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YXRU KV1BfJKG0O04h cNF nA0oEV A 23euBrlMQ5 YYZeee EO7VkgBF 5710Wx7HGROUP II ENGINESECTION S FUEL Carbureters are used onthousands of tractor and industrial engines andhave been designed to provide many years oftroublefree service however as in the case ofall mechanical devices they do in time requireproper service and repairs An their construction and how they operate aswell as an understanding of their function withrespect to the engine will not only avoid manyfalse leads on the part of the service man in diagnosing socalled carbureter complaints but willcreate customer satisfaction and a for the progressive service shopTo understand a carbureter it is necessary torealize that there is only one thing that a carbureter is designed to do and that is to mix fuel andair in the proper proportion so that the mixturewill burn efficiently in an engine It is the function of the engine to convert this mixture intopowerThere are three major factors in an enginewhich control the change of fuel and air intopower 1Compression 2Ignition has been listed last because it isabsolutely necessary for the engine to have and good ignition before it can havegood the average person thinks of carburetion they immediately think of the carbureteras a unit Carburetion is the combined functionof the carbureter manifold valves piston andrings combustion chamber and cam shaftIt can be readily seen that carburetion is afar deeper subject than consideration of the carbureter alone and expecting the carbureter tocure faulty ignition compression valves etc willonly result in wasted time and effort on the partof the service man and added expense to thecustomerIt must be remembered that the function ofthe carbureter does not extend beyond delivering the proper mixture of fuel and air to themanifold and the other factors which effectpower and economy cannot be changed or corrected by the carbureter Inability to understandall the factors that effect engine operation is thereason many service mechanics change from factory standards and attempt to improve on theengine setup by their own methods or standards All that any service mechanic should evertry to do is to make the particular engine he isworking on as good as the manufacturer intended it to be but he can make it a lot worseFar too many engines are running below theirstandard of performance in service todayFor the carbureter to accomplish its functionit must be able to vary the mixture strength dependent upon the engine demands It mustsupply a mixture strength that will allow the engine to give maximum horsepower whenever thethrottle is fully opened while at part it must lean out the mixture so thatmaximum economy can be obtained In additionit must have flexibility throughout the entirerange of operating speeds from idle and partthrottle to full power wide open throttle position The carbureter must also have an accelerating well with enough fuel capacity to starthandling sudden maximum loads In other wordsthe carbureter not only varies the volume of fueland air that enters the engine but also varies theamount of fuel that goes in with a given amountof air in order to produce the proper for any condition under which theengine is operating at any timeIn order to understand the function and operation of the MarvelSchebler Tractor and Industrial Carbureters it is well to consider the systemsthat make up each carbureter These systemsare The Float System The Idle System ThePower Fuel Feed System The Back System and The Choke SystemA thorough knowledge of each system willhelp the service mechanic to quickly locate andcorrect legitimate carbureter complaints as wellas to inspect repair and put back to standardany carbureter that requires an overhaulR 1258254S5GROUP II ENGINESECTION S FUEL SYSTEMFloat SystemIt should be an established policy that whenever the carburetor is disassembled for whatevercause the service manmake following checksFLOAT VALVESEAT4FUEL BOWL1Fig S 13The float system controls the level and supplyof gasoline in the fuel bowl throughout the operating range of the engineWhen the fuel bowl 1and lever 23under pressure from the fuel pump or the float valve 3bowl 1the correct operating level it raises the float andlever 2valve and cut off the flow of fuel into the bowlAs fuel feeds through the carburetor jets intothe engine the fuel level 5additional fuel to enter the fuel bowlUnder actual operating conditions the fuellevel 52position themselves so that the inward flow ofgasoline to the carburetor is equal to the outward flow of gasoline to the engineAs can readily be seen the float system underthe most favorable of operating conditions issubjected to a certain amount of wear Undersevere conditions or conditions that result in excessive vibrations being transmitted to thecarburetor float valve and float valve seat wear isaccelerated1 Examine float valve forany signs of wear If itis not absolutely true oris grooved and hasnt aperfect taper a newfloat valve and also anew float valve seatmust be used Thesefloat valves and seatsare supplied in matched sets and are testedat the factory for leaksAlways use a new floatvalve seat gasket tomake sure of a float for any signs of failure Totest metal float submerge float in pan ofhot water and if air bubbles are with new floatExamine cork float for bare places orcracks in coating If either are found or iffloat shows evidence of having been soggyreplace with new one Do not attempt torecover float with shellac or varnish3Set float height to the proper the particular model carburetor beingserviced Make certain that the entireassembly works free and that there is nobinding4Wash fuel strainer assembly in gasolineand clean screen with air under pressureIf the screen or the threads on the strainerare not in good condition install a newassembly When reinstalling fuel always use a new strainer gasketif a gasket is used to obtain a sealIt has been proven with few exceptionsthat with a float system in good flooding only occurs when dirtor foreign matter becomes lodged betweenthe float valve xx S SEAT12FUEL JET11iI 15rt SECONDARYIDLE ORIFICE8IDLE FUEL BOWLIDLE FUEL