like the semi-fixed jet, the full-loadfixed jet may be installed as aseparate component or be inte-grated as a nozzle bore (2) in the

main jet. There is no direct connec-tion between the separate fixed jetand the venturi. It is connected tothe main jet via a passage (3).

The throttle shutter (1) alters thecross section of the venturi andthus regulates the flow of intakeair.

It is operated by the user via thethrottle rod.

The idle position of the throttleshutter is adjusted by means ofthe idle speed screw.The throttle shutter is almostclosed. Air flows through a narrowgap between the throttle shutterand venturi. Fuel is drawn fromthe primary idle jet (2).There is only a slight vacuum inthe area of the main jet (3) so nofuel flows at this stage.

Idle jets - bypass bores

There are two or three idle boresin the area of the throttle shutter.

When the throttle shutter is closedthere are two bores (secondaryidle or bypass) (1 and 2) in front ofit and one (primary idle) (3) behindit (viewed in the flow direction).

In this position the air flow in the

venturi is very slow and the vacu-um is slight. Fuel is drawn onlyfrom the primary idle jet at the airgap between the throttle shutterand carburetor body.

As a result of the difference inpressures (pressure in front ofthrottle shutter is close to atmos-pheric pressure), air from theventuri passes through thesecondary idle jets (bypass bores)(1 and 2) into the idle chamber (4).

This air mixes with the fuel and isdrawn out of the primary idle jet(3) as a fuel-air mixture at the lowpressure side.

In this process more air is addedto the relatively small amount ofair which flows through the smallnotch or hole in the throttle shutterand only the quantity of fuel re-quired for idling is drawn off.

When the throttle shutter isopened the vacuum acts on thearea of the secondary idle jets(1 and 2). Fuel is then drawnfrom these bores as well.

5.2.2 Idle

915RA184

4

2

BL

31

915RA185

53

BL

21

4

12 Carburetors


13/65

As the quantity of air that flowsthrough the venturi is now greater,this fuel is necessary to enrich themixture during acceleration.

The amount of fuel drawn fromidle jets (1, 2 and 3) can be in-creased or decreased by means

of the low speed screw (5) whosepointed tip varies the size of thepassage to the idle chamber (4).

Idle system independent ofmain jet system

In these carburetors the main jetand idle systems are arranged inparallel.Changes in the idle fuel volumemean a change in the full-load fuel

volume. If the idle fuel volume isaltered with the low speed screw(2), it is necessary to readjust thehigh speed screw (1).

When the idle setting is madeleaner, the overall fuel-air mixturebecomes leaner too. This in-creases engine speed as well asthe risk of engine damage as aresult of overheating.

Idle system dependent onmain jet system

In these carburetors the idlesystem branches off the full-loadsystem after the high speed screw(1). It is not directly connected tothe metering diaphragm chamber.

Full-load fuel flow is determinedby the high speed screw.

Adjusting the low speed screw (2)to change the idle fuel volumedoes not result in a change to theoverall fuel volume.

915RA185

53

BL

21

4

Carburetors 13


14/65

Idle

In the idle positon, the throttleshutter is situated between the2nd secondary idle jet (5) and theprimary idle jet (6).

The pressure in front of the throttle

shutter is close to atmosphericpressure, while the vacuumcreated by the engine prevailsbehind the throttle shutter.Vacuum acts on the idle chamber(3) via the primary idle jet (6). As aresult of the pressure differential,air flows from the venturi throughthe secondary idle jets (4 and 5)into the idle chamber (3).At the same time, fuel is drawnout of the metering diaphragmchamber (1) and into the idle

chamber (3) via the fixed idle jet(2).A primary mixture is created inthe idle chamber (3) which issucked through the primary idlebore (6) into the area at the engineside of the throttle shutter.

The idle air is drawn through anotch or a hole in the throttleshutter and mixes with the primarymixture to form the idle mixture.

The proportion of primary mixturein the idle mixture can be adjustedwith the idle screw (7) (primarymixture control).The machine will run with thebasic setting (approx. 1 turn open).The idle mixture is made richer by

opening the idle screw (7) orleaner by closing the screw.If necessary, the idle speed canbe adjusted with the idle speedscrew (LA).

Transition from idle topart load

When the throttle shutter isopened, vacuum acts on thesecondary idle jets (4 and 5).Fuel is then drawn from all three

idle jets (4, 5 and 6).

Transition from part load tofull load

The main jet begins to deliver fuelwhen the throttle shutter hasopened about 20 degrees. Theidle and full-load system deliverthe optimum amount of fuel to suitgiven opening conditions when thethrottle shutter is opened fully.

Advantages over conventionalcarburetors

The fixed idle jet (2) limits enrich-ment for acceleration and theamount of fuel that flows via theidle system during full-load opera-tion. As a result, the setting ofthe idle screw (7) has no effectat all on the volume of fuel atfull load.

Reduced sensitivityto temperature

Carburetors with fixed idle jetsensure a much steadier idlingbehavior at low temperatures:Fuel becomes viscous at lower

temperatures.

The more viscous the fuel is, thegreater the influence the shape ofthe throttle orifice has on the flowrate.Conventional adjusting screwscreate an annular gap (1) at thethrottle orifice which can becomeblocked within a relatively shorttime.Fixed jets have a round cross sec-tion (2) at the throttle orifice whichallows a constant flow of fuel evenat low temperatures. A constant

fuel flow rate is the preconditionfor a steady idling behavior.

5.2.3 Carburetor with Fixed Idle Jet and Primary Mixture Control

VA

915RA292

2

1

14 Carburetors


15/65

The graph shows the air-fuel ratio

(Lambda ) or the carbon mon-oxide content (CO) in the exhaustgas plotted against the idle RPM:

Carburetors with a balanced idlesystem (BIS) guarantee an idealidle mixture at all idle speeds.

Benefits

These carburetors arecharacterized by:- Smooth, uniform idle speed- Good response and acceleration- Lower emissions under all

operating conditions- Stable adjustment over a wide

ambient air temperature range- Only one adjusting screw

makes adjustment simpler.

Special features

A fixed idle jet and an air bypasssystem with calibrated bores aretypical features of carburetors witha balanced idle system.

The throttle shutter is completelyclosed in the idle position.

Most of the air required to form theidle mixture flows through the holein the throttle shutter.The air required to adjust the idlespeed is sucked in via the idlechamber - or air bypass port.

The amount of bypass air can beadjusted with the idle speed screw(LD screw).

Low engine idle speed,the LD screw restricts theair bypass port:

The vacuum Pu behind thecompletely closed throttleshutter (7) causes -- air to flow through the hole (8) in

the throttle shutter- vacuum P3 in idle chamber (3)

via the primary idle bore (6)- vacuum P11 in air bypass cham-

ber (11) via the bypass port (10).

