carburetion 1975

8/4/2019 Carburetion 1975

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MOTORCYCLECARBU ION

HONDA TECHN CALSERIESCopyright 1915 by:AMERICAN HONDA MOTOR CO., INC. MOTORCYCLE SERVICE DEPARTMENT


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FOREWORD

This manual is designed to provide motorcycle owners, students, and mechanicswith a complete understanding of the construction and operating principles ofmotorcycle carburetors. A troubleshooting chart is included for the diagnosisand correction of carburetion problems.The manual starts w ith a simplified description of the basic principles. Subse-quent sections describe specific carburetion circuitry, adding technical detail asthe reader's comprehension grows.Special care has been exercised in preparing clear, simple illustrations as an aidin visualizing the complex carburetion systems described in the text.The variety of carburetor models in use, and frequent design changes, precludethe listing of service and repair specifications. Refer to the factory shop manualfor service information on specific carburetor models.

AMERICAN HONDA MOTOR CO,. , INC.Moto rcy cle S erv ice Department


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HONDA MOTORCYCLE CARBURETION

CONTENTS

SUBJECTAIR-FUEL RATIOS

BASIC PRINCIPLES OF CARBURETOR OPERATION.Atomizer PrincipleVenturi Effect. . . . . . . . . . .Choke Effect . . . . . . . . . . .FLOAT SYSTEM. . . . . . . . . . . .Float System Components and OperationEccentric Float Chamber.Concentric Float ChamberFloat Adjustment. .THROTTLE VALVE ....Disc Throttle Valve. . .Slide Throttle Valve. . . .

Maintaining Correct: Air-Fuel Mixture RatiosThrottle Valve SynchronizationAI R-BLEEDS . . . . . . . . .Main Jet Air-Bleed . . , ..idle and Low Speed Air-Bleed.AI ti tud e Com pensator. . .IDLE AND LOW SPEED SYSTEM.Fuel Discharge OrificesIdle and Low Speed Air-Fuel Mixture AdjustmentIdle Speed Adjustment . . . . . . . . . .INTERMEDIATE SYSTEM WITH SLIDE TYPE THROTTLE VALVE

Throttle Valve Cutaway . . . . . . . . . . . . . . . .Jet Needle . . . . . . . . . . . . . . . . . . . . .INTERMEDIATE SYSTEM WITH DISC TYPE THROTTLE VALVE ANDVACUUM PISTON .Vacuum Piston ConstructionVacuum Piston OperationAcceleration. . . . . . .Jet Needle . . . . .Main Jet Fuel Circuits .INTERMEDIATE SYSTEM WITH COMBINATION SLIDE TYPETHROTTLE VALVE/V ACUUM PISTONConst ructi on

Operation. . . .HIGH SPEED SYSTEMFUEL MIXTURE CONTROL IN RELATION TO THROTTLE POSITIONCOLD STARTING SYSTEMChoke Valve. . . . . . . . . . . . .Mix tu re En richener. . . . . . . . . .CARBURETOR TROUBLESHOOTING CHART

Ameri can Honda Mator Co , , Inc. 1975

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MOTORCYCLE CARBURETION

The function of the carburetor is to atomize fuel and mix it with air in proper proportions to suit engineope rati ng cond iti ons. I n operation, the carbu retor sprays gasol in e into th e air passi ng th rough it. Th eatomized gasoline (a mist of liquid fuel) is then vaporized by engine heat and heat of compression toprovide a uniform and efficiently combustible air-fuel mixture.

The quantity of gasoline dispensed by the carburetor is controlled by metering circuits within the carbu-retor body, providing exactly the right air-fuel ratios. A throttle valve controls the amount of air-fuelmixture delivered to the engine, regulating the engine's power output.

AJR-FUEL RATIOS

The th eore tical Iy perfect ai r-fu el ratio i515 parts of ai r to 1 part of gasol i ne} by weight. When th ere isa uniform air-fuel 'ratio of this proportion, the mixture burns completely without leaving an excess ofeither fuel or air.

The air-fuel mixture may still burn effectively when the ratio is as rich as 7: 1 or as lean as 20: 1. Theactual limits of combustion will vary according to combustion chamber shape, pressures, temperatures,fuel characteristics, and mixture uniformity.

Extremely rich and extremely lean fuel mixtures both result in loss of power. An extremely rich fuelmixture burns slowly and incompletely, because there is not enough oxygen in the air to combinewith the fuel. Incomplete fuel combustion causes spark plug fouling and carbon build-up in the com-bustion chamber. An extremely lean fuel mixture burns slowly and does not use all the oxygen in theair. If the lean fuel mixture is still burning when the exhaust valve opens, the valve head is exposed toprolonged high temperatures and oxygen, which may result in a burnt valve. Prolonged high cylindertemperatures may also lead to pre-ignition and may melt the piston crown.

Under actual operating conditions, fuel vaporization and combustion are less than perfect, Consequently,maximum power is usually developed with an air-fuel ratio of about 12:'J rather than the theoreticaloptimum of '15:1.

A very rich air-fuel mixture is required for cold starting because a cold engine reduces vaporization andcauses fuel condensation on the intake ports and cylinder walls.

2


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A venturi is a co nstriction In the carburetor bore in the immediate area of the fuel jet nozzle. Thisconstriction makes the atomizer principle work more effectively. Because the carburetor bore is narrowerat the ventu ri, th e vol u me of ai r passi ng th rough th e bore must move faster at th is poi nt than at otherareas. I ncreasi ng th e speed of th e ai rstream at th e fu el jet nozzl e decreases ai r pressu re against th enozzl e, causi ng atm ospheric pressu re in th e fuel reservoi r to force more fuel into the carb uretor bo reo

BASIC PRINCIPLES OF CARBURETOR OPERATION

A tomize r P rin cip le :

Atmospheric pressu re provides the force thatenables the carburetor to function.

Each time the piston moves down on the intakestroke, a vacuum is created in the cylinder, and airunder atmospheric pressure rushes through thecarburetor to fill that vacuum.

It is a law of physics that a moving stream of airexerts less sideways pressure than air at rest. Thus,the airstream moving through the carburetor boreexerts less p ressu re agai ns t the fu el jet nozzle thanatmospheric ai rat rest exerts on the gasol ine in thefloat chamber. Consequently, atmospheric pres-sure forces gasoline up the jet, spraying it into theai r stream in the carb u retor bore.