PASSAGE10GROUP II ENGINESECTION 5 FUEL SYSTEMThe Idle SystemFig 514The idle system controls the flow of fuel atidle speed and at slow speeds until the throttleis opened wide enough to allow the power fuelfeed system to functionWhen the throttle valve 6tion the edge of the valve is between the primaryidle orifice 78sure manifold vacuumorifice 7fuel bowl chamber 9the fuel bowl 110As the fuel travels through the idle fuel passage10the idle jet 11with air entering through the idle seat 12helps to atomize the fuel and this process isrepeated at the secondary idle orifice 8fuel travels through the idle fuel passage 10As this rich mixture of fuel and air emerges fromthe primary idle orifice 7rect proportions by the air which passes aroundthe throttle valve 6this valve must be slightlyopen to permit the engineto idle The resultant mixture is correct for operating engine at idle speedprovided the idle adjusting needle 13erly adjustedAs the throttle valve 6is slowly opened from theslow idle position it gradually subjects the secondary idle orifice 8take manifold vacuumand the secondary idleorifice 8air to the idle fuel quantity of fuel intothe engine This is proper since the throttle valveis now open wider and will admit a greateramount of air to blend with this additional fuelto maintain the correct proportions of fuel andair for the engineAs the throttle valve 6the idle fuel delivery begins to fade out however the throttle valve at this point is far enoughopen for the power fuel feed system to idle system as described above is the mostpositive and satisfactory of idle systems as it isworking under very high suction and the mixtureflows through the small passages and orifices atvery high velocities It is necessary to bear inmind however that there are times when thesesmall holes may become plugged with particlesof dirt or foreign matter and will require cleaning At such times the passages jets and smalldrilled holes should only be cleaned with acleaning fluid such as gasoline and air underpressure Never use drills or wires as a changein size of these small openings will change theentire calibration of the carburetorS 7GROUP II ENGINESECTION S FUEL SYSTEMPower Fuel Feed SystemNOZZLE AIR bt1I N 14111111 11111111t1r4 11of NOZZLE AIR VENT21THROTTLE VALVE6 FUEL BOWLFUEL JET20venturi 17At the sametime the fuel that is storedin the accelerating well16the nozzle air bleeds 15into the nozzle 14because the size of thepower jet 20position of the power adjusting needle 18the amount of fuel whichcan enter the nozzle 14the fuel in the accelerating well 16exhausted and air willthen enter through thenozzle air bleeds 15mix with the fuel passingthrough the nozzle 14The amount of air that canenter into the nozzle 14is limited by the size ofthe nozzle air vent 21POWER ADJUSTING NEEDLE SEAT19POWER ADJUSTING NEEDLE 18 tFig S 15With the throttle valve 6slow idle position fuel rises up through thenozzle 14and out the nozzle air bleeds 15to fill the accelerating well 16mately the height of the fuel level in the fuelbowl 1As the engine speed is increased from theslow idle position the air flow through the venturi17is gradually increased and as the idlesystem begins to diminish the velocity throughthe venturi 17sure at the tip of the nozzle 14slightly lessthan the pressure in the fuel bowl chamber 9and the accelerating well 16feeds Horn the fuel bowl 1ing between the power load18and the power adjusting needle seat 19through the power jet 2014to be discharged into the air stream at theThe result of air bleeding into the nozzle 14to help atomize or breakup the fuel into finer particles to regulate the quantity and the rate of discharge of the fuel fed from the 16the correct mixture proportions for full the throttle valve is opened toward thewide open position the velocity through the venturi 17pressure at the nozzle 14additional fuel being supplied to the engine asthe speed is increasedWhen the throttle valve 6denly from slow or just off slow idle positionthe fuel stored in the accelerating well 16forced out through the nozzle air bleeds 15very rapidly and serves to provide the extrarichness required by the engine to meet thesudden load When the throttle valve 6closed fuel again fills the accelerating well 16ready for the next II ENGINESECTION S FUEL SYSTEMBack Suction Economizer Systemamount of air that is drawn out of the fuelbowl chamber 9of the economizer jet 23tECONOMIZER 24valve 6tECONOMIZER JET23tmanifold vacuum or suction on the economizer orifice 246is opened from the fast idle position thetBOWL orifice 24tXNNtfrom the fuel bowl chamber 9posed to manifold suction and air flowsthe economizer jet 23mizer orifice 24placed by air entering through the bowlvent 2222enter the resultant pressure in the fuelbowl chamber 9ing the difference in air pressure betweenthe nozzle 149tarded so that the exact economy mixtureratio will be delivered to the engine at thisparticular throttle opening Opening thethrottle valve 6economizer orifice 24resulting in additional air being removedfrom the fuel bowl chamber 9out the mixture ratio to the correct proportionsfor this new throttle position After the economizer orifice 24suction the amount of air that is drawn out ofthe fuel bowl chamber 9manifold vacuum or suction at any given throttlevalve 6as the throttle approaches wide open positionless air is drawn out of the fuel bowl chamberand additional fuel flows to the engine to providethe extra richness required for operation atheavy loads where maximum horsepower isnecessaryThe Back Suction Economizer System assures the proper metering of fuel to the the service life of the carburetor asthere are no moving parts to wear out or adjustments to get out of order It is essential however that the system remain free of dirt andforeign matter because any foreign substancein the system will restrict the flow of air improper pressures in the fuel bowlchamber and resulting improper fuel deliveryto the engineFig S16The amount of fuel supplied to an engine iscontrolled by the size of the power jet the position of the power adjusting needle and thedifference in air pressure between the fuel bowlchamber and the venturi However in manyengines