If pressure P11 in air bypasschamber (11) is -- lower than the atmospheric

pressure in front of throttleshutter (7), air flows through by-pass port (9) into the air bypasschamber (11)

- greater that pressure P3 in idlechamber (3), air flows from by-pass chamber (11) through theconnecting passage (4) to theidle chamber (3).

The idle chamber (3) receives airvia the secondary idle bore (5) aswell as the connecting passage (4).At the same time, fuel is drawn outof the metering diaphragm cham-ber (1) via the fixed idle jet (2).

A primary mixture is formed in theidle chamber (3) which is suckedthrough the primary idle bore (6)into the engine side of the throttleshutter.

Primary mixture, air from the by-pass port (10) and the air whichflows through the hole (8) in thethrottle shutter mix together toform the idle mixture.

Higher engine idle speed

The LD screw is turned clockwiseto increase idle speed.

Note: The LD screw has a left-hand thread in order to retain thefamiliar directions of rotation foradjustments:- Rotating the screw clockwise

increases engine speed.- Rotating the screw counterclock-

wise reduces engine speed.

Turning the LD screw clockwisereduces the pressure P11 in theair bypass chamber (11). Thisincreases the air throughput inbypass port (10). The differencebetween P3 and P11 becomessmaller so that less air flowsthrough the connecting passage(4) to the idle chamber.

5.2.4 Carburetor with Balanced Idle System(Fixed Idle Jet and Air Bypass System)

Carburetors 15


16/65

The flow of air in connectingpassage (4) stops when P3 andP11 are equal.

Turning the LD screw furtherclockwise reverses the pressuregradient. P11 is then lower than P3so that primary mixture from theidle chamber (3) is added to thebypass air flowing through bypassports (9 and 10) via the connectingpassage (4).

Part load means all stages ofengine operation with a partlyopened throttle shutter.

Fuel is drawn from the primaryidle jet (1) and the secondaryidle jet (2).

Part-load fixed jet

When working in the part-loadrange (e.g. limbing) the throttleshutter is often in a position be-tween idle and full throttle. In thissituation the fuel-air mixture maybecome over-lean ("part-load flatspot") and there is then a dangerof seizure.

The part-load fixed jet (1), whichopens into the venturi between theidle jets and the main jet, preventsthe mixture becoming over-lean.

The low pressure causes addi-tional fuel to be drawn off andenrich the fuel-air mixture inresponse to the throttle shutterposition.

5.2.5 Part Load

16 Carburetors


17/65

Hot startCompared to engine idle RPM, thespeed achieved by cranking theengine on the starter is very low.The gas flow speed and the vacu-um in the venturi are thereforelimited. Fuel and air mix poorly in

this situation.Moreover, fuel is more sluggishthan air. This means than consider-ably more air than fuel is suckedin, the mixture becomes too leanand does not burn well.

Cold startDuring a cold start the problemsdescribed for a hot start are furtheramplified.Cold components cannot transferheat to the mixture. As a result,

the mixture gasifies poorly anda large proportion of the fuelcondenses on the walls of thecomponents it flows through.

The mixture in the carburetorhas to be highly enriched for thecold start: the choke shutter (1)is closed.

The full vacuum acts on the idlejets (3 and 4) and main jet system(2) in the metering diaphragmchamber. Fuel is drawn in and mi-xed only with the air that flowsthrough the notch or hole in thechoke shutter.

As soon as the engine has fired itis necessary to make the mixtureleaner, i.e. open the choke shutter.The heat generated by initial com-bustion evaporates part of the fuelthat had previously settled on thecomponent walls. This has anenriching effect while the engineis warming up.

Note: Once the engine is running,open the choke shutter immedi-

ately to avoid an over-rich fuel-airmixture (the engine would other-wise stall again).

Automatic choke

Some machine versions areequipped with an automaticchoke. It sets the throttle andchoke shutters to the correctpositions for an optimum mixture.Choke shutter and throttle shutter

with bellows and control valve areconnected via a lever mechanismon the carburetor.

5.3 Problematic Operating Conditions5.3.1 Starting

VA

915RA02

6

Carburetors 17


18/65

The automatic choke uses cylindertemperature as a parameter.Cross section "A" is changed as afunction of cylinder temperature.This, in turn, alters the time re-quired to vent the bellows andthus the opening speed of thechoke shutter.

The regular changes in pressurewhich occur in the crankcase areused to operate the automaticchoke.

These pressure waves are fed viaa hole in the crankcase andthrough a port in the cylinder tothe control valve (1).

The control valve directs the lowpressure waves at a predeter-mined interval to the bellows (2)which contract and thus open thechoke shutter (3).

The period for which the bellowsopen the choke shutter dependson cylinder temperature. It variesbetween 3.5 seconds on a coldengine to approx. 0.1 seconds ona hot engine.

A lever mechanism connectsthe choke shutter to the throttleshutter.

The throttle shutter (1) is at anangle of 37 degrees to thecarburetor mounting flange justbefore the choke shutter (2) opensfully. The throttle shutter returns tothe idle position when the chokeshutter is fully open.

Following a starting attempt, thebellows are filled with air from thecarburetor box and returned to theneutral position. The air requiredto pressurize the bellows is drawnin from the carburetor box via afoam filter, a polymer sinteredfilter, a hose and control valve.

The pressurizing process isaccompanied by a low whistlingnoise.

A depression is maintained inthe bellows while the engine isrunning, i.e. the bellows arecontracted; the choke shutter isopen.

VA

915RA023

VA

915RA027

37

1 2

18 Carburetors


19/65

Icing can occur in the area of theintake if humidity is high and tem-peratures are below + 10C (50F).

Cause:Air can absorb a certain amount ofmoisture. The higher the air tem-

perature the greater its capacity toabsorb moisture.

If the engine draws in damp coldair, pressure and temperaturedrop and so does the ability to ab-sorb moisture. Moisture conden-ses on the intake passages, espe-cially in the carburetor. Thisresults in ice forming, which blocksthe jets and causes engine run-ning problems.

If the air filter is dirty, the air flowrate and pressure in the venturi(Pi) drop.

Carburetor heating

Some models are equipped withan electric carburetor heatingsystem for operation in extremelycold climates, in which the intakeair from around the engine is stilltoo cold.

The heating element (1) is posi-tioned on the carburetor mountingstuds.

The thermostatic switch (1) islocated on the pump end cover.Power supply takes place automat-ically via the heating generator.

Accelerator pump

Some carburetors are equippedwith an accelerator pump. Thepiston (1) is located in a borebehind the throttle shaft (2) andis held in its neutral position by aspring (3).

When the throttle shutter isopened quickly for acceleration,the air is able to adapt rapidly tothe new flow conditions becauseof its low mass.

The reaction of the fuel, however,is sluggish by comparisonbecause it first has to be set inmotion by the vacuum. Thismeans that a certain time lapsesbefore the fuel has also adapted to

the new conditions in the venturi.A flat spot would normally occurbecause of the over-lean fuel-airmixture.

The accelerator pump offsets thiseffect.

