Carburetors 4-9-2010

OriginalEquipmentWhenyourModelHtractorleftthefactory,itwasequippedwithDLTX26fromH1000thruH26999.BeginningwithH27000,DLTX46wasinstalled.Themaindifferenceisinthechokeassembly,the46havingachokeleverthatcouldbecontrolledremotelybyasmallrodfromtheoperatorspaneltoaccommodateEstarttractors;EstartoptionofferingatH27000.Figure800illustratesbothcarburetors,eachwiththechokeinengineoperatingposition.DLTX46cansuccessfullybeusedonallmodelHtractors,andislistedasthegenuineJDreplacementforallHtractors.

Figure 801. DLTX26 Figure 802. DLTX46

UniversalReplacementsTowardtheendofthe1960s,MarvelSchebler/ZenithcollapsedalltheirDLTXseriescarburetorsforJohnDeereintothreecarburetorstheDLTX107U,designedtoreplacethesmallborecarburetors(fortheBandH),theDLTX108Uforthelargeborecarburetors(fortheA,DandG),andaDLTX109Uduplexcarburetorforallthenumberedseriestractors.Andso,locatingaModelHtractorwithaDLTX107Uisentirelypossible,andwouldbeawelcomefindindeed!(Credit:DuaneLarson).

Theory of Operation

What is a carburetor? What does a carburetor do? Bydefinitionacarburetorisadeviceformixing(orcharging)airandgas;specifically,adeviceinwhichairismixedwithgasolinespraytosupplyavaporized,explosivemixtureforaninternalcombustionengine.Themixisnolongerplain,rawgasoline,butamorevolatilegaseouscompound.(WebstersCollegiateDictionary).About Your John Deere "H" Carburetor MostalltwocylinderJohnDeeretractorcarburetorbodiesaremadeofcastiron,andthisistruefortheHtractor.Theyhavemovingpartsthatmayhavebecomeworn,orificescloggedwitholdfuel,andthefloatmayhavebeenabusedand/ormayhavedevelopedaleak.Theinternalfuelandairpassagesoncecoatedwitharustpreventative,nowarenolongerbeingsoprotected.Thus,asidefromweartomoving

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parts,themainbodyispronetobecomefilledwithrust,cloggingthoseairandfuelpassages,andyoucannotboilrustout.Ifyoustartedoutwitharunningtractor,youareaheadofthegamebecauseyouknowthecarburetorisbasicallyfunctional,andyoucanjudgeitsperformance.WhileIhesitatetoeverrecommendacustomrebuildkitfortheDLTXcarburetor,suchasolutionmayindeedbeappropriateforrunningtractors.Ingeneral,however,Iprefertoletthecarburetortellmewhatisneeded.Andthisinformationisevidentonlyafterdisassemblyandareallycloseinspectionoftheoperatingparts.Asanexample,ifwiththenakedeyeyouareabletoseewear,ordeformation(duringyourcloseinspection)ofeitherofthetwooperatingneedlevalvetapers(IdleorLoad),itstimetorenewsuchaneedle!Anotherareatopaycloseattentiontoishowmuchplayyoufindinthethrottleshaftversusitstwobushings.Thisshaftanditsbushingsseealotofaction,movingeachandeverytimethegovernordrivesthethrottle.Thetopbushingispronetofasterwearbecauseitisclosesttotheactingforces.Thethrottleshaftfitintoitsbushingsneedstobeveryclosetotight,butnotabindingtightfit;amachinistssnugfit.Suchaclosefitisnotnearlyascriticalwiththechokeshaftaswearedealingwithaironlyonthechokesideofthecarburetorsventuri,andleakageherewillhavenoeffectonfuelairmixratios.(Note:Venturiisthenarrowedplaceinthecarburetorsthroatorbarrel).Ontheotherhand,startingwithatractorthatyouhaventheardrun,thatmaynothaverunforyears.andisnotnowrunning,putsyouatadisadvantageinsofarasthecarburetorisconcerned.Youhavenoclue.Youarebeginningwithwhatmustbeassumedtobeadysfunctionalcarburetor.Youcleanandrebuild,replacingunserviceablepartsasfound.Thisapproachassumesplentyofwear,andthatanypassagewaythatcouldberustedshutISrustedshut;andyouproceedfromthispointforward.

