Technical descriptionGeneral

Single carburettor (Stromberg)Twin carburettorsSingle carburettor (Pierburg)

Float systemCold-start deviceFast idlingIdling (Stromberg)Acceleration (Stromberg)Normal driving (Stromberg)Idling (Pierburg)Acceleration (Pierburg)Normal driving (Pierburg)

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Full-load operation (Pierburg)Engine overrunIdling shut-off valveAir cleanerPreheatingFuel pumpFuel tankFuel tank venting andoverfill protectionFuel supply andfuel return lineRoll-over valve

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GeneralThe engine is equipped with one or two Strom-berg horizontal carburettors.

As from 1987 models, the engine is equippedwith a Pierburg carburettor.

Single carburettor (Stromberg)The carburettor, made of light alloy metal, com-prises three main sections: the vacuumchamber, the carburettor body (casting) and thefloat chamber.The vacuum chamber, which is the top part of thecarburettor, has a diaphragm at the bottom, towhich a piston is attached, and communicateswith the inlet port of the carburettor through twodrillings in the piston.

The fuel jet is press-fitted in the carburettorbody, which is the middle section of the carburet-tor. The cross-sectional area of fuel flow insidethe jet orifice is varied by a moving taperedneedle, attached to a piston whose position isvaried by variations in the depression inside thecarburettor body. The piston also regulates thecross-sectional area of the flow of induction air.Thus, the engine always receives the correctamount of fuel and air under all load conditions.

The bottom section of the carburettor, the floatchamber, houses a float which opens and closesthe float valve by means of a tab on the float arm.

Twin carburettors

Carburettor (Stromberg)

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Damper and oil cap assemblyVacuum chamber coverSpringRetaining ring for diaphragmDiaphragmRetaining clipAdjusting screwMetering needleVacuum pistonSet screw with spring loaded plungerJetCarburettor bodyFloat valve

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Float and armFloat chamberThrottle cam leverDeceleration valve (not Sweden spec.)Temperature compensatorCold start device with cam leverArm, float chamber ventilationIdling adjusting screwAdjusting screw, float chamber ventilationSpindleSpacer (single carburettors only)CO adjusting screwDeceleration valve (1985 models onwards)

Twin carburettorsThe carburettors used on twin-carburettor en-gines are similar in principle to those on single-carburettor engines.

Located in front of the two carburettors is an airbox, which serves both carburettors and is con-nected to the air cleaner by means of a hose. Theinlet manifold passages from the rear carburet-tor go to number 1 and 2 cylinders and from thefront carburettor to cylinders number 3 and 4. Aconnecting passage links the two manifolds andthis serves to correct any minor variations in thefuel/air mixture from the two carburettors.

Twin carburettors

Pierburg 175 CDUS carburettorThe 175 CDUS carburettor is a horizontal, con-stant-depression (CD) carburettor with a mixingchamber diameter of 45 mm (1 3/4").

In a constant-depression (CD) carburettor, thevacuum is the mixing chamber does not vary, i.e.the pressure of the air stream is practically con-stant, regardless of the engine speed or load.

The piston and thus the jet are controlled by avacuum, which varies with the throttle opening,the engine speed and the engine load. The in-teraction of these factors and the atomization ofthe fuel (due to the nearly constant vacuum re-sulting from a constant air speed at the jet) corn-prise the basic principle of a constant-depres-sion (CD) carburettor The combination of a vari-able jet system and an air-controlled vacuumpiston provides step-less adjustment of the fuel/air mixture supplied to the engine, from idling tofull load.

,4s from 1987 models, the engine is equippedwith a Pierburg carburettor.

Because of the central location of the jet system,the carburettor can be installed horizontally, orinclined at the angle of up to 26°.

The fuel supply to the jet is regulated by thedouble float, and is kept at a constant level, sothat centrifugal, braking and acceleration forceswill not affect the fuel/air mixture.

The CDUS carburettor is equipped with a chokedisc controlled by a manual choke control, a de-celeration valve (overrun braking valve), whichreduces exhaust emissions during overrun brak-ing and gear-changing and a device which allowsthe idling mixture to bypass the throttle.