5.3.2 Temperatures/Humidity 5.3.3 Acceleration

VA

915RA291

1

VA

915RA029

3

2

1

Carburetors 19


20/65

The accelerator pumps piston (1)is located in a bore behind thethrottle shaft. The space below thepiston is filled with fuel.

When the throttle is opened, thecontrol face on the throttle shaftpushes the piston inwards. The

fuel below the piston is theninjected into the venturi via thevalve jet (3).

This enriches the fuel-air mixtureto ensure a smooth transitionwithout a flat spot during accelera-tion. When the throttle shaft isturned back, the spring (2) returnsthe pump piston to its neutral posi-tion. A vacuum is created andfresh fuel is drawn from the meter-ing system into the space below

the piston. Compensator

1 Air filter2 Carburetor3 Valve jet4 Cylinder5 Metering diaphragm6 Inlet control valve7 Fuel pump8 Fuel tank9 CompensatorPa Atmospheric pressurePi Internal pressure

Pr Pressure in meteringchamber

The compensator connects theclean side of the air filter to thefuel metering system in thecarburetor. This means that thepressure applied to the meteringdiaphragm is identical to theinternal pressure Pi in the intake -not atmospheric pressure Pa.

In this way fuel feed is dependenton the current internal pressure Piand the amount of air or oxygenthat is actually available.

The air flow volume is reducedwhen the air filter is dirty. Thiscauses the pressure Pi at theclean side of the air filter to drop,and also the pressure appliedto the outside of the metering dia-phragm. The metering diaphragmmoves outwards and the inlet

control valve reduces the crosssection.

The flow of fuel is reduced sothat the mixture cannot becomeover-rich.

5.3.4 Dirty Air Filter/Compensator

VA

915RA030

3

2

1

7

3

1

9

2

5

6

8

4P

915RA031VA

a Pi

Pi

Pr

20 Carburetors


21/65

A carburetor equipped with a com-pensator therefore keeps the fuelcontent in the mixture constant atall times, irrespective of air filtercontamination.It is no longer necessary to alterthe setting of the high speed

screw as contamination of theair filter increases.

However, engine power drops asa result of the reduction in airvolume and can only be restoredto normal by cleaning the air filter.There is no need for any furtheradjustment after cleaning.

A conversion from a HD (textile)filter to a standard (wire mesh)filter or vice versa does not neces-

sitate readjustment of the mixtureeither.

Power curve

P = Powern = RPMa = C lean air filterb = dirty air filter,

with compensatorc = dirty air filter,

without compensator

The mixture cannot become over-rich. And it is no longer necessaryto alter the setting of the highspeed screw as contamination ofthe air filter increases. However,engine power drops due to thereduced quantity of air and fuel.Optimum power can only be resto-red by cleaning the air filter.

Control valve

Carburetors that are directlyflange-mounted to the cylinder(without a resilient manifold toinsulate vibrations) may beequipped with a control valve.

This valve (1) opens in responseto resonance at a predeterminedengine speed. Additional fuel isdrawn into the venturi through thepassage (2) and the outlet bore.The fuel-air mixture is enriched sothat the engine speed cannotincrease any further.

5.4 Special Features

915RA298VA

VA

915RA033

2 1

Carburetors 21


22/65

Speed governor (vane)

A torsion spring (1) acts via thegovernor lever and rod to keepthe vane (2) in its neutral position(choke shutter is open). As enginespeed increases, the force of thecooling airstrean overcomes

spring tension and deflects thegovernor vane. This movement istransmitted to the choke shutter.

When a predetermined limit RPMis reached, the governor vane isdeflected to a maximum degreeand the choke shutter closes to apoint where the richer mixtureprevents any further increased inRPM.

Note: The cut-off speed can be

adjusted by changing the springspreload.

VA

915RA034

1

2

22 Carburetors


23/65

Condition Cause Remedy

Carburetor floods; Inlet needle not sealing. Remove and clean or replaceengine stalls Foreign matter in valve seat inlet needle, clean fuel tank,

or cone damaged pickup body and fuel line ifnecessary

Inlet control lever sticking on Free off inlet control leverspindle

Helical spring not located on Remove inlet control levernipple of inlet control lever and refit correctly

Perforated disc on diaphragm Fit new metering diaphragmis deformed and pressesconstantly against inlet controllever

Inlet control lever too high Set inlet control lever flush(relative to design position) with top of body or bottom

of metering chamber

Poor acceleration Idle jet "too lean" Back off low speed screwslightly

Main jet "too lean" Back off high speed screwslightly

Inlet control lever too low Set inlet control lever flush(relative to design position) with top of body or bottom

of metering chamber

Inlet needle sticking to valve Remove inlet needle, cleanseat and refit

Connecting bore to atmosphere Clean boreblocked

Diaphragm gasket leaking Fit new diaphragm gasket

Metering diaphragm damaged Fit new metering diaphragmor shrunk

6. Troubleshooting Chart

Carburetors 23


24/65

Condition Cause Remedy

Engine will not idle, Throttle shutter opened too wide Reset idle speed screwidle speed too high by idle speed screw correctly

Idle speed screw (LD) opened Adjust idle speed screw correctlytoo far

Idle speed screw (LA) opened Adjust idle speed screw correctlytoo far

Machine leaking Locate and repair leak

Engine stalls at idle speed Idle jet bores or ports Clean jet bores and portsblocked with compressed air

Idle jet "too rich" Screw down low speed screwslightly

Setting of idle speed Set idle speed screwscrew incorrect correctly- throttle shutter completely

closed

Setting of idle speed Set idle speed screwscrew (LD) incorrect correctly

Small plastic plate in valve Clean or renew valve jetjet does not close

Engine speed drops quickly Air filter plugged Clean air filterunder load - low power

Tank vent faulty Clean or replace tank ventif necessary

Leak in fuel line between Seal or renew connectionstank and fuel pump and fuel line

Pump diaphragm damaged Fit new pump diaphragmor fatigued

Main jet bores or ports Clean bores and portsblocked

Fuel pickup body dirty Clean pickup body, fit newfilter

Fuel strainers dirty Clean fuel strainers

24 Carburetors


25/65

The carburetor can be testedfor leaks with the carburetorand crankcase tester (1)1106 850 2905.

- Check the tester for leaks.

The following test values refer

to carburetors that have beenflushed with fuel and havemoist diaphragms. Dry carbure-tors must not be subjected topressures of more than 0.35 bar(5 psi).

Flush dry carburetors with fuelbefore starting the test.

Carburetors can only seal properlywhen the fuel passages and allsealing points are wetted with fuel.

The pressure applied duringthe test must not exceed 1 bar(14.5 psi).

If higher pressures are applied,the control valve will open andallow the pressure to enter themetering chamber and possiblyover-stretch the meteringdiaphragm.

Push fuel line (2) 1110 141 8600

with nipple (3) 0000 855 9200onto the carburetors intakefitting.

Connect testers pressure hoseto nipple.