A spray gun used in painting, and a perfumeatomizer, operate on the same principle.

V enturi E ffect:

\FIG.l

3


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ASIC PRINCIPLES OF CARBURETOR OPERATION (continued)

ome carburetor models have a fixed venturi built into the bore as illustrated in F ig. 2, and other modelsave a variable venturi a rea contra lied by a th rottl e sl id e or vacu u m piston, bu t a II ca rb u retors makesc of the ven tu ri effect inane form or another.

FUEL RESERVOIR

FIG. 2 E lementary C arburetor with F ixed V enturi

Effect:

choke IS a device which obstructs the carburetor bore. The obstruction reduces the volume of airat can flow through the bore to fill the vacuum created in the engine cylinder, thus maintaining aow pressure area beyond the obstruction.

n this manual, the term choke will be used to refer to an obstruction on the upstream side (air cleaneride) of the fuel jet nozzle, as opposed to the term throttle which will refer to a centra! or downstream

ecause the choke is located on the upstream side of the fuel jet nozzle, the nozzle is within the lowressure area, causing atmospheric pressure in the fuel reservoir to force more fuel into the carburetor

he most familiar application of a choke is the choke valve used to enrich the cold starting mixture,ut the choke effect may also be utilized in other systems of the carburetor; for example, the upstreamdge of a 5 1 ide type throttle valve exerts a choking effect (see Throttle V alve Cutaway, page 17)_


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5

BASIC PRINC IPLES OF CARBURETOR OPERATION (continued)

- FUEL RESERVOIR

FIG .3 E lementary Carburetor with Choke ValveAI tho ugh th e venturi and the choke both increase fuel flow by red uc ing ai r pressu re agai nst the fueljet nozzle, there are im porta n t struc tu ral and fu n cti ana 1 d i fferen ces:

A venturi is a carburetor bore constriction in the immediate area of the fuel jet nozzle. A venturireduces air pressure against the fuel jet nozzle by locally increasing airstream velocity .

A choke is a carburetor bore obstruction on the upstream side of the fuel jet nozzle. A chokereduces air pressu re against the fuel jet nozzle by preventing atmospheric air from filling inductionport vacuum.

The elementary carburetor illustrated in Fig. 2 would be capable of sustaining an engine at a constanthigh rpm, as long as the operator supplied fuel to the reservoir at the base of the jet. However, sucha simplistic design lacks the features needed for control of engine rpm and flexible response to differento pe ra tin g c onditio ns .

For practical operation, the carburetor must have a float system to control the fuel level at the baseof the jets (or an alternative system such as the pressure pulse diaphragm), a throttle valve to regulatethe volume of air-fuel mixture delivered to the engine, fuel metering systems to suit all operating con-ditions, and a choke valve or mixture enrichener to facilitate cold starting.


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FLOAT SYSTEM

! Float valve seat washer2 Float valve seat3 Float valve needle4 Float pin5 Float6 Float bowl gasket7 Float bowl8 Drain plug washer9 Float bowl drain plug10 Float bowl retaining el i p

FIG . 4 Float System Components

FLOAT FLOAT VALVE

FLOAT ARMFIG .5 Float V alve C losed

6

The pu rpose of the float system is to m ai n tai na constant and correct level of fuel in the car-buretor's fuel reservoir.

A float, made of brass or plastic, rises and fallswith the level of fuel in the float chamber,

W hen the float rises, the arm on which the floatpivots presses against a valve, shutting off the fuelsupply to the float chamber (Fig. 5).

W hen the float falls, the float arm releases thevalve, allowing fuel to enter the float chamber(Fig.6).

The float arm is adjusted to close the valve whenthe fuel reaches exactly the right level. If thefuel level shou Id ri se above the correct level,too much fuel may be released into the carburetorbore, resulting in a rich air-fuel mixture. If thefuel level shou Id fall below the correct level,too little fuel may reach the carburetor bore,resulting in a lean m ixture.

FLOAT VALVE

FIG .6 Float V alve O pen


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A single float, located to one side of the fuel jet (F ig. 7) maintains a constant fuel level in the jet whilethe machine stands in an upright position but causes the fuel level in the jet to rise or fall when themachine is tilted to one side. If the carburetor uses a separate float chamber, mounted to one side ofthe carburetor, the variation in fuel level is even more pronounced.

FLO AT SY STEM (continued)

E ccentric F loat. C ham ber:

JI BFIG . 7 Eccentric F loat Chamber

7

In Fig. 7B , the motorcycle is standing upright. W hen fuel enters the float chamber, the float rises tothe position illustrated and closes the float valve. The fuel level is maintained at the height shown.

W hen fuel enters the float chamber with the motorcycle tilted as shown in Fig. 7 A , the float will beraised to the closed position before the fuel level rises to the correct height in the jet.

W hen fuel enters the float chamber with the motorcycle tilted as shown in Fig. 7C , the float will notbe raised to the closed position until the fuel level rises above the correct height in the jet.


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FLOAT SYS TEM (co ntin ue d)Fuel levels shown in Fig. 7A , B , & C apply when the motorcycle is at rest or operated in a straight line.U nder those conditions, gravity is the controlling force. W hen the motorcycle negotiates a curve in theroad, however, centrifugal force also affects the level of the fuel.W hen ridden through a curve in the road, the motorcycle is leaned to the inside of the curve to a pointwhere th e com bi na ti on of gravi ta ti anal force and cen tri fugal fa rce acts th rough the pi ane of th e wheels.F o rtu n atel v, th is co m b ina tio n of forces also serves to kee p the fuel level in th e ca rb u reto r perpend icu la rto the jet, as shown in Fig. 7B .B ecause a motorcycle is leaned to the inside of the curve when cornering, an eccentric float chamberlocated to one side of the fuel jet w ill maintain the correct fuel level in the jet under most operatingconditions.If an eccentric float chamber, located to one side of the jet, were to be used in an automobile, centrifugalforce while cornering would pose a problem. B ecause an automobile does not lean to the inside of thecu rve, fu e l is 5105hed to a ne sid e of the float cham ber. If the fl oa t ch am ber we re located to on e si de ofthe fuel jets, the air-fuel mixture would become very rich or very lean, depending upon the direction inwhich the au tornobi!e t u r ned.W hen an eccentric float chamber is used with an automobile carburetor, the problem is lessened by locatingthe float chamber forward of the fuel jets rather than to one side. W ith the float chamber in the forwardposition, the air-fuel mixture becomes richer while ascending a hill and leaner while descending.C oncentric F loat C ham ber:Double floats, located on opposite sides of the fuel jet (Fig. 8), maintain a constant fuel level in the jetregardless of the attitude at which the machine is tilted. W hen the machine is tilted to one side, increasedpressure against one float is always balanced by decreased pressure against the opposite float.