At idling speed, the mixture bypasses the almostclosed throttle, via the idling shut-off valve, andflows direct to the inlet manifold. This provides ahigher effective vacuum for delivery of the mix-ture and thus guarantees smooth idling.

Another feature of this carburettor is the temper-ature compensation function of the main jet. Thejet holder contains bimetal washers, whichmove the jet axially when the temperaturechanges. This, in turn, changes the flow area ofthe jet. The fuel flow is thus adjusted to suit theprevailing operating temperature.

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Carburettor bodyFloat chamber coverCarburettor top coverChoke discCoverOil filler plugThrottle spindleThrottle butterfly

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Throttle leverChoke leverChoke cable guideDeceleration valveCO adjusting screwPlug (connection for idling speed adjustmentvalve for cars equipped with AC)Spigot for vacuum line to distributor

Carburettor designThe carburettor consists of four main parts, heldtogether by screws:

Carburettor bodyFloat chamber coverCarburettor top coverChoke mechanism

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Carburettor bodyFloat chamber coverCarburettor top coverChoke discCoverDamper piston and oil cap assemblyPiston diaphragmDamper pistonVacuum pistonVacuum piston springThrottle spindleThrottle butterflyThrottle leverChoke leverFast idling adjusting screwChoke cable guide

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Modulator valve (lean-mixture valve)FloatFloat valveJetJet holderNeedleBimetal washersSpringsIdling by-pass passageDeceleration valveCO adjusting screwDamper oilPlug (connection for idling speed adjustmentvalve for cars equipped with AC)Spigot for vacuum line to distributor

Auxiliary devicesShut-off valve for idling mixture

The induction system incorporates a shut-offvalve for the idling mixture to prevent the enginerunning on after the Ignition has been switchedoff. The valve, a solenoid valve, fitted in theunderside of the inlet manifold, opens when theignition is switched on.

Idling control valve

On cars with AC, the carburettor is equipped withan idling control valve instead of plug 29. Thisvalve opens when the AC compressor cuts in,thereby enriching the mixture and preventing adecrease in the engine speed.

Float systemStrombergFuel enters the float chamber through the floatvalve. The float, which is double, is mounted onthe float chamber by an arm and spindle, whichfits into two retaining clips. As the fuel levelrises, the float rises with it, and when the correctlevel is reached, the float valve is closed by atongue on the float arm. Fuel is also drawn Intothe jet, where the level will be the same as in thefloat chamber (engine at standstill).

Float chamber ventilation, twin carburettors1 Throttle stop and fast idle lever2 Throttle relay lever and idle adjust screw3 Ventilation valve4 Ventilation outlet, throttle shut5 Ventilation outlet, throttle open

The carburettors are fitted with a special floatchamber vent valve. When the throttle valve isclosed, a iris expelled directly through a vent holein the carburettor. When the throttle valve isopened, ventilation of the float chamber will beby means of the air cleaner connection.

PierburgThe float system controls the fuel flow by meansof the float, which operates the float valve via thefloat arm, thereby keeping the fuel level in thefloat chamber constant under all conditions.

The fuel supplied by the fuel pump flows throughthe supply tube and the open float valve in thefloat chamber. As the fuel level rises, the floatalso rises, pressing the float valve needleagainst its seat. When the preset fuel level hasbeen reached, the float valve closes and will notreopen until the fuel level in the float chamberhas again fallen.

When the engine is running, the float chamber isvented via the valve in the chamber to the air fil-ter (internal venting).

With the engine at a standstill, venting is via thevalve in the float chamber to the engine compart-ment {external venting).

Float chamber ventilation1 Ventilation through air cleaner2 Direct atmospheric ventilation

The valve is located in the air inlet flangeWhen the engine is running, this valve is closed,and the internal vent passage open. When the ig-nition is switched off and the engine has stop-ped, the electrical supply to the valve is cut off,causing the passage for internal venting to closeand the passage for external venting to open.

Float chamber vent valve

Cold-start device (choke)The carburettor is equipped with a cold-start de-vice to assist starting and running the enginefrom cold.