Close vent screw (4) on therubber bulb (5) and pump air intothe carburetor until the pressuregauge shows a reading ofapprox. 0.8 bar (11.6 psi).

If this pressure remains constant,the carburetor is airtight. However,if it drops, the likely causes are:

The inlet needle is not sealing(foreign matter in valve seat orsealing cone of inlet needle is

damaged or inlet control leversticking).

Check the metering diaphragm fordistortion and replace if necessary.

In these cases the carburetor mustbe serviced.

- After completing the test, openthe vent screw and remove thefuel line from the intake fitting.

Carburetor problems are oftencaused by blocked jets, bores andfuel strainers. Even water droplets,which get into the carburetor withthe fuel, can cause trouble. Owingto its high surface tension, waterrestricts the flow of fuel in the fine

jet bores.

Water droplets may freeze inwinter and completely block fuelpassages.

To clean the carburetor, strip itdown and remove the adjustingscrews, jets and inlet needle. Usea brush or a clean leather clothand fresh fuel to clean the carbure-tor body. Then blow out all portsand jets with compressed air.

Warning: Never use needles,wires or similar tools or fibrouscloths for cleaning the carburetoror jets.

7. Servicing7.1 Leakage Test 7.2 Cleaning

Carburetors 25


26/65

Take out the screws.

Remove the end cover.

Remove the gasket (1) andpump diaphragm (2) from theend cover or carburetor body.

Note: If the gasket and diaphragmare stuck, remove them carefully.

Carefully separate thediaphragm and gasket.

- Inspect diaphragm and gasketand replace if necessary.

Note: The diaphragm and the inletand outlet valves are exposed tocontinuous alternating stressesand the material eventually shows

signs of fatigue. The diaphragmdistorts and swells and has to bereplaced.

Place the gasket (1) on the endcover (2).

Wet the pump diaphragm (3)with fuel and place it on thegasket.

- Fit the end cover on thecarburetor body.

Note: Pump diaphragm, gasketand end cover are held in positionby the integrally cast pegs on the

end cover.

- Insert screw(s) and tighten downsecurely.


- Remove the end cover.

- For carburetors with manual fuelpump see 7.9.

Remove metering diaphragmand gasket from carburetor bodyor end cover.



7.3 Pump Diaphragm 7.4 Metering Diaphragm

VA

915RA039

3

1

2

VA

915RA036

VA

915RA037

21

VA

915RA038

26 Carburetors


27/65


Note: The diaphragm is exposedto continuous alternating stressesand the material eventually showssigns of fatigue. The diaphragm

distorts and swells and has to bereplaced.

Place gasket on carburetor body.

Place metering diaphragm ongasket.

Note: Gasket and metering dia-phragm are held in position by the

integrally cast pegs.

Fit pump diaphragm so thatperforated plate (1) pointstowards the inlet controllever (2).

If the peg (1) is on the end cover,place the metering diaphragm(2) and gasket (3) on the endcover.

- Fit end cover in position.

Carburetors with compensator: Install end cover with connector.

Fit screws and tighten downsecurely.

- Remove metering diaphragm -see 7.4.

Take out the screw.

- Remove inlet control lever withspindle or plate with inlet controllever and gasket. Pull out the

inlet needle.

If there is an annular indentationon the sealing cone of the inletneedle, it will be necessary toreplace the inlet needle becauseit will no longer seal properly.

7.5 Inlet Needle

VA

915RA045

1

2

VA

915RA048

VA

915RA046

2

1

3

VA

915RA047

VA

915RA043

VA

915RA044

Carburetors 27


28/65

Fit the inlet needle (1).

Fit helical spring (2) in the blindhole.

Insert spindle (3) in the inletcontrol lever (4).

- Engage clevis of inlet controllever in annular groove on thehead of the inlet needle.

- Press down the inlet controllever and secure it with thescrew.

Note: Make sure that the helicalspring locates on the control lever-s nipple.

- Check easy action of the inlet

control lever.

Important: The top of the inletcontrol lever must be level with thebottom of the metering chamber(1) or the top face of the carbure-tor body (2).

- If this is not the case, the inletcontrol lever is bent and must bereplaced.

- Install metering diaphragm -

see 7.4.

If the plate of the check valve inthe main jet no longer movesfreely, install a new main jet.


Use a suitable pin punch topress the main jet out of its seatin the direction of the venturi.

WS-26A carburetor:Use a screwdriver to push the

valve jet out of its seat frominside the venturi.

- Wash the main jet in white spirit.

Install the main jet exactlyvertical in the bore. Press ithome until it is flush with thebottom of the metering chamber.

7.6 Main Jet

VA

915RA058

VA

915RA05

6

VA

915RA05

4

1 2

VA

915RA057

VA

915RA051

2

1

3

4

28 Carburetors


29/65


Note: The part-load fixed jet (1)cannot be replaced, it can only beblown clear with compressed air.

Use a pin punch to press the full-

load fixed jet (2) out of its seat inthe direction of the venturi.

HS-262B carburetor: Unscrew the full-load fixed jet.

HD-5 carburetor: Use a scriber to remove the

retaining ring (1).

Tap the carburetor body againsta wooden base to remove thefull-load fixed jet (2) and 0-ring(3).

- Wash full-load fixed jet in whitespirit and blow out with compres-sed air.

Note: Replace damaged full-loadfixed jet.

- Install full-load fixed jet exactlyvertical in the bore. Press ithome until it is flush with thebottom of the metering chamber.

HD-5 carburetor:- Fit a new 0-ring in the bore.

- Press home the fixed jet (flatside up) as far as stop andsecure in position with retainingring.

- Install metering diaphragm -see 7.4.

WT-227E and WT-264carburetors:- Remove the manual fuel pump -

see 7.9.

Unscrew the control valve (1).

Take the copper washer (2) out

of the bore (3).

- Fit a new copper washer in thebore for the control valve.

- Screw in new control valve about2 full turns.

- Coat the thread that still projectsfrom the carburetor body withLoctite - see 8.2.

- Carefully screw home control

valve as far as stop and tightendown to 4 Nm (3.0 lbf.ft).

7.7 Full-Load Fixed Jet/ 7.8 Control ValvePart-Load Fixed Jet

VA

915RA060

2

1

VA

915RA062

3

2

1

VA

915RA063

123

VA

915RA061

Carburetors 29


30/65

Take out screws (1).

Remove end cover with pumpbulb (2).

Remove the pump bulb.

Remove the flange.

Take valve out of flange.

- Check valve and replace ifnecessary.

Note: If the circuit for filling themetering chamber is malfunction-ing, if the valve is dirty, distorted or

damaged, install a new valve.

Check metering diaphragm andsealing ring and replace ifnecessary - see 7.4.

Assemble in the reverse sequence.


Fit a 2.5x13 mm self-tappingscrew in the check valves boreand use it to pull out the checkvalve.

Use a 4 mm (approx. 5/32") pinpunch to press the check valvesquarely into the meteringchamber as far as stop.