the mixture must be leaned out additionally during part throttle operation to obtainmaximum economy To provide this leanermixture MarvelSchebler Tractor and make use of the Back System With this method of metering fuel the air pressure in the fuel bowlchamber is regulated and controlled accordingto load conditions by a combination of bowlvent and economizer passages the throttle bore of the regulations of the air pressure in thefuel bowl chamber the fuel flow through thecarburetor can be controlled to provide the proper mixture proportions for the engineAll the air that enters the fuel bowl chamber9the bowl vent 2222can enter the fuel bowl chamber 9FUEL BOWL II ENGINESECTION S FUEL SYSTEMChoke S17The choke system is used during cold starting valve 25loaded relief valve 26This valve opens automatically with enginespeed and load and eliminates a great deal ofmanipulation of the choke on the part of the engine has obtained normal operating temperature the choke valve 25fully opened to assure maximum power andeconomy In addition extended use of the chokeresults in more gasoline being supplied to theengine than can be burned A large percentageof the unburned gasoline is lost through the exhaust system The remainder of the raw gasolineis forced between the pistons and cylinder wallswashing away the protective oil film and increasing engine wear and enters the crankcase whereit dilutes the engine oilAny adjustments that are necessary on thecarburetor should never be attempted until theengine has obtained its normal operating temperature and the choke valve 25placed in the wide open positionand the warmup period Under these cold conditions it is necessary to supply an additionalrich mixture of fuel and air as only the lightends or more volatile portions of the fuel willvaporize with the manifold and air temperatures at these cold conditions Consequently itis necessary that a large quantity of fuel beavailable so that there will be enough lightends to combine with the air to form a combustible mixture for starting the engineThe function of the choke valve 25restrict the amount of air that can enter thecarburetor and to increase the suction on the nozzle14so that additional fuel will be drawn intothe manifold As soon as the engine fires andruns the rich mixture must be rapidly reduced toprevent stalling This change in mixture is accomplished by the operator positioning the chokevalve to provide the proper mixture Howevera few degrees movement of the choke valve 25will make a big change in the mixture strengthand to help reduce the sensitivity of the ADJUSTING NEEDLE FUELIDLE ADJUSTING NEEDLE JETBOWL DRAIN ItPOWER ADJUSTING VENTFig S 19 Back Suction Economizer SystemGROUP II ENGINESECTION S FUEL SYSTEMModel TSX Carburetertive hole plugs and alsoto prevent vapor lock orpercolation of the fuelwhen the carburetor isoperated under extremely hot in hard starting or erratic engine operationThe Model TSXcarburetor is completely sealed against dust or dirtAll air entering the fuelbowl of the carburetormust first pass throughthe air cleaner Thethrottle shaft bearingsand choke shaft bearings are sealed to eliminate dust and dirt entering at these pointsThe back suction economizer system Fig S19 is provided with are movable economizerjet The size of this jethas been carefully established by engineering tests to provide theexact fuel maximum economy at part throttle operation Always use the economizer jet specified inthe individual carburetor service parts list toassure proper engine operation On some carbuFig S 18 Idle and Power Fuel Feed SystemsThe MarvelSchebler Model TSXCarbureter is manufactured in threeSAE nominal sizes 78 inch 1 inchand 114 inch In addition to these variations in size there are also by the specific requirement of the engines on which the carbureters are used Many engines forinstance require special throttle andchoke operating levers and for purpose of calibration they may have different size jets nozzles venturii etcFor this reason when ordering partsrefer to the individual carburetor service parts list for the engine on whichthe carburetor is installedThe Model TSX Carburetor consistsof only two major castings1The throttle body casting whichforms the cover for the fuel bowl2The fuel bowl casting which contains the air inletCast iron material is used for ruggedness It will be noticed Fig S18that all passages whenever possibleare drilled from the top face of thefuel bowl casting to prevent any fuelleaks to the outside of the of shrunken gaskets or defecR VALVEFLOAT VALVECHOKE VALVEGASOLINE DRAIN STRAINERGROUP II ENGINESECTION S FUEL SYSTEMModel TSX CarbureterFig S20 Float and Choke Systemsreter models the proper fuel requirements areestablished without the use of an economizer jetand the fixed economizer orifice machined in thecarburetor throttle body regulates the fuel supplied to the engine In addition there are engineand carburetor combinations that do not requirethe back suction economizer system In the economizer orifice has not beenmachined in the throttle body castingTo provide additional economy in addition tothe back suction economizer system some carbureters are provided with two adjusting needlesthe low speed or idle adjusting needle and thepower or load adjusting needle However thepower adjusting needle is not always requiredand for applications of this nature the fixed jettype carburetor is used in which the power jetcontrols the amount of fuel that is supplied tothe engineThere are two variations in carbureters having the power adjusting needle commonly calledthe adjustable jet type carburetor In Fig S18is shown these two arrangements The adjustment of either type is accomplished in the samemannerA large percentage of the Model TSX Carbureters are provided with an idle adjusting needlewhich alters the fuel and air proportions of themixture which enters the carburetor bore fromthe idle passagetThis is known as anair adjusting idle needle The upperinset in Fig S18 shows an idle adjusting needle which alters the amount offuel and air mixture which enters thecarburetor bore from the idle passageThis is commonly known as a fuel adjusting idle needle It is