8

BFIG .8 Concentric F loat Chamber


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FLOAT SYS TEM (co ntin ued )

F lo at Ad ju stment:

Th e CO ITect floa t cham ber fu e[ Ievel is establ ish ed by th e carb u retor man ufactu rer In accorda nce withthe design, c haracterist ics, an d a p pl icat ion of the pa rtic u Iar carbu retor mod el ,

B ecause it is not possi b Ie to see the actual level of the fu el w ith in th e fl oa t cham ber (u n Iess one attach esa sigh t tu be), float adj ustme n t spec ifica ti ons are usual Iy give n in terms of th e distance between thecarburetor body and the bottom of the float when the float arm just closes the valve (F ig, 9).

F loa t heigh t, adjusted to th e man ufac tu rer's spec-ifications, maintains the correct fuel level in theflo at c hamb er.

A djustm en t I nstruc ti ons:

1, H old the carburetor w ith its bore in thevertical position, so the float arm tang willjust close the float valve without compressingthe spring loaded pin in the end of the floatvalve needle (F ig. 9),

2. Position the float height gauge against thecarburetor body as shown in Fig. 10. Ifthe gauge has been set to the spec ifi ed floatheight, it shou Id just touch the float w ithou tcausing the float to move,

3 . I f float height is fou nd to be i ncorrec t,carefu IIy bend the fl oa t arm tang toward oraway from the float valve until the specifiedfloat height is obtained.

W hen adjusting carburetors equipped with doublefloats, measure both floats to be certain they areof eq u al heigh t. 51 igh t m isal ign m e n t betweendouble floats can be corrected by carefully tw istingthe float arm. F loats which are damaged orseverely m isaligned m ust be replaced.

FLOAT VALVE

T ARM TANG

FLOAT ARM

~----- CARBURETOR BODY

FIG .9 F loat H eight

FIG.10 Float Height G auge9


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THROTTLE VALVE

The th rott Ie valve regu lates th e engi nes powe r au t pu t by can tro II ing the vol u me of ai r-fu el mix tu redel ive red to th e engi ne cyl i nd er. Red ue ing the e h arge in th e cy Iinder red u ces th e power it can generateand vice versa,

Throt tie val ves used in motorcycle carburetors are common Iy ei the r disc (b u tterfl y) ty pe or c yl ind ricalslide type. O ther throttle valve shapes (e.g. flat slide) have been utilized but are not commonly en-countered.

FIG . 11 D isc Throttle Valve

FIG . 12 Slide Throttle Valve10

D isc Throttle V alve:

B ecause the disc throttle valve rotates on a centralaxis, it is balanced against pressure. This typeof th rottl e valve a pens and cIoses free Iy regard lessof induction pressure fluctuations or pressuredifferences on either side of the valve. Discthrottle val ves are used in automobile carb u retorsand in som e m otorcycle carburetors.

Slide Throttle V alve:

Slide type throttle valves require fluctuating in-duction pressures in order to work freely andeasily without unusually strong closing springs.5 tead y ind uction pressu res press th e th rottl e sl id eagainst one side of its guide, which increasesc lo sin g f ric tio n.

For this reason, slide type throttle valves arenot suitable for the multi-cylinder manifolds usedin automobile engines but work very well inmotorcycle applications where each carburetorfeeds only one or two cylinders. Manifold vacuumfl uc tua tes grea tl y where there is no over! ap be-tween intake strokes in the engine.


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The air-fuel mixture ratio is affected by changes in venturi air speed. When the throttle valve opening]nc reased and eng] ne rpm al so ]nc reases, air w i II rush th rough th e carb uretor bore at a greater ratU nfortu natel y, the rate of fuel flow th rough a fi xed iet 'does not i ncrease proportiona tel y wi th an icrease ina i r speed th rough a fixed ven tu ri. At high speeds, the air-fue I mix tu re rati 0 tend 5 to becomricher. It is therefore desirable to provide some means of varying the venturi size and metering fueflow in order to maintain correct air-fuel mixture ratios over a wide range of operating speeds. Alternatively, this objective can also be achieved solely by the use of compensating jets and air-bleeds, asdone in many automobile carburetors.

THROTTLE VALVE (continued)

Mainta ining Correct Air-F uel Mix tu re Rati os:

The use of a sli de type th rottl e valve hel ps to slmp Iify carbu retor design because ita Iso acts as a vari abven tu ri. As the 51de is lowered, it cons tri cts the carb u retor bore at the fuel jet. Til e sl id e is alsovery convenient mount for holding a metering needle to control fuel flow at the jet.

A disc th rottl e valve is located beyond the ventu riarea and obvi ausl y creates no canst ri ct i on at til atpoi nt. If a carb u retor usi ng a disc throttle val vehad only one fixed jet and one fixed venturi,as in Fig. 11, it could deliver the correct air-fuelmix tu re ratio at on Iy one ai r speed. Th erefore,when a disc th rottl e valve is used, the carb u retormust also be equipped with compensating systems,or a vacuum piston to vary venturi size and holda fuel metering needle (Fig. 13) in the manner ofa sl ide th rattle. The vacuum piston possessessam especial advantages, however, which will be ex-plained in later sections of this manual.

Th rattle Valve S y nch ronization:FIG.13 Disc Throttle Valve

and Vacuum Piston

On m uIti-cy Ii nde rand twi n cy Ii nder engl nes u51 ng more than one carbu retor, the moveme nt of thtil rattle valves must be aceuratel y sync h ron ized to ensure th at each cy Ii nde r receives an iden I : i cal amouof fuel mixture.