- Install metering diaphragm -see 7.4.

7.9 Manual Fuel Pump 7.10 Check Valve

VA

915RA064

2

1

1

VA

915RA067

VA

915RA069

VA

915RA065

VA

915RA068

VA

915RA070

VA

915RA066

30 Carburetors


31/65

- Remove pump diaphragm -see7.3.

- Remove inlet needle - see 7.5.

- Remove valve jet - see 7.6.

- Remove full-load fixed jet, iffitted - see 7.7.

- Remove check valve, if fitted -see 7.10.

Use a scriber to pry out fuelstrainer and then clean it.

Important: Always replace adamaged fuel strainer.

Take out the high speed screw(1), low speed screw (2) and, iffitted, the idle speed screw (3).

Caution: Idle speed screw (LD)has a left-hand thread.

Use a pin punch to press thecenter of the sealing plug (1) orsealing plate (2) until it bucklesand can be taken out.

Important: The sealing plug orsealing plate is destroyed duringremoval.

- Wash the carburetor body infresh white spirit and blow allports and bores clear withcompressed air.

- To replace the carburetor body,remove the throttle shaft and, if

fitted, the choke shaft - see 7.12and 7.13.

- Place the new sealing plug orplate in the bore with its convexside facing up.

- Use a large pin punch to applylight pressure until the sealingplug or plate is flat.

- Secure new sealing plate orplug with Loctite - see 8.2.

(Apply Loctite to gap betweencarburetor body and sealingplate or plug.)

Assemble all other parts in thereverse sequence.

7.11 Carburetor Body

VA

915RA071

VA

915RA074

2 1

3

VA

915RA075

Carburetors 31


32/65

- For carburetors with automaticchoke see 7.15.1.

Take out the screw.

- Remove the throttle shutter orpull it out of the throttle shaft.

On C1Q-S11G, detach torsionspring (1) from groove in throttleshaft (2).

Remove the E-clip.

- Take off the washer, if fitted.

On some carburetors, removethe screw from the lever.

- Remove the lever.

Remove spacer sleeve.

On some carburetors, take outthe screw (1).

Pull the clip (2) off the throttleshaft.

Carefully withdraw the throttleshaft.

- Check for excessive play whichmay be a source of secondaryair.

- For carburetors with accelerator

pump see 7.14.

- On some carburetors it isnecessary to remove the pumpend cover before withdrawingthe throttle shaft - see 7.3.

Pull off the torsion spring.

7.12 Throttle S haft

VA

915RA077

VA

915RA080

VA

915RA078

2

1

VA

915RA081

VA

915RA079

VA

915RA085

2

1

VA

915RA086

VA

915RA087

32 Carburetors


33/65

On throttle shafts with a slottedpin (1), remove the E-clip (2).

Take slotted pin out of thelever (3).

After pushing home the throttleshaft, check that torsion springis correctly positioned.

Note: The illustration shows theWT-227F carburetor. The positionof the torsion springs on othercarburetors is similar.

- Install throttle shutter so that itsentire circumference locatesagainst the wall of the venturi.

- Coat screw with Loctite, see 8.2,and tighten down securely.

- Fit E-clip or clip in the groove.

- Secure clip with screw.

On C1Q-S11G, attach torsionspring (1) to groove in throttleshaft (2).

On WT-230, push spacer sleeve(1) into position.

Fit lever (2) so that the slottedpin (3) points towards meteringchamber end cover (4).

On HD-3A, fit the spacersleeve (1).

Fit lever (2) so that slotted pin (3)points towards intake fitting (4).

On WT-253, fit lever (1) so that itpoints towards intake fitting (2).

On C1Q-SK6, fit lever (1) so thatslotted pin (2) points towards themetering chamber end cover (3).

VA

915RA088

22

1

3

VA

915RA090

2

1

VA

915RA089

4

3

2

915RA091VA

1

VA

915RA092

1

23

4

VA

915RA093

2

1

VA

915RA094

1

2

3

Carburetors 33


34/65

- For carburetors with automaticchoke see 7.15.1.

Pull off the lever, if fitted.

On C1Q-S11G, detach torsionspring (1) from groove in throttleshaft (2).

Take out the screw.

- Remove the choke shutter or pullit out of the choke shaft.

Remove the E-clip, if fitted.

Withdraw the choke shaft withtorsion spring.

Carefully withdraw choke shaftwithout torsion spring so thatthe ball does not pop out and belost.

- Take out the ball and spring.

Remove the torsion spring.

- Fit the torsion spring.

After fitting the choke shaft,check that the torsion spring iscorrectly positioned.

7.13 Choke Shaft

VA

915RA099

VA

915RA078

2

1

VA

915RA101

915RA102VA

VA

915RA103

VA

915RA104

VA

915RA105

VA

915RA106

34 Carburetors


35/65

On choke shafts without torsionspring, fit the spring (1) and ball(2).

Push home the choke shaft untilthe ball engages the groove (3).

- Install choke shutter so that its

entire circumference locatesagainst the wall of the venturi.

- Fit screw with Loctite, see 8.2,and tighten down securely.

- On choke shafts with torsionspring, fit the E-clip.

On C1Q-S11G, attach torsionspring (1) to groove in throttleshaft (2).

- Refit the lever, if removed.

- Remove the throttle shaft -see7.12.

Knock the pump piston (1) andspring (2) out of the bore (3).

Take the sealing ring (4) off thepump piston.

- Inspect pump piston and springand replace if necessary.

Fit new sealing ring (4) in grooveof pump piston (1).

Place spring (2) in the bore (3).

Fit pump piston (1) in the bore,closed end facing outward.

- Install throttle shaft - see 7.12.

- Press down the pump piston sothat throttle shaft can be pushedhome.

VA

915RA090

2

1

7.14 Accelerator Pump

3

VA

915RA107

3

11

2 2

VA

915RA109

3

2

4

1

Carburetors 35


36/65

Removal:

Carefully pry bell crank "G" andlever "H" off the shafts.

- Remove the lever system fromthe shafts.

- Take the torsion spring off thethrottle shaft.

Installation:

Note: Bell crank "G" and lever "H"have to be pressed onto theshafts. To do this, clamp the car-buretor body in position so that thepressure used to install the leversis transmitted to the carburetorbody via a short part of the shaftand the retaining ring. This pro-

cedure ensures that the shaft isnot loaded beyond its bucklingstrength.

Carefully push the link "C" intoposition to connect levers "A"and "B".

Fit the torsion spring "D" on thethrottle shaft "E" so that its bentend points away from thecarburetor.

Push lever "A" on to the throttleshaft and, at the same time,lever "B" on to the choke shaft.

Rotate lever "B" on choke shaftcounterclockwise as far as stop.

Fit the bell crank "G" (with flatface pointing to lever "B") overthe choke shaft so that the pinon lever "B" is between the armsof bell crank "G".

Rotate bell crank counterclock-wise until its right arm butts

against the pin on lever "B".