important toremember in setting the idle mixturethe air adjusting idle needle must beturned in or clockwise to enrich theidle mixture and the fuel adjustingidle needle must be turned out to enrich the idlemixtureA dual float mechanism Fig S20is used in a fuel bowl that almost completely surrounds the nozzle This design and construction is such that thetractor or engine can be operated atany angle up to 45 degrees affecting the fuel and airratio and without flooding because themean level at the nozzle tip is practically constant at any angle of operationSome carbureters are equipped with a springloaded governor control lever to permit manualclosing of the throttle to an idle position forengines equipped with certain type governorsAn example of this type lever is shown in Figure4 however there are other variations of thistype dependent upon the particular there are many variations produced bycombining the different types and sizes into aspecific application all Model TSX the same engineering principles andare alike from a functional standpointFig S21 SpringLoaded Governor II ENGINESECTION 5 FUEL SYSTEMService Instructions for Model TSX AFixed JettType BAdjustable JettType CAdjustable JetThe following procedure for service of all Model TSX Carbureters is for a complete overhaul Afterremoving carburetor from engine wash thoroughly with cleaning fluid such as gasoline to of external parts for damage For type carburetor being serviced see illustrations apply to all types unless specified Power AssemblyType B2Remove Bowl CoverScrews and Lock Float ValveBowl Gasket and VenturiIf Valve is grooved ordamaged replace Valveand Float Valve Seat4Remove Float Valve Seatand GasketRemove Economizer JetNOTE Not required in allcarbureters Check serviceparts list or repair kit ofcarburetor being Idle JetNOTE Not required in allcarbureters Check serviceparts list or repair kit ofcarburetor being Idle and SpringReplace with new Needleif grooved or damaged8Remove Throttle ValveScrews Valve andThrottle Shaft andLever with new shaftand lever assembly if excessive looseness betweenshaft and throttle bodyS139Remove Throttle ShaftPacking Retainerand PackingForce out Retainerwith small screwdriveror punch10Remove Main Nozzleand GasketType AType BtRemove Choke ValveScrews Valve ChokeShaft Lever AssemblyChoke Return Springand Choke Power JetType AType B1217Remove Power Throttle ShaftNeedle and RetainerType CtAssemble new retainerand packing on not havingtshaft Insert shaft inadjustable needle and tap lightlypower jettuntil retainer is flush withcasting face1318Remove Main NozzletInstall Throttle Valveand Gaskettand ScrewsType C Install valve with mark facingflange face of carburetorTap valve lightly to center in throttle bore Tighten screws Retainer Plugand Gasoline can only be replaced on carbureters having a curled hair or felttype strainer Only replace when impossible toclean with gasoline andcompressed air Porousmetal type strainer cannot be replaced Choke ShaftPacking Retainerand PackingForce out retainer withsmall screwdriveror II ENGINESECTION S FUEL SYSTEMBefore assembling carburetor clean castings channels and parts with carburetor cleaning fluidand air under pressure Make certain all small holes and channels are open and free from carbonand dirt Do not use wire or small drills to clean out small holes as a slight change in size of theseholes will affect the carburetor operation To assure a successful overhaul always replace all wornor damaged parts and any parts that are questionable Always use all new gasketsS14tR 1258254L To I32t4pj I momlayGROUP II ENGINESECTION S FUEL Economizer Jet20Install Idle Jet21Install Idle and SpringSet approximately oneturn from seat forpreliminary Float Valve Seatand GasketUse new Float Valve andSeat Bowl CoverGasket and Venturiin CastingInstall Float and LeverAssembly andFloat Lever PinSet floats 14 from gasketface to nearest edge offloat keeping edge of floatparallel with gasket Adjust by using bending Choke ShaftPacking Retainer retainer as shownin illustrations belowNote On some the packing is retained by choke bracket inplace of packing retainer25AOn carbureters counterbored Vs to h deep install retainer with cupfacing towards castingTap lightly until flush withcasting face25BOn carbureters counterbored to 14 deep install retainer with cupfacing away from castingTap lightly until flushwith casting face26Install Choke BracketChoke Return SpringChoke Shaft and LeverAssembly Valve valve in castingbefore tightening Power JetType AType BInstall Main Nozzleand GasketType AType BUse new gasketInstall float Main Nozzleand GasketType CUse new Power AssemblyType CUse new gasketSet approximately oneturn from seat forpreliminary Gasoline DrainStrainer Retainer PlugStake retainer plug inplace with center punch toinsure secure CastingsInvert throttle body andlower fuel bowl over floatstaking precaution thatventuri guides bodies Bowl Cover Screwsand Lock WashersTighten screws graduallyuntil all are tight34Install Power AssemblyType ASet approximately oneturn from seat forpreliminary settingGROUP II ENGINESECTION 5 FUEL ADJUSTMENTSSet throttle stop screw so that throttle valve is openslightly Make certain that fuel supply to carburetor isopen Close choke valve Start engine and partially releasechoke After the engine has been run sufficiently to bringup to operating temperature throughout see that choke isreturned to wide open positionLOW SPEED OR IDLE ADJUSTMENTSet throttle or governor control lever in slow idleposition and adjust throttle stop screw for the correctengine idle speed On a new stiff engine this speed mustbe slightly higher than required for a thoroughly runinengineto falter or roll from richness then turn needle in theopposite direction until the engine runs smoothlyNOTE It is better that this adjustment be slightly toorich than too leanIDLE ADJUSTING NEEDLEAIR ADJUSTINGTo richen the idle mixture turn the idle adjusting needleto the right or clockwiseIDLE ADJUSTING NEEDLEFUEL ADJUSTINGTo richen the idle mixture turn the idle adjusting needleto the left or OR LOAD ADJUSTMENT TYPE B TYPE CWith the engine running at governed speed under loadturn power adjusting