As the engine warms up, the choke control ispushed in to maintain the correct fuel/air mixtureas the fast-idling speed decreases.

StrombergWhen the choke control is pulled out, a disc (4) isrotated and fuel flows from the float chamberthrough one, two, three or four of the holes in thedisc. The fuel flow through the disc is determinedby the number of the holes that are not blankedoff. Additional air to the disc is drawn in throughair jet (3), to form an emulsion with the fuel. Theadditional fuel/air mixture then flows into themixing chamber through passage (6a).(Refer to Fig. A.)

To enable the engine to run smoothly under alldriving conditions, the quantity of choke fuel isoptimized to meet the requirement during accel-eration or at full throttle (Fig. A). This quantity offuel is much greater than that needed when thecar is travelling at a constant speed.

A leaner mixture is obtained as follows (Fig. B).

At constant speed (constant throttle opening), adepression is present in the inlet manifold. Viaconnecting passage (8) the depression acts onthe diaphragm (1), once the force of the spring(2) has been overcome, allowing air to be drawnthrough passage (9) into the fuel inlet passage(5). The fuel/air mixture flows through the disc(4), where additional air is drawn in through airjet (3). This lean mixture (6b) then flows into themixing chamber, providing additional fuel/air.

On renewed acceleration (opening of the throt-tle), the mixture is automatically enriched (load-sensing choke), since there is a reduction in thedepression in the inlet manifold, which allowsthe spring loading on the diaphragm to close theair-bleed port.

Cold-start device (Stromberg)

Operating principle of the cold-start device(Stromberg)

B

At constant speed

A

On starting, during acceleration and at fullthrottle

= Fuel

= Air (at atmospheric pressure)

— Strong depression

- Weak depression

= Fuel/air mixture

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DiaphragmDiaphragm springAir jetDiscFuel inlet passage (from float chamber)a Fuel/air passage (rich mixture to mixing

chamber)b Fuel/air passage (lean mixture to mixing

chamber)

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Air inlet from atmosphere (as from 1984 models)Passage to inlet manifoldAir-bleed passageAir-bleed passage inlet from atmosphereFast-idling cam

PierburgThe choke disc, which is controlled by the chokecontrol, permits reliable starting and warm-up ofthe engine, regardless of the ambient tempera-ture. When a car is started from cold, a rich fuel/air mixture is required at first. When the chokecontrol is withdrawn, the throttle (12) is partiallyopened (fast idling) and the choke disc (4) isturned to a position in which the fuel aperturesare opened.

4 Choke disc12 Throttle butterfly17 Modulator (lean-mixture) valve

During starting, fuel is metered from the floatchamber (A) via the fuel apertures in the chokedisc (4).

Cold startingA Fuel supply from float chamber

Immediately after the engine has started, themodulator valve (lean-mixture valve) (17) opensin the inlet manifold. Thus, the air required for aleaner mixture flows through a passage (B), fromthe clean-air side of the air cleaner.

Warm startingB Air supply

Fast idlingWhen the choke control is pulled out, a cam,which acts on the throttle, is rotated. The furtherthe choke control is withdrawn, the faster will bethe fast idling speed.

Idling (Stromberg)The carburettor does not have a separate idlingsystem. At idling speed there is a depression inthe vacuum chamber. In this position thethickest section of the tapered metering needleis in the jet orifice and only a small quantity offuel, sufficient for idling, is inducted into thecylinders. The air/fuel mixture should be setwhile the engine is idling by adjusting the re-lationship between the metering needle and thejet aperture. This is effected by altering theorifice adjusting screw which raises or lowers thetapered metering needle and it is this settingwhich then covers the entire range of enginespeeds. The idling speed is changed by adjust-ing the setting of the throttle stop screw (idle-ad-just screw). On twin-carburettor engines thethrottles can be synchronized by adjusting theclamping bolt on the coupling assembly betweenthe two throttle spindles. The carburettor isequipped with a temperature compensator tomaintain a constant fuel/air mixture regardlessof engine temperature. The temperature com-pensator consists of an atmospheric valve con-trolled by a bi-metallic strip. The valve starts toopen when the temperature of the air at thetemperature compensator reaches 68°F (20°C)approx. Additional air is introduced through avent which discharges behind the air valve shaft.