With the bell crank in this posi-tion, carefully press it on to thechoke shaft.

Caution: Take care not to bend

the choke shaft.

VA

915RA110

H

G

VA

915RA112

E D

VA

915RA11

3

A

B

VA

915RA11

1

AC

B

VA

915RA114

B

7.15 Lever Mechanism on Carburetorswith Automatic Choke

VA

915RA115

B

G

VA

915RA116

36 Carburetors


37/65

Turn the idle speed screw "LA"counterclockwise until thethrottle shaft is clear of the taperon the idle speed screw.

Open the choke shutter "I" andhold it steady in that position.

Swing lever "A" counterclock-wise as far as stop.

Important: The choke shuttermust be held open for the nexttwo operations.

Fit lever "H" on the throttle shaftand rotate it counterclockwiseuntil it butts against the pin onlever "A".

Press lever "H" on to the throttleshaft.

Caution: Take care not to bendthe throttle shaft.

Installed positions of levers:

In the idle position the chokeshutter must be closed and thethrottle shutter at an angle of 37

degrees to the carburetor moun-ting face.

- Open the choke shutter byslowly turning the right-hand endof the choke shaft. The throttleshutter must jump to the closedposition just before the chokeshutter is fully open.

Note: This is accompanied by adefinite clicking sound.

Attach bent end of torsion springto pin on lever "H".

Push the straight end of thetorsion spring between thecarburetor body and pin onlever "A" and locate it behindthe levers pin.

VA

915RA117

L A

VA

915RA119

A

H

VA

915RA118

A

I

VA

915RA120

H

VA

915RA059

37

VA

915RA121

H

VA

915RA122

A

Carburetors 37


38/65

Checking installed position oftorsion spring:

- Open throttle and choke shutterssimultaneously and hold themsteady.

The torsion spring must move

levers "A" and "B" into thepositions shown in the illustration.

- Remove lever mechanism -see7.15.

- Remove the throttle shaft -see7.12.

Note: The throttle shaft is longer

than on carburetors withoutautomatic choke.

- Remove the choke shutter -see 7.13.

- Ease the E-clip off the chokeshaft.

Pull out the choke shaft.

Remove torsion spring (1),choke lever (2) and torsionspring (3) from the chokeshaft (4).

Install in the reverse sequence.

Slip the grommet, if fitted, off thecarburetor adjusting screws.

Remove screws (1) from intakemanifold, if fitted.

Remove the intake manifold (2)and gasket (3).

- Mark position of intake fittingrelative to fuel pump body.

Take out the screw.

- Remove the end cover.

7.15.1 Throttle Shaft/ 7.16 HL-324A/HL-327D/LA-S8AChoke Shaft Carburetors

B

VA

915RA123A

VA

915RA12

4

VA

915RA125

1

23

4

VA

915RA126

VA

915RA127

3 2

1

VA

915RA128

38 Carburetors


39/65

Take out the gasket (1) and fuelstrainer (2).

- Clean the fuel strainer.


- Remove fuel pump body.

Remove pump diaphragm andgasket.

Note: If diaphragm and gasket arestuck, remove them carefully.



Note: The diaphragm and the inletand outlet valves are exposed to

continuous alternating stressesand the material eventually showssigns of fatigue. The diaphragmdistorts and swells and has to bereplaced.

Remove diaphragm cover.

Remove the metering diaphragmand gasket from the carburetorbody or diaphragm cover.




Note: The diaphragm is exposedto continuous alternating stressesand the material eventually showssigns of fatigue. The diaphragmdistorts and swells and has to bereplaced.

VA

915RA129

21

VA

915RA132

VA

915RA130

VA

915RA133

VA

915RA134

VA

915RA135

Carburetors 39


40/65

Take out the screw.

Remove inlet control lever (1)with spindle (2).

Remove the helical spring (3).

Pull out the inlet needle (4).


Use a thin-walled 8mm socketwrench to unscrew the valvebody.

- Take out the gasket.

Unscrew the high speed screw(1), low speed screw (2) and idlespeed screw (3).

Remove washer (1) and 0-ring(2) from the carburetor body orthe adjusting screws.

On HL-327D, unscrew thecontrol valve.

- Take the copper washer outof the bore.

Use a 5 mm (approx. 3/16") pinpunch to press the valve jet outof its seat in the direction of theventuri.

- Wash valve jet in white spirit.

Apply a 3 mm (1/8") pin punch tothe center of the sealing plugand press down until it bucklesand can be taken out.

VA

915RA136

VA

915RA139

VA

915RA137

2

43

1

VA

915RA140

3

1 2

VA

915RA138

1 2

VA

915RA141

VA

915RA142

VA

915RA143

VA

915RA144

40 Carburetors


41/65

- Wash the carburetor body andall serviceable parts in freshwhite spirit and blow clear withcompressed air, paying specialattention to the bores and ports.

- To replace the carburetor body,

remove the throttle shaft andchoke shaft -see 7.16.1 and 7.16.2.

- Place the new sealing plug in thebore with its convex side facingup.

- Use a 8 mm (5/16") pin punchto apply light pressure until thesealing plug is flat.

- Secure new sealing plug with

Loctite - see 8.2. (Apply Loctiteto gap between carburetor bodyand sealing plug.)

Install the valve jet exactlyvertical in the bore. Press ithome until it is flush with thebottom of the metering chamber.

Fit a new gasket (1).

Fit valve body (2) and tightendown securely.

Insert inlet needle (1).

Fit helical spring (2) in blind hole.

Fit spindle (3) in inlet controllever (4).


- Press down the inlet controllever and secure it with the

screw.

Note: Make sure that the helicalspring locates on the controllevers nipple.

- Check easy action of the inletcontrol lever.

Important: The top of the inletcontrol lever must be level with thebottom of the metering chamber.

- If this is not the case, the inletcontrol lever is bent and mustbe replaced.

Fit the gasket and meteringdiaphragm (1).

Note: They are held in position bythe integrally cast pegs (2) on thecarburetor body.

On the LA-S8A, the locatingpegs (1) are on the diaphragmcover. Therefore, the meteringdiaphragm (2) and gasket (3)should be fitted on the

diaphragm cover.

VA

915RA146

2

1

VA

915RA148

VA

915RA149

2

2

2

1

VA

915RA145

1

1

3

1 2

VA

915RA150

Carburetors 41


42/65

Fit the diaphragm cover.

Fit the gasket (1) and pumpdiaphragm (2).

Note: The gasket and pumpdiaphragm are held in position bythe integrally cast pegs on thediaphragm cover.

Fit the fuel pump body.

- Fit screws and tighten themdown alternately in a diagonalpattern.

- Fit fuel strainer and gasket in thefuel pump body.

Note: Always replace a damagedfuel strainer.

- Fit the end cover.

- Line up the intake fitting accord-ing to mark made prior to dis-assembly.

- Fit screw and tighten downsecurely.