needle to the right or clockwise alittle at a time until the power drops appreciably Thenturn the needle to the left or until theengine picks up power and runs smoothly This will givean economical part throttle mixture and due to theeconomizer action the proper power mixture for fullthrottle operation Due to variations in temperature orfuels it may be necessary to richen up this mixture bybacking out the power adjusting needle a small amountat a time until good acceleration is obtainedNOTE Carbureters TSX107 TSX330 TSX339 TSX355 TSX385 and TSX398 use the fuel adjusting type idle needle All other Model TSX Carbureters use the air adjusting type idle needleS16tR 1258254GROUP II ENGINESECTION S FUEL SYSTEMService IN OPERATIONThe idle construction used in and Industrial Carbureters is the mostpositive and satisfactory of idle systems becauseit is working under very high suction and themixture flows through the small passages at veryhigh velocities It is necessary to bear in mindhowever that there are times when these smallholes may become plugged with particles of dirtor lint but very seldom If idle trouble is experienced first check the manifold to cylinder headgasket and the carburetor to manifold gasketfor air leaks At slow idle an engine requiresonly approximately 20 to 25 lbs of air per hourand a slight leak will result in a very erratic orrough idling engineOther causes for a rough idling engine are uneven compression caused by sticky or leakingvalves leaking valve seats tappets with improper clearances leakage past pistons andrings cylinder head gasket leaking weakspark or spark plug points not spaced correctly ignition cable covering cracked and thus grounding spark and cable not assembled properly inthe distributor cap which causes corrosion andweak sparkThe spark timing of the engine is most important and should also be checked very carefully and set exactly on the mark as called forin factory standard specifications In fact allof the above items must be checked very carefully to factory standards and not just given acasual inspection with the common looks OK You can KNOW definitely that the tractor is up to the standards setby the AND are received from the field thatthe engine will not pull or develop its or that it develops good power butuses far too much fuel Too often a service manwill at once change the carburetor to correctthese complaints but by so doing he may not besuccessful in overcoming the difficultyIt must be clearly understood by all servicemen that when a new engine is designed anddeveloped the management first decides whathorsepower they want this engine to produce ata definite rated speed The engineering department develops the new engine to pull the required horsepower In the design there are certain fixed dimensions that never change Forinstance the bore and stroke the ratio diameter of valves lift ofvalves diameter of intake passage Thecarburetor engineer works out the diameter of throttlebore venturi size and provides for means ofadjusting and regulating the power fuel mixtureratio as well as the idle Now in service consider that the compression ignition and timinghave been checked and found to be 100 inthis engine If the air intake temperature andthe water temperature is held constant then theonly variable we have that affects is the fuel mixture ratioIf compression ignition and timing whichare variable are first properly checked by aservice man and set to factory little difficulty will be experienced in adjusting the carburetor to give the and economyA great deal has been said regarding the importance of engine tuneup and the reasons forservice men being exact in their service work onengines The reasons why a carburetor may notR to MeasureTSX 78 1 1 Ai t14 tFrom the gasket to the nearest surface of thefloatGROUP II ENGINESECTION S FUEL SYSTEMfunction properly when everything else has beenchecked and set to factory standards will nowbe coveredWith the present type carburetor on MarvelSchebler tractor and not very much can go wrong withthe possible exception that it may foul with dirtThere are only two places that are subject towearthe throttle shaft and bushings and thefloat valve and seat The wear on the throttleshaft and bushings and resultant air leak therefrom results in a lean idle and to compensatefor the air leak more fuel must be turned on foridle Wear on the float valve and seat results ina high fuel level in the fuel bowl and Both faults can be easily observed bythe service man and corrected by replacingworn parts with new ones The proper functioning of the carburetor is obtained by a series ofholes drilled to exact size and location whichdo not wear or change location in service Itmust be realize that if the carburetor at first when passed by the inspectorsat the tractor factory it will always function thesame provided these passages are all free fromdirtOn a carburetor complaint from the field theonly thing a service man can do to thecarburetor is to disassemble it BE SURE that thepassages are open and free from dirt that thereis no wear on the throttle shaft and bushingsthat float valve and seat are OK that the floatheight is correct and that a good airtight sealexists around the bowl gasket If such does not correct the complaints a complete check of the engine must again be madeTo check the float setting the casting must be held in an inverted position so that thefloat lever is in contact with the float valve and the float valve seatedNOTE Changing the float setting from our standard in an effort to improve theoperation of the carburetor or in an effort to prevent flooding will only result infaulty carburetor with permission Products Division BorgWarner INLETFUEL VALVESEATFUEL BOWLIrt II ENGINESECTION S FUEL SYSTEMZENITH 61 AND 161 SERIES CARBURETORSFig S22The Zenith 61 and 161 Series carburetors areof updraft single venturi design They are madein 58 and SAE barrel sizes with 58 781 and 114 SAE flange sizes available Theyare made with selective fuel inlet with or without a back suction economizer and a main are balanced and sealed and thesemi concentric fuel bowl allows operation toquite extreme angles without flooding or