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Damper piston and oil cap assemblyDiaphragmCompensating apertureDamper pistonFloat chamber vent.Float valveFloatVacuum pistonStarting-fuel apertureThrottle butterflyVacuum apertureNeedleJet orificeFloat chamber

15 Constant-depression chamber (CD chamber)16 Aperture for additional air (temperature compen-

sation)

Acceleration (Stromberg)A damper piston is provided in the centre of theair valve in order to provide a richer mixture whenthe throttle is opened rapidly (acceleration). Thedamper consists of a piston, which runs in oil, at-tached to a rod. When the throttle is openedquickly the depression in the vacuum chamberincreases rapidly. When the air valve rises thepiston damper is forced against its seat prevent-ing the oil from flowing past which retards themovement of the air valve. This causes a tempor-ary increase in the depression above the jetorifice and the air/fuel mixture is enriched. Thedownward movement of the air valve is spring-assisted.

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Damper piston and oil cap assemblyDiaphragmCompensating apertureDamper pistonFloat chamber vent.ValveFloatVacuum pistonStarting-fuel apertureThrottle butterflyVacuum apertureJet orificeNeedleFloat chamberConstant-depression chamber (CD chamber)Aperture for additional air (temperature compen-sation)

Normal driving (Stromberg)When the throttle is opened, the pressure in thevacuum chamber, which is in communicationwith the top of the diaphragm, falls, causing thepiston to rise to a new position, stabilizing thedepression in the vacuum chamber. As theneedle rises with the piston, the flow of fuel isadjusted to the flow of air.

Idling (Pierburg)This carburettor is not equipped with a separateidling system. Instead, the idling mixture de-pends on the position of the throttle butterfly(12), the vacuum piston (9) and the taper needle(22) in the jet, which controls the fuel flow.

Turning the adjusting screw (27), which is in con-tact with the jet holder (21), alters the position ofthe jet (20), thus also changing the annular gapbetween the jet and the needle.

When the adjusting screw is turned clockwise,the annular gap is reduced, thus providing aleaner idling mixture. Turning the screw anti-clockwise has the opposite effect.

When the engine is running, the resultingvacuum causes fuel to be drawn into the mixingchamber, where it is mixed with the air flowingpast the top of the jet.

The carburettor is equipped with a by-pass de-vice for the idling mixture. Most of the idling mix-ture flows from the mixing chamber, past thethrottle which is set to a minimum opening andinto the inlet manifold. The rest of the idling mix-ture flows through the throttle gap, into the en-gine.

The bimetal washers (23), located below the jet,contract when the temperature of the fuel andthe surrounding metal is low.

As the temperature increases, the bimetalwashers expand axially in the jet holder, causingthe annular gap between the jet and the taperneedle to decrease. This results in a leaner mix-ture at higher temperatures and a richer mixtureat lower temperatures.

This temperature compensation helps to ensurethat the optimum fuel/air ratio will be main-tained, regardless of the viscosity of the fuel.

Idling

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Vacuum pistonThrottle butterflyFloat valveJetJet holderNeedleBimetal washersAdjusting screw

Acceleration (Pierburg)When the accelerator is suddenly depressed, aricher fuel/air mixture is required briefly. Thistemporary enrichment of the mixture is achievedby means of vacuum piston spring (10) anddamper piston (8) in the damper oil (28).

When the throttle is suddenly opened, thedamper piston prevents an immediate upwardmovement of the vacuum piston (9). The vacuumof the jet (20) briefly increases, thus enrichingthe fuel mixture.

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Carburettor coverDiaphragmDamper pistonVacuum pistonVacuum piston springJetDamper oil

Normal driving (Pierburg)When the throttle is opened the vacuum acts ona chamber in the carburettor cover (3), throughthe compensating passages in the bottom of thevacuum piston, and thus also on the diaphragm(7) on the vacuum piston (9). Due to the differ-ence between the vacuum in the chamber andthe atmospheric pressure on the underside ofthe diaphragm, the piston is raised by an amountproportional to the air flowing past the throttle,and the cross-sectional area of the intake is in-creased. The air velocity and the vacuum at thejet (20) thus remain practically constant, andprovide reliable atomization of the fuel at all en-gine speeds.