The Basic Systems of the Natural-Draft Carburetor 1. Idling System

Figure 803 - Idling System Diagram

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Whenthethrottlevalveisclosed,theidlingsystemsuppliesjustenoughfuelairmixturetokeeptheenginerunning(slowidle).Theclosedthrottlereducesairflowthroughtheventuritoalevelthatcannotovercometheresistancetoflowofloadfuelupthroughthefuelnozzle,butalsomeansthatafairlysignificantvacuumoccursontheenginesideofthethrottlevalve.Thismanifoldvacuumissufficienttopullthefuelairmixturethroughaseriesofsmallchannelsandopeningsthatbypassthethrottlevalve.Airentersthroughasmallpassageinthecarburetorbodyattheventuriridge(notthrutheventuri)andpassesacrossanidleadjustingneedleseat.Atthispoint,theairpicksupfuelandthefuelairmixtureisdeliveredtotheenginethroughaseriesofdrilledpassagewaysleadingtoanopening(theprimaryidleorifice)ontheenginesideoftheclosedthrottlevalve.Asecondaryidleorificefeedsairintothefuelairstream,thinningdownthefuelairmixturetosatisfythesloweridlerequirements.Theamountoffuelenteringtheairstreamcanberegulatedbytheidleadjustingneedle.Themanifoldvacuumontheenginesideoftheclosedthrottlevalveprovidestheneededpressuredifferentialacrossthethrottlevalvetocausefuelairmixtoflowinthatdirection.TheamountoffuelisdeterminedbyadjustmentoftheIdleneedlevalve.Observethatairflow,anenginefunction,isdeterminedbytheconditionofthevalvesorpistonrings,ortheenginesabilitytodrawtheairfuelmix.Theresultisanentirelyseparatecarburetorcircuitthatbyitselfoperatestofeedtheenginewhenthethrottlevalveisclosed.Asthethrottleisopenedupslightlyfromthefullyclosedposition,thesideoftherotatingthrottleplateopeningtowardtheventuri,exposesthesecondaryidleorificetomanifoldvacuumallowinggreaterflowofaleanerfuelairidlemix.Andatthesametime,alsoprovidingatransitiontometeredfuelflowthroughtheregularopenthrottle(load)circuit.Asthethrottleisprogressivelyopened,themanifoldvacuumreducessincethereislessrestrictiononairflow,reducingtheflowthroughtheidlecircuit.Thisiswheretheventurishapeofthecarburetorthroatcomesintoplay,duetotheBernoullieffect(i.e.asthevelocityofagasincreasesthroughafixedorifice,itspressurefalls).Asthethrottlevalvecloseswhenloadisrelaxed,theairflowthroughtheventuridropsuntilthereducedpressureisinsufficienttomaintainloadfuelflowandtheidlecircuittakesoveragainasdescribedabove. 2. Load System Thissystemmetersfuel,anddeliverstheproperfuelairmixturetotheengineinallgovernedrangesofspeedandloadaboveidling.Theamountoffuelenteringthenozzleisregulatedbyaloadadjustingneedle.Acarburetorbasicallyconsistsofanopenpipe,thecarburetorsthroatorbarrel,throughwhichairpassesintotheinletmanifoldoftheengine.Thepipeisintheformofaventuri,thatis,itnarrowsincrosssectionandthenwidensagain,causingairflowtoincreaseinspeedthroughthenarrowestpart.ThisiswheretheBernoulliprinciplecomestolife;i.e.,thatmovingairhaslowerpressurethanstillair,andthatthefasterthemovementoftheair,thelowerthepressure.Generallyspeaking,thethrottleoracceleratordoesnotcontroltheflowofliquidfuel.Instead,itcontrolstheamountofairthatentersthecarburetor.Fasterflowofairandmoreairenteringthecarburetordrawsmorefuelintothecarburetorduetothepartialvacuumthatiscreated.Thethrottlevalveislinkedtothegovernorthatisdesignedtoregulateenginespeedbysensingsmallchangesinenginespeed(flyingweightsystem);openingthethrottlevalveasspeedslows,andclosingthevalveasspeedrises,andreachesitsregulatedrate.