Slip spring (1), washer (2) andnew 0-ring (3) over high speedand low speed screws.

- Carefully screw high speed andlow speed screws down ontotheir seats and then back themoff one full turn.

- Fit idle speed screw.

- On HL-327D, fit new copperwasher in bore for control valve.

- Screw in new control valve about2 full turns.

- Coat the thread that still projectsfrom the carburetor body withLoctite - see 8.2.

- Screw home control valve andtighten down to 4 Nm (3.0 lbf.ft).VA

915RA151

VA

915RA152

2

1

VA

915RA154

123

VA

915RA153

42 Carburetors


43/65

Take out the screw.

- Remove lever or throttle lever.

Take out the screw.

- Pull the throttle shutter out of thethrottle shaft.

Take out the screw (1).

Pull off the clip (2).

Remove screws (1) from cableguide (2), if fitted.

- Lift away cable guide.

Pull out the throttle shaft.

Remove the torsion spring (1).

After pushing home the throttleshaft, check that torsion springis correctly positioned.

- Install throttle shutter so that itsentire circumference locatesagainst the wall of the venturi.


- Fit clip in the groove.

- Insert screw and tighten downfirmly.

- Install cable guide, if fitted.

On HL-327D, fit lever so that theslotted pin (1) points towardsdiaphragm cover (2).


On LA-S8A and HL324A, fitthrottle lever (1) so that roundend (2) point downwards toconnecting flange (3).


7.16.1 Throttle Shaft

VA

915RA162

VA

915RA165

1

21

VA

915RA163

VA

915RA166

1

VA

915RA168

2

1

VA

915RA164

12

VA

915RA167

VA

915RA169

2

3

1

Carburetors 43


44/65

- Remove the intake manifold, iffitted - see 7.16.

Ease the E-clip (1) off thegovernor rod (2), if fitted.

- Remove the governor rod.

On LA-S8A and HL327D, takeout the screw (1).

Remove the governor lever (2).

Take out the screw.

- Pull the choke shutter out of thechoke shaft.

On HL-327D and HL-324A, takeout the screw (1).

Pull off the clip (2).

Withdraw the choke shaft.

Note: On HL-327D, carefully with-draw choke shaft so that the balldoes not pop out and be lost.

- Slide choke shaft into thecarburetor from the adjustingscrew side.

Note: On HL-327D, fit the spring(1) and ball (2).

Push home the choke shaft untilthe ball engages the groove (3).

- Fit clip in groove in choke shaft.

- Secure clip with screw.

On LA-S8A and HL327D, rotatechoke shaft so that slot (1) ishorizontal and the flat side (2)points down.

7.16.2 Choke Shaft

VA

915RA155

2

1

VA

915RA158

2

1

VA

915RA16

0

2

1

3

VA

915RA156

12

VA

915RA159

VA

915RA157

VA

915RA161

2 1

44 Carburetors


45/65

On HL-324A, rotate choke shaftso that the pin (1) points forward.

Fit the choke shutter so that thehole (2) points forward and theindentations (3) face the endcover.


- On LA-S8A and HL327D, closethe choke shutter.

Fit governor lever (1) so that thepin (2) lines up with the center ofthe high speed screw (3).

- Fit screw and tighten down

securely.

Take out the screws (1).

Remove the end cover (2) withcap.

Pull out the cap.

Remove the flange.

Take valve out of flange.

- Check valve and replace ifnecessary.

Note: If the circuit for filling themetering chamber is malfunction-

ing, if the valve is dirty, distortedor damaged, install a new valve.

Remove metering diaphragmand gasket.


VA

915RA055

2 1

3

VA

915RA192

2

1 1

11

VA

915RA191

VA

915RA193

VA

915RA171

3

12

VA

915RA194

VA

915RA196

7.17 WYL-63 / WYL-73Rotary Valve Carburetors

Carburetors 45


46/65


- Inspect diaphragm and replaceif necessary.

Note: The diaphragm is exposedto continuous alternating stresses

and the material eventually showssigns of fatigue. The diaphragmdistorts and swells and has to bereplaced.

Remove pump housing.

Remove the screw (1).

Remove inlet control lever (2)with spindle (3) and spring.

- Pull out the inlet needle.


Use a scriber to pry out fuelstrainer and then clean it.

Important: Always replace adamaged fuel strainer.

Remove gasket and pumpdiaphragm.

Note: If gasket and diaphragm arestuck, remove them carefully.


- Inspect diaphragm and replace ifnecessary.

Note: The diaphragm is exposedto continuous alternating stressesand the material eventually showssigns of fatigue. The diaphragmdistorts and swells and has to bereplaced.

VA

915RA195

VA

915R

A197

1

23

VA

915RA201

VA

915RA199

VA

915RA20

2

VA

915RA198

VA

915R

A200

46 Carburetors


47/65

Remove the spring.

Use a scriber to carefully easethe main jet out of its seat.

- Remove the sealing ring.

Take out the screws (1).

Pull out the control valve (2).

If necessary, pry off the retainingring (1) and pull out the slottedpin (2) with washer (3).

Take out the idle speedscrew (4).

- Wash the carburetor body and

all serviceable parts in freshwhite spirit and blow clear with

compressed air, paying specialattention to the bores and ports.

Install the control valve so thatthe idle speed screw (1) points tothe left - viewed from stub (2).

- Insert screws and tighten downsecurely.

Slip 0-ring (1) over the mainjet (2).

- Press main jet into carburetorseat as far as stop.

Note: Check size of main jet(number on main jet):40 = WYL-6336 = WYL-73

VA

915RA203

VA

915RA206

3

4

2

1

VA

915RA204

40

VA

915RA205

1

1

2

VA

915RA207

2

1

VA

915RA208

1

2

Carburetors 47


48/65

Fit the spring (1).

Fit a new gasket (2).

Place pump diaphragm on thegasket.

Note: Gasket and pumpdiaphragm are held in placeby the integrally cast pegs.

Fit the inlet needle (1).

Fit helical spring (2) in the blindhole.

Insert spindle (3) in the inletcontrol lever (4).


- Press down the inlet controllever and secure it with thescrew.

Note: Make sure that the helicalspring locates on the controllevers nipple.

Important: Measure distance "a"between upper edge of inlet con-trol lever and pump housing. Itmust be 1.5 mm (0.06").

- If the distance more or less thanspecified, the inlet control leveris bent and must be replaced.

Fit the pump housing.

Fit a new gasket.

Fit diaphragm on the gasketso that perforated plate pointstowards the inlet control.

2

VA

915RA209

1

VA

915RA21

3

VA

915RA210

a

915RA212VA

VA

915RA21

4

VA

915RA21

1

1

2

3

4

VA

915RA21

5

48 Carburetors


49/65

Note: Gasket und pumpdiaphragm are held in placeby the integrally cast pegs.

- Insert the valve in the flange.

Fit the flange in position.

- Fit the cap in the end cover.

Fit the end cover.

- Insert screws and tighten downsecurely.