Full-load operation (Pierburg)The more air drawn through the carburettor, thehigher position of the vacuum piston (9) and theneedle (22). The piston reaches its highestposition at full load and maximum engine speedand the proportional increase of the annular gapat the jet (20) matches the fuel supply to theamount of air being drawn in. The shape of theneedle ensures that the fuel/air ration is step-lessly adjusted to the correct value under alloperating conditions.

Full-load operation9 Vacuum piston

20 Jet22 Needle

Engine overrunSweden specificationA mechanical dashpot delays the closure of thethrottle when the accelerator is released.

Sweden specification 1984 and earlier models

Europe specification (1984 and earliermodels)The carburettor is equipped with a diaphragmvalve which is affected by the manifold depres-sion during overrun, thereby opening the throttleby-pass and permitting the correct air/fuel mix-ture to reach the engine.

Deceleration valve, normal driving

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Adjusting screwRubber ringCoverNutSpringPassage to diaphragm upper side

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DiaphragmValveThrottleAir/fuel mixture inlet passageAir/fuel mixture outlet passage

1985 models onwardsA poppet valve is incorporated in the throttle but-terfly. On engine overrun, the valve opens a portin the butterfly, allowing the correct mixture offuel and air to be admitted to the engine.

Deceleration valve, normal driving

Deceleration valve, engine overrun1234

Poppet valveSpring seatSpringSpring seat

Idling shut-off valveCarburettor engines are equipped with an idlingshut-off valve to eliminate the engine running onafter the ignition has been switched off.

Single carburettor engines obtain their air/fuelmixture at idling speed through a small aperturein the throttle butterfly and through a throttle by-pass passage. When the ignition is turned off theby-pass passage is blocked by a spring-loadedsolenoid which is then deprived of its electriccurrent. The air/fuel mixture can now only passthrough the aperture in the throttle butterfly,which is insufficient to keep the engine running,which therefore stops.

Twin carburettor engines incorporate a solenoidvalve which regulates communication betweenthe section of the float chamber above the fuellevel and the constant-depression (CD)chamber. When the ignition is switched off, atime relay closes a circuit which causes the sol-enoid valve to open the connection to the floatchamber, giving rise to a depression above thefuel level. This eliminates the pressure differ-ence, needed for fuel to be drawn through theneedle valve, and the engine therefore stops.Current to the solenoid flows for a limited timeonly; after six seconds, therefore, with the en-gine-idle, the relay will be de-energized.

Idling shut-off valve, single carburettor

Idling shut-off valve, twin-carburettors

Stromberg carburettors (as from 1984models)Preheated air for cold-start device (choke)

As from 1984 models, the induction air for thecold-start device is preheated. The preheated airenters the air intake hose to the carburettor andflows through a hose to the cold-start device.

When the air is preheated, condensation is re-duced, with a consequent reduction in the risk ofmoisture freezing in the system.

Outlet for EGR valveIn conjunction with the introduction of the EGRsystem, the carburettor has been equipped withtwo vacuum outlets. The outlet marked 'E' is con-nected to the pressure upstream of the throttlebutterfly. This outlet is for control of the EGRvalve. The other outlet is in communication withthe pressure downstream of the throttle butterflyand is connected to the vacuum control unit onthe distributor.

Stromberg1 Outlet for EGR valve2 Vacuum control unit (distributor)

Pierburg1 Outlet for EGR valve2 Vacuum control unit (distributor)

Air cleanerThe air cleaner is positioned at the front of theleft wheel housing and is connected to the car-burettor by means of a hose. Its purpose istwofold: to clean the air inducted into the engineand to reduce the noise caused by the inductionsystem. The air cleaner element, which is madeof a special grade of paper, must not be washedor wetted, but should be replaced at thespecified service interval.