starvingThis design makes them particularly adaptable tosmaller farm tractors and a great variety ofagricultural machines and industrial unitsMODEL and bowl castings cast ironStylesA Throttle and choke shafts with degasser assemblyEElbow air intakeJ Backsuction through air intakeX Flange next size larger than standardXX Flange second size larger SizeSAE Standard558787 ortIi781023 or 1A78X7781023 or 1121XX7781023 or 1A114811181 or 1A1FUEL SUPPLY SYSTEMThe fuel supply system is made up of thethreaded fuel inlet the fuel valve seat fuel valvefloat and fuel bowlThe fuel supply line is connected to the threaded inlet The fuel travels through the fuel valveseat and passes around the fuel valve and into thefuel bowl The level of the fuel in the fuelchamber is regulated by the float through itscontrol of the fuel valve The fuel valve does notopen and close alternately but assumes an opening regulated by the float sufficient to maintaina proper level in the fuel chamber equal to theFUEL SUPPLY SYSTEMFig S23demand of the engine according to its speed andloadThe inside bowl vent as illustrated by the passage originating in the air intake and to the fuel bowl is a method of ventingthe fuel bowl to maintain proper air fuel mixtureseven though the air cleaner may become restricted This balancing is frequently referred toas an inside bowl ventS19PRIMING PLUGIDLE PLATEIDLE 10510N1IDLE AIRFROM BEHINDIDLE JETIDLE FUELPICKUP t Awe r II ENGINESECTION 5 FUEL SYSTEMIDLE SYSTEMThe idle system consists of the idle dischargeport idle air passage idle adjusting needle idlejet and fuel passageThe fuel for idle is supplied through the mainjet to a well directly below the main dischargejet The pickup passage is connected to thiswell by a restricted drilling at the bottom of thispassage The fuel travels through this channelto the idle jet calibration The air for the idlemixture originates back of or from behindmain venturi The position of the idle in this passage controls the suction on theidle jet and thereby the idle mixture Turningthe needle in closer to its seat results in a greatersuction with a smaller amount of air and therefore a richer mixture Turning the needle outaway from its seat increases the amount of airIDLE SYSTEMFig S 24and reduces the suction and a leaner mixture isdelivered The fuel is atomized and mixed withthe air in the passage leading to the dischargeport or priming plugat this pointHIGH SPEED SYSTEMThe high speed system controls the fuel mixture at part throttle speeds and at wide openthrottle This system consists of a venturi controlling the maximum volume of air admitted intothe engine the main jet which regulates theflow of fuel from the float chamber to the maindischarge jet the well vent which mixture ratio under changing suctionand engine speeds and a main discharge jetwhich delivers the fuel into the air streamThe main jet controls the fuel delivery duringthe part throttle range from about onequarterHIGH SPEED SYSTEMFig S25to full throttle opening To maintain a propermixture ratio a small amount of air is the well vent into the discharge jetthrough the air bleed holes in the discharge jetat a point below the level of fuel in the passage of fuel through the high speedsystem is not a complicated process The fuelflows from the fuel chamber through the mainjet and into the main discharge jet where it ismixed with air admitted by the well vent andthe airfuel mixture is then discharged into theair stream of the carburetorBACK SUCTION ECONOMIZER SYSTEMFig S 26ECONOMIZER SYSTEMThe economizer system consists of a milledslot in the throttle shaft which acts as a valveto open or close the system a vacuum passagefrom the throttle bore to the slot in the throttleS20tR I258254GROUP II ENGINESECTION 5 FUEL SYSTEMshaft and a vacuum passage from the slot in thethrottle shaft to the fuel bowlThis system allows economical fuel mixtureratios for part throttle operation while still permitting the richer mixture ratios that are neededfor full load operationThe economizer system performs its functionby establishing a back suction on the fuel inthe fuel bowl during most of the part throttlerange of operation This back suction is createdby manifold vacuum through the channels connecting the throttle bore with the fuel bowl Thisretards the flow of fuel through the and thus permits the carburetor to operate on leaner part throttle mixture ratiosThe rotation of the throttle shaft controls theeconomizer system During part throttle operation from about onequarter to the passages are open and the pressurein the fuel bowl is lowered This retards the flowthrough the main jet and a leaner mixture issupplied pn full throttle opening the passagesare closed and the main jet flows to full capacityto supply the richer mixture requiredCHOKE SYSTEMThe choke system consists of a valve mountedon a shaft located in the air entrance and operated externally by a lever mounted on the shaftThe choke valve is used to restrict the air entering the carburetor This increases the suction onCHOKE SYSTEMFig S27the jets when starting the engine The chokevalve is of a semiautomatic type having apoppet valve incorporated in its design which iscontrolled by a springThe poppet valve opens automatically when theengine starts and admits air to avoid overchokingor flooding of the engine The mixture requiredfor starting is considerably richer than thatneeded to develop power at normal temperaturesAs the engine fires and speed and suction are increased the mixture ratio must be rapidly reduced This change is accomplished through adjustment of the choke valve and the of the poppet valve to admit more airwhen the engine firesSERVICE AND REPAIR to top of float bowl cover against specification chart The inside number nextto the rivet is the Zenith outline assembly numberand the one next to the outer edge of the disc is thevehicle VIEWaThe disassembled view will identify the various component parts and show their relation to assemblyUse the disassembled view with the identifying partnumbers to identify and locate parts when performing the disassembly and reassembly OF TOOLS AND REPAIR PARTS KITaThe use of the proper Zenith tools and the