Adjusting idle speed andidle mixture

Note: If either the carburetor orthe control valve has been re-placed, it will be necessary toadjust idle jet needle in the controlvalve.

Set the idle speed to 3100

200 rpm with the idle speedscrew (1).

Note: Use a tachometer.

Pull plug (2) out of the controlvalve.

- Start the engine.

Use a small screwdriver (3) torotate the idle jet needle (4)

clockwise or counterclockwiseuntil maximum engine speed isreached.

- From that position, turn the idlejet needle one eight turn coun-ter clockwise to reduce enginespeed by 200 - 500 rpm.

- Remove the screwdriver andonce more set idle speed to

3100 200 rpm with the idlespeed screw.

Note: Adjustment of maximumengine speed is not necessarybecause it is determined by themain jet.

- Fit plug in the control valve.

VA

915RA194

VA

915RA

216

VA

915RA217

1

4

2 3

Carburetors 49


50/65

WT-283B, WT-323, WT-325A,WT-326C, WT-360, WT-396,WT-403A, WT-493, WT-498A,WT-503

These carburetors have a fixed jetin place of a high speed adjusting

screw (H screw).

Maximum engine speed is presetand no longer adjustable. It is onlypossible to correct idling speedwithin certain limits.

These carburetors guarantee anoptimum fuel-air mixture in alloperating conditions.

Standard setting

To readjust the carburetor, startwith the standard setting.

Carefully screw down the lowspeed screw (L) clockwise until itis against its seat. Then back itoff one full turn counterclockwise.

- Check chain tension.

- Check air filter and clean ifnecessary.

- Check spark arresting screenand clean or replace if necessary.

- Start the engine and allow it towarm up.

Adjusting idle speed

A correction at the low speedscrew (L) usually necessitates achange in the setting of the idle

speed screw (LA).

Note: Turn screws very slowly andcarefully - even slight movementsproduce a noticeable change inengine running behavior.

Engine stops while idling:

- Check standard setting.

- Turn idle speed screw clockwiseuntil the chain begins to run -then turn it back one quarter turn.

Chain runs while engine isidling:

- Check standard setting.

- Turn the idle speed screwcounterclockwise until the chainstops running - and then turn itabout another quarter turn inthe same direction.

Erratic idling behavior, pooracceleration -even though low speed screw (L)is one turn open:

- Idle setting is too lean.

- Turn the low speed screw (L)counterclockwise until the engineruns and accelerates smoothly.

VA

915RA294

L

VA

915RA295

LA

7.18 Standard Setting on Walbro Carburetors7.18.1 Fixed Jet Carburetors

50 Carburetors


51/65

HD-13A, HD-16, HD-17,HD-18A, HD-21A, HD-22,WY-69, WT-329, WT-330,WT-367, WT-393, WT-489,WT-492, WJ-69

Turn the cap to line up the lugwith the slot.

Screw puller (1) 5910 890 4500into cap counterclockwise untilit comes off the the head of theadjusting screw (left-handthread).

Note: Limiter caps are damagedduring removal. They must not beused again.

- If necessary, take out the lowspeed and high speed screws.

Carefully screw high speedscrew (1) and low speed screw(2) with sealing rings (3) downonto their seats.

- Refer to flow chart for carburetoradjustment - see 7.21.

Preparations

- Check air filter and clean ifnecessary.

- Inspect spark arresting screenand clean or replace asnecessary.

On blowers:

- Fit blower tubes and nozzles.

On hedge trimmers:

- Clean and oil cutting blades.

One brushcutters:

- Mount STIHL Autocut orPolymatic cutting head.

- Adjust line length so that itextends as far as limiter blade

on deflector.

Basic setting(all models)

Starting with the adjustingscrews (H) and (L) firmly againsttheir seats, open them by thenumber of turns specified below.

Model H L

BR 320 LBR 320, 400FC 44FS 36, 40, 44

FS 72, 74, 76FS 88FS 550HS 72, 74, 76BG 72MS 044, 046MS 029, 039MS 066

1.5241.5

1

111

42.5

4

1

111

7.18.2 Carburetors with Limiter Caps

VA

915RA218

1

VA

915RA219

2 3

1

VA

915RA225

VA

915RA293

Carburetors 51


52/65

MS 029/039 and MS 044/046

Fitting limiter caps

Insert the new caps in the bores,making sure the lugs arecorrectly positioned (arrows).

Push the caps on to the adjust-ing screws until they are flushwith the insert (1).


Note: Use a tachometer.

Set idle speed to 2800 rpm(029/039) or 2500 rpm (044/046)with the idle speed screw (2).

Corrections are now possibleonly within a range of270 degrees because theintegrally molded stops in thebores limit any further adjust-

ment.

- This enables the mixture to bemade leaner, if necessary, foroperation at high altitudes andprevents it being enrichedbeyond the emission limitsspecified by EPA.

BR 320 L, BR 320, 400,FC 44, FS 36, 40, 44,FS 72, 74, 76, FS 88, FS 550,HS 72, 74, 76, BG 72


- Refer to flow chart for carburetoradjustment - see 7.21.

Use the low speed screw (L) toset maximum idle speed.

Turn low speed screw (L)counterclockwise until enginespeed drops about 300 rpm.

Use the idle speed screw (LA)

to set engine speed as specifiedbelow.

Model RPM

BR 320 LBR 320, 400FC 44FS 36, 40, 44FS 72, 74, 76FS 88FS 550HS 72, 74, 76BG 72

31003100310031002800

Full-load setting

On blowers:

Running at full load, turn highspeed screw (H) slowly clock-wise until maximum enginespeed is reached.

Turn high speed screw (H)counterclockwise until enginespeed drops about 100 rpm.

VA

915RA229

VA

915RA22

7

270 270

H L

VA

915RA228

52 Carburetors


53/65

On brushcutters(with cutting head and deflectorfitted):

Running at full load, turn highspeed screw (H) slowly clock-wise until no further increasein engine speed is achieved

within a speed range of 6500 -8500 rpm.

Note: If the required speed rangeis not reached, adjust the length ofthe nylon line until it is.

Turn high speed screw (H)counterclockwise until enginespeed drops about 100 rpm.

On edgers:

Running at full load, turn highspeed screw (H) slowly clock-wise until engine speed of9000 rpm is exceeded.

Turn high speed screw (H)counterclockwise until an enginespeed of about 8000 rpm isreached.

Turn high speed screw (H) onehalf turn clockwise (leaner).

Checking running behavior

- Open the throttle several timesto check that engine acceleratessmoothly.

- Engine must run uniformly at

idle speed.

Fitting limiter caps

Fit cap (1) on adjusting screw sothat the integrally molded lug (2)locates in the gate (3) in the bore.

- Push caps into bores until theyare flush with the carburetorbody.

Corrections are now possibleonly within a range of 90degrees because the integrallymolded stops in the bores limitany further adj

Documents

Stihl Carburetor Manual