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TH ROTTlE VALVE (continued)

FIG . 14 Checking Throttle V alveSynchronization with aVacuum Gauge Set

AIR-BLEEDS

Throttle valve synchronization in twin cylinderengines can be judged by placing one's hands atthe tips of the mufflers. If the ex h aust pressu refeels equal in both pipes, the th rattle val ves aresynchronized within acceptable limits. A moreaccurate measure of synchronization can be madewith a vacuum gauge set if the engine is tappedto receive vacuum gauge attachments, or with anair flow meter if a vacu u m gauge set ca n not beattached.Throttle valve synchronization in Honda fourcylinder engines requires the use of a vacuum gaugeset. F au r th rott1 e valves can not be accu ra tel ysynchronized by feel, and it is inconvenient to usean air flow meter on these models.5 pecific instructions for th rattle valve sy nc h ro-n iza t i on adj ustment a re given in the a ppl icab Iesho p m an uals.

Air under atmospheric pressure is bled into carburetor fuel passages to im prove fuel atomization, stabilizefuel height in the jets, and provide corrections in the air-fuel mixture ratio. Air jets and/or air-bleedadjustment screws control the relative amount of atmospheric air drawn into the fuel systems.

FIG .15 Main Jet Air-B leed12

Main J et A ir-B leed:Fig. 15 illustrates a typical main jet air-bleed.low ven tu ri pressu re, wh lch causes fue I to riseth rough the ma in fue I jet, also ca uses atmosph eri cair to flow through the air jet. Air and fuel meetand mix togeth er ina perforated tu be above th ema in fuel jet. The aerated fuel re I~ased into th eventuri is more easily atomized than a denseu n aerated stream of fuel.Aerated fuel al so h as Iess tend enc y to fall bac kdown the jet tube between intake strokes, thusstab iii zi ng fuel heigh tin the jet tube. Th e sameeffec t can be observed when d r i n ki ng beveragesthrough a straw. W hen you remove your mouthfro m th e straw) a froth y beverage tends to remainin the straw, but an unaerated beverage will fallback down the straw into the cup.


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AIR-BLEEDS (continued)

Idle and Low Speed Air-Bleed:

Fig. '16 illustrates an idle and low speed ai r-bleedprovi ded with an ad iustmen t sc rew fa r can troll i ngth e flow of air del ivered to the perforated tu beabove the low speed fuel jet. The screw can betu rned to canst ri c t th e a ir-b leed passage, prod uc-ing a richer pre-m ix tu re, and vice versa.

Th e maj ori ty of motorcycl e carbu retors are pro-vided with this type of air-bleed screw for adjustingidle mixture. However, some models have afixed idle and low speed air-bleed and use anadjustment screw to meter the flow of aeratedfuel instead.

Altitude Compensator:

The atmosphere is less den se at high al titud esthan at sea level. Th e weigh t of ai r in the ai r-fuelmix tu re at high al titudes is therefore red uced ,causi ng the mix tu re to becom e excessivel y ri c h.

When a veh i cle is operated at high al t itudes, theair-fuel mixture ratio can be corrected by installingsrna Iler diameter fue I jets to red uce fu el flow inproportion to the reduced weight of air. Alter-na t ivel y, ai r-bl eeds can be provi ded to correctthe ai r-fuel mix tu re ra ti a wi thou t necessita t i ngfue I jet repiacemen t.

The carburetor on Honda CT-90, ST-90, andA TC-90 mode Is IS equ i pped wi than altitudecompensator for use at 6000 ft. above sea levelor higher. When th e com pensator knob is pull edoutward, an auxiliary air-bleed comes into oper-ation and supplies additional air to both mainand low speed fuel systems.

SLOW SPEED JETWITH PERFORATEDTUBE

13

AIR ADJUSTMENT SCREWFIG. 16 Idle and Low Speed Air-Bleed

ALTITUDE 1OMPENSATINGCARBURETORPUll"DB.HI' "I''''''u H R 1 00 0 f l ' IT .

FIG. 17 AI titude Com pensator


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IDLE AND LOW SPEED SYSTEM

If the engine were equipped with one of the carburetors illustrated In Fig. 11 , 12, or 13, it would ceaseto ru n whe n the th rottl e va Ive ap preached the cIosed posi ti o n.

W hen the throttle valve is at or near the closed position, very little air can flow through the carburetorbore. The air moves too slowly across the mal n jet for the atomizer princi pie to work effectively, andno fuel is de Iive red from that jet. It is therefore necessary for th e carbu retor to have a sepa ra te fuelsystem for idle and low speed operation.

FIG . 18 Idle and Low Speed Systemwith Air-B leed Adjustment Screw and

Single Fuel D ischarge O rifice

FUEL DISCHARGEORIFICE --_.../

14 FIG .19 Enlarged V iew of System

F uel D ischarg e O rifices:

Some carbu retors use a si ngl e fuel dischargeorifi ce for both id Ie an d low speed 0pera ti o n(F ig. 18 & 19), while others have separate idleand low speed orifices (Fig. 20 & 21). Ineither case, fuel is metered through the lowspeed jet and aerated before it reaches the fueldischarge orifice or orifices.

C arb u reto rs USI ng a si ngl e fuel d ischa rge orifice(F ig. 18 & 19) have that orifice located at thelip of the throttle valve, where carburetor boreconstricti on and ai r speed are greatest.

C arburetors using separate idle and low speedorifices (Fig. 20 & 21 ) have the idle orificelocated beyond the throttle valve, relying onintake port vacuum for fuel delivery. In thisoperating mode, the atomizer principle is notuti I ized; the th rottle val ve acts as a choke. W he nthe throttle valve is opened slightly, the idleorifice continues to supply fuel, and the low speedorifice (or orifi ces) at the lip of the th rattle val veare brought into play, supplying additional fuelto match the inc rea s ed ai r flow.


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IDLE AND LOW SPEED SYSTEM (continued)

Idle and Low Speed Air-Fuel M ixture Adjustment:

A screw type needle valve is provided for adjustment of the air-fuel mixture ratio. The adjustmentscrew can be located in the air-bleed passage leading to the low speed fuel jet or in the passage betweenthe low speed fuel jet and the idle fuel discharge orifice.

If the adjustment screw is located in the air-bleedpassage, as in F ig, 18 & 19, it w ill control the flowrate of air del ivered to the perforated tube abovethe low speed fuel jet. The final air-fuel mixtureratio is achieved when the aerated pre-mixture iscombined with the air in the carburetor bore, Theadjustment screw is turned clockwise to enrichthe mixture by reducing aeration, and vice versa.