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Figure 804 - Load System Diagram

Fuelisintroducedintotheairstreamthroughthefuelnozzlepositionedatthenarrowestpartoftheventuri.Therateoffuelflowinresponsetoaparticularpressuredropintheventuriisafunctionoftwovariables,oneistheloadadjustingneedlevalve(theequivalenttoafixedfueljetinanautomotivecarb),andtheotherisfuelheldinreserveintheacceleratingwellwhichsurroundsthefuelnozzle. 3. Accelerating System Wheneverthethrottleisopenedquicklytogiveextrapowerforasuddenload,anextraamountoffuelisrequiredforamomentarilyricherfuelairmixture.Theinertiaofliquidfuelbeinggreaterthanthatofairmeansthatwhenthethrottleissuddenlyopenedinreactiontoasuddenload,airflowwouldincreaseaheadoffuelflow.Werethistooccur,thistemporaryleanconditionwouldcausetheenginetohesitateorbogdownunderacceleration.Inautomotivecarburetors,thissuddendemandforarichfuelairmixissatisfiedusingasmallplungertypeacceleratorpumptoinjectextrafuel,fuelabovethemeteredamounttocoverthisleanperiod.Inthis,andinmostcarburetorsusedbyDeere,anacceleratingwellprovidestheextrafuel.

Figure 805 - Accelerating System Diagram

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Whentheengineisidling,fuelrisesinsidetheloadnozzleandpassesthroughholesintheside(wall)ofthenozzleintothechamber,orwell,surroundingthenozzlewhereit(thefuel)accumulates.Whenthethrottlevalveissuddenlyopened,thefuelstoredintheacceleratingwellgushesthroughtheholesinthesideofthenozzlewithoutbeingmeteredbytheadjustingneedleandcombineswiththenormalflowinthenozzle.Thetwoquantitiesoffuelentertheairstream,makingamuchricherairfuelmixturetosatisfythesuddenrequirementformorepower.Asthefuelsupplyintheacceleratingwelldiminishes,theholesuncoveredbythefuelleavingbecomeairbleeds,satisfiedbytheLoadFuelAir(in)passageatthetopoftheacceleratingwell.Aftertheacceleratingwellisdrained,itremainsemptyuntilthefuelconsumedbytheengineislessthanthefuelflowingintotheacceleratingwell(throughtheLOADadjustingneedle),atwhichtimeitrefillswiththisexcessfuel.Thisoccurswhenthethrottlereturnstoa1/4loadortofastidleposition. 4. Economizer System - A System of Pressure Differentials (Figures 806 & 807) Theeconomizersystemretardstheflowoffueltotheengineatpartthrottlespeeds,whenfullcapacityofthefuelnozzleisnotrequired.Fuelisdeliveredoutofthemainnozzleopeningbyadifferenceinairpressureatthetopofthenozzleandthepressureonthefuelinthebowl.Toregulatethispressureinthepartloadrange,apassagewayisdrilledfromtheaircleanersideofthecarburetorbody,throughatrapconstructiontowardthecenterofthebodyandcontinuingtothemanifoldsideofthecarburetor.Thispassagewayhasanopeningintothebowlnearthetrapsection(bowlvent).