properrepair parts kits is essential if the best service andrepair procedure is to be preformed on the carburetor The following list of Zenith special tools andgeneral hand tools will best perform the service jobbZenith Special Jet Discharge WrenchC161711 Line ReamerC161721 Bushing DriverC161731 Counter Bore Valve Seat Jet WrenchcGeneral Hand Tools716 Open End Wrench12 Open End Wrench14 Blade Screw DriverLong Nosed Pliers6 Depth Gage14 Round FileLight HammerLong Rod or II ENGINESECTION 5 FUEL SYSTEMdtor except model 61A8SRD is No K501 A properrepair job cannot be performed however by usingthe basic kit as such The basic kit must be tailored to fit the particular outline of the serviced by the addition of the parts listed inlarge print on the label of the basic kit container781 tt3Fig S28D SEPARATE CARBURETOR BODIESaRemove the hex plug or filter screen 15of throttle body 2bRemove the four assembly screws 52washers 512the fuel bowl 49cSeparate the throttle body 2assembly 49E DISASSEMBLE THROTTLE BODYa1Press screwdriver against float axle 22slotted side of float hinge bracket and forcethrough hinge bracket2Remove float axle 22from opposite side and remove float 21b20c23chined surface of the throttle body Zenith Tool No of throttle body 220g17spring 162h17washers 8141Unscrew throttle stop screw 9end is flush with lever 132Make match marks with file on throttle body2ble these parts in the same position as removed3Loosen throttle clamp lever screw 11andremove lever 1014Some 161J Series Carburetors have the throttlelever and the throttle stop lever riveted together Omit Step No 3used4File off the riveted or peened end of the throttleplate screws 7NOTE When such screws are riveted or peened thethreaded end of the two screws must befiled flat before removal to avoid breakageor stripping of threads in the shaft Insome cases it may be necessary to use asmall 14the surface of the shaft because of a slightcounter bore around the screw holeBe sure to avoid striking and cutting the side of thethrottle body bore or the throttle plate when filingthe screws5Remove the screws 7plate 16Remove the throttle shaft and stop lever assembly 142i4retainer 5follows1Screw a 516 fine thread taper tap into packing retainer 52Insert long punch or rod through opposite shafthole and drive punch against the end of the tapuntil retainer for other packing and retainerNOTE Do not disassemble the throttle plate 1throttle shaft and stop lever assembly 14throttle packing 452throttle shaft is bent or otherwise damagedor unless there is damage to any of theother component parts of the throttle II ENGINESECTION S FUEL SYSTEMF DISSASEMBLE FUEL BOWL BODYa56and fibre washer 55body 49NOTE Some models will have a 12 plug hexplace of the the drain plug hex50bottom of fuel bowl 49C16110Remove main jet 5453threaded passage in bottom side of fuel bowl 49with Zenith Tool No C1611Remove main discharge jet 2526face of fuel bowl 49Remove well vent jet 27opening in machined surface of the fuel bowl 49with a small choke as follows1Remove the bracket spring 37lever 35282Make match marks with a file on air shutterbracket 284935in the same position as removed3Remove the choke shaft nut 33washer 324Remove the choke lever 355Remove the choke bracket screw 3612 open end wrench and remove choke the shaft hole plug 44washer 457Remove the choke plate screws 42washers 4140and choke plate 43NOTE Some models of the Zenith 161J Series carburetor employs choke shaft packing washers 3938in the choke shaft holes around the chokeshaft The disassembly of these packingwashers and retainers should be performedin the same manner as the disassembly ofthe throttle shaft packing washers and retainers which is described in detail in thedisassembly of the throttle bodyNOTE Do not disassemble the choke 283540plate 43one of the above mentioned parts or damageto any of the other component parts of thechoke AND INSPECTION OF PARTSA CLEANING PARTSaClean all metal parts thoroughly with and rinse in solventbBlow out all passages in the air intake and fuelbowl casting 492sure all carbon deposits have been removed fromthrottle bore and idle port It is advisable to reverse flow of compressed air in all passages toinsure that all dirt has been removed Never use awire or drill to clean out jetsB INSPECTION OF PARTSaFloat Assembly Replace float assembly 21loaded with gasoline damaged or if float axle bearing is worn excessively Inspect top side of floatlever for wear where it contacts fuel valve needleNOTE Such wear can affect the float levelbFloat Axle Replace if any wear can be on the bearing surfacecFuel Valve Seat and Needle Assembly Alwaysreplace fuel valve seat and needle 20both parts wear and may cause improper float leveldIdling Adjusting Needle and Spring Inspect pointof needle Plate Inspect plate 1damaged edges Never clean a throttle plate witha buffing wheel or sharp Plate 43damaged edgesgChoke Shaft Check bearing surfaces for wearsee that shaft 40hGaskets Replace all gaskets and fibre washersevery time the carburetor is Shaft Replace if throttle shaft 6evidence of wear on the bearing surfacesjCheck Specifications Use the outline and verify the correctness of the followingparts Numbers shown on chart will be found onparts The following calibrated parts should bechecked Venturi Main Jet Discharge Jet WellVent Jet Idling Jet and Fuel Valve FUEL BOWL BODYa1Install the two choke shaft packings 39and retainers 3849follows Use bushing driver tool packing 3938place completed assembly on bushing drivertool with packing facing small end of toolS23GROUP II ENGINESECTION S FUEL SYSTEM3Insert small end of tool into choke shafthole start retainer 38body 4938body 49face4Insert choke shaft 40495Insert choke plate 4349NOTE Be sure the choke plate 43in the same position in the air intake 49as regards the poppet valve as when itwas choke plate screws 42washers 417Install the shaft hole plug 44washer 4512 open the choke bracket 28on the air intake 49bracket screw 36open end wrench9Place the choke lever 354033using Zenith Tool No C1612510Attach the choke lever spring 37choke bracket 2835NOTE Use the Match Marks put on the chokelever 3528take body 49erly align the choke assembly