If the adjustment screw is located In the passagebetween the low speed jet and the idle fueldischarge orifice, as in F ig. 20 & 21, it w illcontrol the flow rate of aerated fuel deliveredto the carburetor bore. The final air-fuel m ixtureratio is achieved when the aerated pre-m ixtureis combined with the air in the carburetor bore,The adjustment screw is turned counterclockwiseto enrich the mixture by increasing fuel flow ,and vice versa.

Major corrections In the idle and low speedair-fuel mixture ratio can be made by replacingthe low speed jet. Replacement jets are availablein graduated sizes to change the fuel flow rate.Select a larger jet diameter (higher jet number)to enrich the air-fuel mixture ratio or a smallerjet diameter (lower jet number) to produce ale an er m ix tu re ,

F IG . 20 Idle and Low Speed Systemwith Fuel Adjustment Screw andSeparate Fuel D ischarge O rifices

AIR-BLEED JETILOW SPEED ORIFICE,

LOW SPEED JET

FIG .21 Enlarged V iew of System15


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IDLE AND LOW SPEED SYSTEM (continued)

FIG . 22 Carburetor with Slide TypeThrottle V alve

THRO TTL ES T OP S C RE WFIG . 23 Carburetor with D isc Type

Throttle V alve

THRO TTL ES TO P S CR EW

FIG .24 Throttle V alve L inkagefo r M ultiple C arburetors

16

Idle S peed A djustm ent:

An adjustable throttle stop controls engine idlerpm by lim iting throttle valve closure. The stopscrew is turned clockwise to increase idle rpmor counterclockwise to decrease idle rpm .

I f the ca rbu retor is eq u i pped wi th a sl id e typethrottle valve, the stop screw will usually beloca ted on the sl ide bore and act d irecti y agai nstthe slide (Fig. 22).

A disc type throttle valve is more delicate andwaul d be da maged if allowed to re pea ted Iy stri kea stop. Therefore, if the carburetor is equippedwith a disc type throttle valve, the stop screwwill usually be located at the throttle valve pivotarm (Fig. 23).

W hen carburetors on twin or multi-cylinder engineshave individual throttle stops, idle speed adjust-ment requires precise equalization of each stopsetti ng in ord er to main tai n an id en t ical th rottl eopening at each carburetor. The idle speedadjustment procedure is greatly simplified in cer-tai n mod el s by p rovid i ng a si ngl e th rottl estopat the th rattle va Ive opera ti ng linkage (F ig. 24).W hen a single throttle stop is used for two ormo re ca rb u retors, th rottl e val ve eq ua I ization isnot affected by idle speed adjustm ent.


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INTERMEDIATE SYSTEM WITH SLIDE TYPE THROTTLE VALVE

When the throttle valve is opened, there is a transition from the Low Speed System to an IntermediateSystem which meters fuel from the main fuel discharge jet (needle jet).

In carb u reto rs w i th 51ide type th rottl e valves, th e interm ed iate system uses a tapered fuel meteri ngrod (jet need Ie) that works in can neet ion with th e vari abl e ven tu ri and cho king action of the th rottl evalve. The jet needle meters fuel flow, maintaining the correct air-fuel mixture ratio through mostof th e carb u retor 's opera ti ng range. At full th rottl e open ing, th e jet need Ie is fu IIy ra ised, an d fuelflow wi I I be con troll ed sol el y by m a i n jet d ia m eter (High Speed System).

Throttle V alve C utaway:

The need Ie jet begi ns to discharge fuel at about'1/8 throttle opening and supplants the low speedsystem as the chief fuel supply. It is importantto have a smooth transition from the low speedsystem to the intermediate system as the throttleis opened. O therw ise, there would be a momen-tary fu e l del ive ry fai l ure causi ng a fl at spot inacceleration. The height of the throttle valvecutaway is c rue ial in obtai n ing smooth systemtransition and good initial acceleration.

W hile the position of the throttle slide determinesth even tu ri constri c ti on in the carb u reto r bore,the shape of the bottom edge of the slide deter-mines the ex t ent to w h ichi nd u ction port vac u u mis maintained at the needle jet.

The bottom edge of the slide, on the upstreamsid e (ai r cI eaner sid e ) of th e need Ie jet acts as achoke, but is cut away to a height that willprod uce the exac t degree of ch o k ing need ed toensu re proper fu e l del ivery fro m the need Ie jetat low throttle openings.

THROTTLE VALVE

JET NEEDLE

NEEDLE JET

THROTTLE VALVECUTAWAY

FIG.25

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INTERMEDIATE SYSTEM W ITH SlI DE TYPE THROTTLE VALVE (continued)

N ee dle p os itio nat low throttleopening.N eed le p os itio nat fu ll th ro ttleopening.

FIG . 26 Needle and Needle Jet

THROTTLESLI DE - -- -- --

FIG . 27 Needle and Retaining C lip

18

Increasi ng th e h eight of the cuta wa y red uces th echoking effect, producing a leaner air-fuel mixtur-eratio, and vice versa.

The throttle valve cutaway controls the air-fuelm ixture ratio primarily at 1/8 to 1/4 throttleopen ings a nd ceases to have a ny effect beyon da 1/2 th rottl e open ing.

J et N ee dle :

A tapered metering rod, called the Jet needle,is attached to the throttle slide and extendsdownward into the fuel discharge jet (needlejet ).

At low throttle openings, the wide base of theneedle fills most of the fuel jet diameter, reducingfuel flow to a minimum. As the throttle valveis opened, raising the needle, the progressivelysmaller needle diameter allows more fuel to flowthrough the jet, matching the increasing volumeof air flowing through the carburetor bore (Fig.26).

Jet needles used with slide type throttle valvesusually have a series of grooves at the needlehead to permit alternate mounting positions. Thejet need Ie can be rai sed or lowered in the th rottl eslide by installing the jet needle retaining clip(Fig. 27) in one of the alternate grooves.

Raising the needle to a higher mounting positionallows a grea te r flow of fuel th rough the need Iejet, creating a richer air-fuel mixture ratio. Lower-ing the needle creates a leaner air-fuel mixtureratio. The jet needle controls the air-fuel m ixtureratio primarily at 1/4 to 3 /4 throttle openings.