Air preheatingA thermostatic valve, situated in the air cleanerintake, regulates the temperature of the induc-tion air.

There are two air intakes in the valve housing:one for cold air and one for heated air. Theheated air is drawn in through an insulated hosefrom a hot spot on the exhaust manifold.

On 1985 and earlier carburettor engines, thevalve is activated by a thermostat in front of thecarburettor. The thermostat senses the tempera-ture of the pre-mixed induction air and maintainsit at 23-37°C (73-98°F) by means of a cable. Inoperation the valve therefore alternates betweenthe non-preheated and preheated position.

As from 1986 models, the valve butterfly in car-burettor engines is controlled not only by a ther-mostat but also by a bimetallic strip in the air in-duction hose upstream of the carburettor. Thebimetallic strip senses the temperature of the in-duction air and uses the depression in the inletmanifold to operate the valve butterfly. When theengine is under full load, and the depression inthe inlet manifold is weaker, the butterfly is con-trolled by the thermostat. This system ensuresthat the induction air to the engine is always atthe correct temperature of 25 ± 5°C {77 ± 15°F).

Fuel pumpThe fuel pump is a diaphragm pump, driven by apush-rod from an eccentric on the camshaft.Apart from the filter, which can be removed forcleaning (up to engine No. D 052892), the pumpcannot be dismantled for overhaul or repair.

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Valve housingCold-air intakePreheated air intakeValve butterflyThermostat bodyBimetallic stripInlet manifoldCarburettorCrankcase ventilation

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Fuel pump bodyAdaptor

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Push-rodSeal

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FilterCover

Fuel tankThe fuel tank, made of injection-moulded plas-tic, houses the fuel gauge sender unit, the pumpinlet line and a connection for the fuel returnline.

The tank is equipped with a breather system andoverfill protection, which allows for expansion ofthe fuel inside the tank.

Fuel tank venting and overfillprotectionWhen fuel is added to the tank air is evacuatedpartially through breather pipe 3. An air cushionis formed at the top of the tank when the level offuel reaches the lower opening of the breatherpipe (3), owing to the action of a restriction (5)positioned in the breather hose for the uppersection of the tank (4). The restriction hindersrapid changes in volume when the car is being re-fuelled but does not affect gradual changes involume occasioned by temperature changes ormovement of the car.

The tank is vented externally through the venthose (6) which runs from the filler pipe up therear corner pillar and along the roof (above theheadlining) down through the left front cornerpillar and opening into the engine compartment.In cars produced as from the latter half of the1985 model year, the vent hose is connected toa spigot on the outer end of the filler pipe.

Under normal conditions, the filler cap makes atight seal with the filler pipe. However, the capincorporates a vacuum valve which will preventthe fuel tank from collapsing as a result of thepressure difference that could arise if the venti-lation system should become blocked.

Fuel linesThe fuel system includes both plastic and rubberfuel lines. Plastic pipes are used for runs throughthe body, and rubber hoses for connections tothe fuel tank, fuel pump and carburettor.

The fuel supply and fuel return lines, which runtogether between the fuel tank and fuel pump,are routed along the rear-seat member and LHside member.

Body lead-troughs are sited to the left in thebulkhead panel and to the right in the floor-panpressing for the rear axle. The lines enter the en-gine compartment through the LH wheel-archbracket.

Return fuelSurplus fuel is returned to the fuel tank via thefuel return line, which is connected to a restric-tion-type branch connector between the fuelpump and the carburettor. As from 1986 models,the fuel return line is connected to the fuel-return outlet on the vapour trap.

Fuel tank ventilation and breather system

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Fuel tankFiller pipeBreather pipeBreather hoseRestriction

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Vent hoseFiller capVacuum valveExpansion spaceFuel return line

Roll-over valve1984 model cars onwards are equipped with aroll-over valve. The valve is connected to the venthose for the fuel tank and prevents petrol escap-ing in the event of the car being involved in a col-lision.

The valve is located on the right-hand side in theluggage compartment, mounted on the rein-forcement panels inside the rear wing.

Location of the roll-over valve

1 Fuel tank2 Roll-over valve