during main discharge jet 252649Zenith Tool No C16125Install well vent jet 2749tighten with a small main jet 5453threaded passage beneath the fuel bowl 49Zenith Tool No C1611Install the drain plug hexbottom of fuel bowl using Zenith C16110 wrenchInstall main jet adjustment 5612 hex plug57B THROTTLE BODYNOTE Any throttle body of a Zenith 161 Series carburetor can have throttle shaft bushings installed to return it to factory specifications asregards fit of the throttle shaft If the fit ofthe throttle shaft is sloppy in the throttle bodyand it is desired to use the same throttle bodyfor reassembly of the carburetor then it isabsolutely necessary to install throttle shaftbushings A poorly fitting throttle shaft upsetsidling of the engine for the throttle plate willnot be correctly located in reference to the idledischarge port and also it is possible for additional air to be admitted into the throttle bodyaround the shaft which will also tend to upsetthe idleThe following procedure should be adhered toto properly install throttle shaft bushings inthe Zenith 161 Series To properly rebush the throttle body of theZenith 161 Series carburetor it is to have available the proper counterbore reamer and line reamer and the bushingdriver tool needed to install the new reamer No C161731 line reamerNo C161711 and bushing driver No C161721are used The bushing itself is CR913 Afterthe new throttle shaft bushing is in place it willbe necessary to redrill the economizer restriction located in the cover and the channel fromthe throttle body bore into the throttle shaftholeTo obtain the correct drill sizes for this operation consult the specification card covering theparticular outline in question To drill the channel from the throttle body bore into the throttleshaft hole it will be necessary to remove thebrass channel plug in the throttle body Thiscan be drilled out using a 332 drill and a newoversize plug No CR13710stalled after the drilling operation is completedThe throttle body should not be rebushed if theextent of wear of the throttle body and shaftdoes not warrant it However if the wear issevere enough to warrant a rebushing job thefollowing procedure should be followed1Place a suitable center in the drill press bedWith one throttle shaft hole on this centerbring the spindle down until the contacts the opposite shaft hole Thereamer in this instance is of a diameter toresult in a press fit for the outside diameterof the throttle shaft bushing2With the casting still in place as described inthe above paragraph set the stop on the pressto the length of the bushing This will giveyou the approximate setting of the hole is then counterbored to accommodatethe bushing4A throttle shaft bushing is driven into placeusing the proper bushing driver tool5And this bushing is then reamed with the linereamer Use the opposite shaft hole as a pilotto align the line reamer in the bushing6Now turn the casting over and prepare theopposite hole to take the bushing It will benecessary to reset the stops on the spindle againas described before Then counterbore the hole7Drive the second throttle shaft bushing line ream the inside diameter as the finalmachining operationThe casting is now ready for II ENGINESECTION 5 FUEL SYSTEMNOTE A lathe may be substituted for the drillpress in performing the counterboring andline reaming 52bushing driver tool Zenith packing 45place completed assembly on bushing drivertool with packing facing small end of tool2Insert small end of tool into throttle shaft holestart retainer 52and lightly drive retainer 52until it is flush with machined surface NOTEThe packing retainer 5machined surface or slightly below to avoidstriking throttle lever the throttle shaft and stop lever assembly 1422Rotate throttle shaft 6insert throttle plate 1position holding the plate in position with fingers3Start throttle plate screws 78sure that the throttle plate 1tered in the throttle body boreNOTE The screw holes in the throttle plate are offcenter Start the side of the throttle platewith the shortest distance between the screwholes and beveled edge into the shaft firstThe throttle plates are made with two opposite edges beveled to fit the throttle bodybore when the plate is closed The throttleplate will not close tightly if installed upsidedown To properly center the plate in thethrottle body bore the screws should bestarted in the shaft and then with the plateclosed it should be tapped on the mountingflange side Pressure on the plate must bemaintained with the finger until the screwsare tightened When properly installed theside of the throttle plate farthest awayfrom the mounting flange will be alignedwith the idle port when the plate is closedInstall throttle clamp lever in same position as removed Refer to match marks placed on lever andthrottle body during disassembly stepInstall idle adjusting needle 1716211h full turnsInstall idle jet 18machined fuel valve seat 2019using Zenith Tool No C16182hPlace new throttle body to fuel bowl gasket 23machined surface of fuel bowl cover 2iInstall fuel valve needle 2020by float 2122Fig S29The A dimension should be 1 532 plus or minus 364jcorrect measurement to obtain proper float levelusing a depth gage Obtain float setting measurement from outline specification chart NOTE Donot bend twist or apply pressure on the float bodies1With bowl cover assembly 2position viewed from free end of float 21float bodies must be centered and at rightangles to the machined surface The float setting is measured from the machined surfaceno gasketat highest point2Bending Float Lever To increase or between float body and machined surface use long nosed pliers and bend lever closeto float body NOTE Replace with new float ifposition is off more than 116k24opening firstC ASSEMBLE CARBURETOR the two completed bodies 2 and 49four screws 5251screws evenly and firmlybInstall the hex plug or filter screen 15passage in throttle body 2C16110 wrenchcHold the throttle lever 13turn the throttle stop screw 9contacts the stop on body 29in 112 additional turnsPrinted with permission ofZENITH CARBURETOR DIVISION AVIATION CORPORATIONS25


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