22/34

~COVER

INTERMEDIATE SYSTEM W ITH DISC TYPE THROTTLE VALVE AND VACUUM PISTON

As the disc type throttle valve is opened, thereis a transition from low speed fuel dischargeorifi ces to an in termed ia te system, si mil ar tothat wh ieh oceu rs in carb u retors usi n g 5 1 id e ty peth rottl e valves. However, there is more aceura teventuri control, as the vacuum piston movesind ependentl y.

W hen a slide type throttle valve is opened, theventuri constriction is widened in direct proportionto throttle opening, as these are functions ofone integral pa rt. If th e sl id e th rottl e val ve ismechanically linked to the throttle grip, venturisize is regulated by the rider, whose wishes willnot necessarily coincide with the needs of theearbu retor. Ani nde pend en tl y control Ied vacu u mpiston (or a vacuum controlled slide throttlevalve, see pages 23-24) ensures correct venturisi ze and air veloci ty th roughout the in termed ia teoperating range.

V acu um P iston C on stru ction:

The bottom of the vacuum piston IS vented toallow air pressu re in the earb u retor bo re and airpressure in the chamber above the vacuum pistonto equal ize. The cham ber below the vac u u mpiston d iaph ragrn 0r ri m is ven ted to the atmos-phere.

V acuum piston design may utilize a diaphragmto se para te th e upper and lower air cham bers(Fig. 28 & 29). Designs which do not utilizea diaphragm rely on a close fit between thepiston and the walls of the carburetor top toseparate the air chambers (Fig. 30 & 31).W hen air pressure in the carburetor bore becomessign ificantl y lower than atmospheric pressu re J a t-mosphe ric p ressu re forces the piston upward.W hen air pressure in the carburetor bore ap-proaches atmospheric pressure, the piston willthen fall of its own weight, sometimes assistedby a retu rn s pr i n g.

RETURN SPRING

FIG .28 COMPONENTSV acuum Piston with D iaphragm

FIG .29 OPERATIONV acuum Piston w ith D iaphragm

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INTERMEDIATE SYSTEM W ITH DISC TYPE THROTTLE VALVE AND VACUUM PISTON (continued)

VACUUM CYLINDER

FIG .30 COMPONENTSVacuum Piston without D iaphragm

VACUUM

20 FIG .31 O PERATIONVacuum Piston without D iaphragm

D iaphragm type vacuum pistons have returnsprings to ensure closure because additional pres-su re is req u i red to fl ex the diaphragm. N an-d iaph ragm ty pe vacu u m piston s fall freel y and donot req u ire retu rn spri ngs for positive closu reoN on-diaphragm type vacuum pistons are sometimesfitted with return springs in order to modify thepiston's operating characteristics. Adding a returnspring has the same effect as increasing the weightof the piston.

V acuum P iston O peration:

During idle and low speed operation, when thethrottle valve is nearly closed, induction portvacuum is confined to the area downstream of thethrottle valve. Air pressure at the vacuum pistonis nearly the same as atmospheric pressure, andth e vacuu m pis ton is in th e full y cl osed posi tion(F ig. 32).

As the throttle valve is opened, the area underinduction port vacuum extends upstream to thevacuum piston. The intermediate system becomesoperative, and the vacuum piston moves in re-sponse to differences between atmospheric pres-sure and vacuum in the carburetor bore. P istonmove ment stab iii zes at a heigh t where the pressu redifferential ba lances p isto n weigh t (F ig. 34).

As engine rpm increases, the volume of air passingthrough the carburetor bore also increases. Th evelocity of air passing through the venturi remainsconstant, however, throughout the operating rangeof the vacuum piston.

An increase inflow vol u me is accomod a ted byincreasing the size of the air passage rather thanby increasing air speed. Piston movement variesthe size of the venturi so that air velocity canremain constant regardless of changes in flowvolume. Carburetors equipped with vacuum pis-tons are therefore frequently referred to as"constant veloc ity ca rb u retors."


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INTERMEDIATE SYSTEM WITH DISC TYPE THROTTLE VALVE AND VACUUM PISTON (continued)

Accel era t ion:

The main advantage of constant velocity carbu-retors is appa ren t d uri ng hard accelerati on. If amotorcyc Ie is eq ui pped wi th conventional slidethrottle carb u retors, sudd en full th rottl e 0peningat low speed may produce poor engine response,as vacuum and venturi air velocity will drop to apoi nt where very little fuel is d rawn from the jets.If the motorcycle is equipped with constantveloc ity carbu retors, the vacu um pi stons ensu resuffi cient ventu ri ai r velocity to prevent a leanfuel mixture on sudden throttle opening.

In auto motive appl ica tio ns, the vacu um pistonmay be equipped with an oil damper to retardupwa rd movemen t of. the piston for a richeracceleration mixture. Alternatively, an autorno-ti ve carb uretor may use an "accelerator pum p"wh ich sq ui rts ad ditional fuel into the carb ureto rbore on sudden throttle opening.

FIG. 32 Low rpm and Low Throttle Opening

Jet Needle:

Because venturi air velocity is held to a constantspeed, air pressure against the fuel jet nozzle(s) isalso constan t. Th is causes the fuel to ten d toflow from the jet(s) at a constant rate, regardlessof engine rpm, so the vacuum piston must carrya meteri ng rod (jet need Ie) to regu Iate fuel flowin proportion to air flow volume.

FIG. 33 Low rpm and Full Throttle Opening

Jet need les used in constan t vel oci ty carb uretorsare simila r to those used in 5 1 id e th rottl e carb u-retors, except that most models of constantvelocity carburetors do not provide any needleheight adjustment. Fuel mixture adjustment forthe in termed iate opera ti ng range is achi eved byrepl aci ng th e mai n j et(s) wi th jets of Iarger orsm aller diameter. I n some cases, repl acemen tneedles with other profiles may be available toa Iter the fuel flow rate.

FIG. 34 Intermediate rpm andIntermediate Throttle Opening

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INTERMEDIATE SYSTEM W ITH DISC TYPE THROTTLE VALVE AND VACUUM PISTON {continued}

Single M ain F uel C ircuit:

The carburetor illustrated in F ig. 32, 33 , & 34discharges fuel from a single jet assembly through-out the operating range of the intermediate andhigh speed systems. Mai n jet and need Ie jetfunctions are sim ilar to those in carburetorsw ith slide type throttle valves.

P rimary and Secondary M ain Fuel C ircuits:

PRIMARY MAINJET The carburetor illustrated in F ig. 35 & 3 6 divides

the main jet system into primary and secondaryci rcu its. The purpose of th is design is to provi dea smoother transition from low speed to inter-F IG .35 Fuel F low During Transition

from Low Speed System to IntermediateSystem

FIG .36 Fuel F low After Transitionto Interm ediate System

22

med iate sy stems.

As the throttle valve starts to open and inductionport vacuum extends upstream, fuel is dischargedfrom the primary main jet nozzle before thesecondary circuit becomes operative (F ig. 35 ).

As th e th rottl e valve 0pens farther, fuel is d is-charged from the secondary main jet nozzle(needle jet), and the vacuum piston begins torise (F ig. 36).

Fuel is delivered from both primary and secondaryci r cu its th roughou t the 0perati ng range of thevacuum piston. Fuel m ixture adjustment can bemade by replacing the main jets w ith jets oflarger or sm aller diam eter.


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INTERMEDIATE SYSTEM W ITH COMBINATION SLIDE TYPE THROTTLE VALVE/VACUUM PISTON

The carbu retor used in Honda 55-125 and C L - 125 twi n cyl i nder motorcycl es incorporates both th rottl eand vacuum piston functions in a single slide valve assembly.

Construction:

Air chambers above the th rottl e sl ide/vac uu mpiston are divided by a diaphragm. The throttlegrip at the handlebar operates a lever which limitsupward movement of the throttle slide and con-trols the position of a plunger valve within theslide. The plunger valve vents the air chamberabove the diaphragm to atmospheric pressure whenthe pi unger is depressed, and to ind uction portvacuu m when the pi unger is rei eased. Th e ai rchamber below the diaphragm is always ventedto th e atm osp here.

Operation:

W hen the lever is fully lowered, the throttle slideis held in the idle position. The plunger valvewithin the slide is depressed, venting the upperai r cham be r to atmospheric pressu re, so the sl id ewill not resist closure (Fig. 37).

W hen the lever is raised, the plunger valve isreleased, venting the upper air chamber to in-d uc t ion po rt vacu L I m. The th rottl e sl id e thennses In response to differences between atmos-pheri c pressu re and vac u u min the carb u retorbore. W hen the th rottle sl id e rises, fu e I isdischarged from the needle jet (Fig. 38).

THROTTLE SLIDE

FIG . 37 Idle Position

FIG . 38 Lever Fully Raised, andThrottle Valve Partially Raised 23


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INTERMEDIATE SYSTEM WITH COMBINATION SLIDE TYPE THROTTLE VALVE/VACUUM PISTON(continued)

With the lever held in the raised position, the throttle slide will rise until it stabilizes at a height where

the pressure differentia I ba lances sl ide we i gh t, or un ti I the pi unger con tacts the lever. 1n th is operati ngmode, the carburetor is self-throttling; the rider controls only the upward limit of throttle slide traveland cannot raise the throttle slide excessively.

A sim i lar self-t hrottl i ng function is in heren t, though less obvi OUS, in carbu reto rs wi th a separate discth rottle valve and vacuum piston. I f the disc th rottl e valve is opened farthe r than pressu re differencespermit the vacuum piston to rise, the vacuum piston temporarily assumes the function of a throttle.

HIGH SPEED SYSTEM

FIG. 39 High Speed System

24

When the throttle valve is fully open, and thevacuum piston (if so equipped) is fully raised,the carburetor functions as an elementary fixedventuri instrument. Com pare the high speedsystem illustrated in Fig. 41 with the elementarycarburetor illustrated in Fig. 2, page 4.

Fuel mixture is controlled by the Size of themain jet. The fully raised jet needle does notapprec iab Iy restrict the flow of fuel at the jetneedle. Fuel mixture adjustment is achieved byreplacing the main jet(s) with jets of larger orsmaller diameter.


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FUEL MIXTURE CONTROL IN RELATION TO THROTTLE POSITION

FIG.40 IDLE TO 1/8 THROTTLE OPENING:Idle and low speed system

controls fuel m ixture.

FIG.41 1/8 TO 1/4 THROTTLE OPENING:T hrottle valve cutaw aycontrols fuel m ixture.

FIG.42 1/4 TO 3/4 THROTTLE OPENING:Jet needle controls fuel m ixture.

FIG.43 3/4 TO FULL THROTTLE OPENING:M ain Jet controls fuel mixture.

The above illustrations show the throttle position where each system predominates in fuel mixturecontrol. The operating ranges of these systems overlap each other to provide a smooth transition fromone system to another.

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COLD STARTING SYSTEM

FIG .44 SlldeTvpe C hoke Valve

FIG .45 P ivoted D isc-Type C hoke V alve26

Fuel does not vaporrze well in a cold engine andtends to condense on intake port and cylinderwalls. B ecause a smaller percentage of the fuelis vaporized in a cold engine, the carburetor mustdeliver a richer m ixture in order for the cornbus-tion chamber to have enough vaporized fuel tomake a combustible air-fuel m ixture. The m ixturemust not be excessively enrichened, however, orthe combustion chamber will become flooded withliquid fuel which may not ignite.

The cold starting system used in Honda motor-cycles may be either a supplemental fuel deliverycircuit (M ixture Enrichener) or a choke valve thatenrichens the mixture by obstructing the carbu-retor bore (see Choke Effect, pages 4-5).

C hoke V alve:

A choke valve is the most commonly used coldstarting device. In motorcycle carburetors, itmay be either a flat slide (Fig. 44) or a pivoteddisc (Fig. 45). The choke valve is connected to alever or knob by which the rider controls thechok e valve position.

W hen the choke valve is closed, it reduces thevolume of air that can flow through the carbu-retor bore to fill the vacuum created in the enginecy lind er, th u s mai n tai n i ng a partial vacu u m atall fu el disc ha rge jets and orifi ces. A t mospheri cpressure in the float chamber than forces morefuel into the carburetor bore. A spring loadedrelief valve within the choke slide or disc limitscarburetor bore vacuum to a certain level toreduce the possibility of flooding.


30/34

COLD STARTlNG SYSTEM (continued)

Mix tu re Enrichener:

A mixture enrichener is used in Honda two-stroke motorcycles instead of a choke valve inorder to improve fuel atomization and furtherreduce the possibility of flooding. A plungervalve, con trolled by an ex te rn al Iever or knob,opens or closes a supplemental fuel deliverycircuit (Fig. 46). The valve is either fully openor closed; th ere is no provision for parti al open j ngpositions.

When the valve is open, the mixture enricheningcircuit functions like an atomizer (see AtomizerPrinciple, page 3). Fuel flows from the float bowlth rough a me teri ng jet, th en th rough a pe rfora tedtube where the fuel is initially aerated. Whenthe aerated fuel reaches the enrichener valvejuncture, it is combined with the airstream In apassage that parallels the carburetor bore. Themix tu re is th en sprayed into the carb u retor bo reat a point downstream from the throttle slide.

The throttle slide must be closed for the mixtureenrich ener c ircu i t to be fu IIy effective. A cI osedthrottle slide maintains a high vacuum at the dis-charge port of the mixture enrichener, and airrushes th rough the mixtu re enrichener ai r passageto fill that vacuum. Raising the throttle slidecauses a loss of vacuumat th e discharge port anddiminishes the mixture enrichener airstream.

FUEL JET

FIG . 46 Mixture Enrichener Circuit

27


31/34

CARBURETOR TROUBLESHOOTING CHARTPROBLEM POSSIBLE CAUSE CORRECTION

HAR 0 STARTI NG. Ine 0 rreet use of eh 0 ke. C orrect use of eh o ke.I nc or re ct a ir- fu el m ix tu re a dju stmen t. Set mixture adjustment screw in

ace ordance with own er's rna n u al ors hop manua l ins tr uc tion s.

Clogged fuel filter. CIean fi Iter.Clogged low speed fuel jets. Disassemble carburetor and cham-

ically clean .C logged vent in fuel tank cap. U nclog vent or replace cap.Float stuc k. Remove float bowl, check float npar-

ati on, and co rrect or re p lace.Float dam aged a r Ieaki ng. Replace float.Inco rract II oat level. Set float hei gh tin ace ordance wi th

shop manua l s peci fi ca tion s.I ntake air leak. Check carburetor mounting flanges

fo r a ir le ak s.I gn it ion p roblem. R epair, replace, or a djust as necessary.Low cyl in der com pressi on. Rep ai r, replace, a r ad iu st a s n ee sss arv .

POOR IDLE OR Idle speed adjustmentlsl set too low. Adjust idle rpm in accordance withSTALliNG. specifications in owner's manual ors hop manua l.

Idie speed adjustments are unequal E q u a li ze th rottl e sto p setti ngs.(twin carburetor models and multi-c arburetor m odels u sing ind iv idua lthrot tl e s top adj us tmen ts ).C Iogge d id I e & low speed air bl eed. D isasse mb Ie carburetor and cham-

ically clean.All causes listed under "HARDSTARTING."

IDLE MIXTURE Idie speed set too high. Ad j ust idl e sp eed in ace orda nee wi thADJU STMENT IS specifications in owner's manual orINEFFECTIVE. shop manua l.C ARB URETO R DO ESNOT RESPOND TO Clogged low speed a ir -b leed s. o isassern ble carburetor and chern-MOVEM ENT OF TH E icall y clean.IDLE MIX TU RE SC REW .

D amage d m ixture adjustm ent needle. Replace mixture adi ustment need Ie.Mixture adjustment needle "0" ring Replace "0" ring.is not sealing (models using "0"ringl.Damaged mixture adjustment needle R epl ace Cil rbu rater.seat.All carburetor problem s listed under"HARD START ING ."

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PROBLEM POSSI BLE CAUSE CORRECTIONSLOW RETURN Idie speed set too high. Adjust idle speed in accordance withTO IDLE. s~ecifications in owner's manual o rs op manual.

Idie sp eed ad justments are u neq u al Eq u al ize th rottl e sto p setti ngs.(twi n ca rbu retor mod el sand m u Iti-carburetor models uSin~ individualthrot tl e s top adj us tmen ts .Th rottl e valve sti cki ng. Clean and inspect throttle valve andreturn spring. Replace if necessary.Throttle linkage sticking. Clean and inspect throttle linkageand return spring. L ubricate, repair,o r r ep la ce a s n ec essa ry .Thr ottl e c ab le bi nd in g. Correct routing or replace cable asnecessary.

ENGINE SURGES Incorrect air-fuel mixture adjustment. Low Speed Low speed jet sizeWHEN CRUIS ING change.AT A CON STANT Intermediate Jet needle height ad-SPEED. justment or primary main jet sizechange.Vacuum piston sticking. Clean and inspect vacuum piston andreturn spri ng. Rep Iace if necessary.

ENGINE DOES NOT Incorrect use of choke. C orrect use of chok e.DEVELOP FULL POWER, C lo gg ed a ir e le an e r. C Ie a n or r e pl ac e.OR MISSES ONACCELERATION. Incorrect air-fuel mixture adjustment. Low Speed Low speed jet sizechange.Intermediate Jet needle height ad-justment.High Speed Main jet size change.

Throttle valves not synchronized Adjus t throt tl e valve synch roniza tion .(models with two or moreca rbu r eto r s) .C lo gg ed fu el f il t er. Clean f il ter.C lo gg ed f ue l je ts . D i sa ss ern b Ie carbu re to r and chemically clean .C logged a ir b leed s. Disassemble carburetor and cham-i c al lv c l ean .F ue l je ts lo ose . T igh te n fu e l je ts.Fuel jet "0" rings leaking (models Replace "0" rings.using '0" rings).Float stuck. Remove float bowl, check floatoperation, and correct or replace.Float damaged or leaking. R epl ace float.I nc orre ct fl oa t le ve l. Set float height in accordance withshop manual specifications.V acuum p ist on sti ck in g. Clean and inspect vacuum pistonand return spring. Replace ifnecessary.V acuum p ist on d ia ph ra qrn ru pt ure d. Replace vacuum p ist on a ssemb ly .Ignition problem. Repair, replace, or a djust as necessary.Low cyl inde r compres sion . R ep air , r ep la ce , o r a dju st a s n ec essa ry.

NOTE: It may be necessary to change carburetor jetsto correct the air-fuel mixture ratios under thefollowing circumstances: E xhaust system modifications Air cleaner alteration or removal Altitude changes T em peratu re and hum id ity c ha nges

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NOTES


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AMERICAN HONDAMOTOR CO ., IN C .

carburetion 1975

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