Figure 806 - Economizer System Diagram Locatedinthisdrilledpassageway(towardtheenginemanifoldend),thereisacalibratedeconomizerplugmountedintoanalsocalibratedannulusorchannel.Theseworktogethertoregulatetheexactpressuredesiredonthefuelinthebowl,soastoprovidemaximumeconomythroughouttheentireoperatingrange.Whenthethrottlebutterflypassestheopeninginthepassageway,thepressureisreduced.Thisreductioninpressureiscarriedbackthroughthepressureregulatingopening,enablingthecarburetortoautomaticallyadjustthepressureonthefuelinthebowlinaccordancewithenginerequirements.Thismeansthatforlightloadsasmallerquantityoffuelwillbedeliveredtotheengine.

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Figure 807 - Economizer Path, Air (left) to Manifold end (right)

RoughTerrainIfyoueveroperatedaJDA,JDBorJDGtractoroverroughterrainsuchasacrosscornrowswhilemakingaturn,youalsoprobablysometimesnoticedthetractorseemedtotakeontoomuchfuel,andwouldsmokealittleandfalterormiss.Thisisduetofuelbeingsloshedupintothebowlventtrapsection.Thefuelupsettheongoingchangesinairpressureonfuelcontainedinthebowlthusfeedingexcessfuelintotheengine.Somewoulddescribethisasatemporarilyfloodingcondition,andthatisbasicallycorrect.InAugustof1953,Deereissuedinstructionsformodifyingthebowlventtrapsoastocorrectthismissingoftheengineastheycalledit.Thiswasprettylateintheproductionofthesetractors,butillustratesthateconomizerproblemswerebeginningtosurface.SeeFieldServiceBulletin(orFSB)No.206,August1953,pages62&63.Blockage,CompleteorPartialForcarburetorsusedonmanyoftheAandBtractors,thereexistapairofbrassbafflesinthebowlventtraptoallowairtopassbutblocksomeofthefuelsloshing.Asinallpartsofthefuelsystemsubjecttocloggingbyvarnishbuildup,thesetoocanbecomerestrictive.Whentheydo,performanceoftheeconomizersystemisspoiled.TroubleShootingSeveralfactorsmayaffectperformanceoftheeconomizersystem.Oneisapartialorfullblockageofthepassagewayconnectingtheeconomizer,bowlventandbowlventinairopening.Accumulationofdebrisintheannuluscanalterthemeteringfunctionoftheannulusandeconomizerplug.Finally,thereistheeconomizerplugitself.Youshouldseealetterstampedontheplug.Comparetheletteryouseewiththeletterlistedforyourcarburetortoensureyouhavetherighteconomizerplug.SeetheCarburetorIDChart(FSB182S,August1951,orwww.jdhpubs.com).Ifthelettercannotbereadduetosandblastingorotherabrasiveprocesses,renewaloftheplugisrecommended.5. Choke Whentheengineiscold,fuelvaporizeslessreadily,andignitionandcombustionaremoredifficulttosustain;thusarichermixture(morefueltoair)isrequiredforstarting.Toachievethis,achokeisused;thisisadevicethatrestrictstheflowofairattheentrancetothecarburetor.Thisfunctionssimilarlytothethrottlebeingclosed,exceptforthefactthatitisclosedoffbeforebothidleandloadcircuits.Here,

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thelowpressurecausedbytherestrictionsucksadditionalfuelthroughbothofthefuelcircuits,idleandload.6. Float Chamber Toensureareadysupplyoffuel,thecarburetorhasafloatchamber(bowl)thatcontainsaquantityoffuelreadyforuse.Thisworkssimilarlytoatoilettank;afloatcontrolsaninletvalve.Whenthefloatdrops,theinletisopenedallowingthefueltoflowintothefuelbowlunderthegravityfeedsystempressure.Andasfuelflowcausesthefloattorise,thisrisingactionclosestheinletvalve.Theleveloffuelmaintainedinthefloatbowlcanbeadjustedbybendingthesmalltabthatmakescontactwiththefloatinletvalve.Thisisafairlycriticaladjustmentcriteriadiffer,butforJDHcarburetors,wehavethreeindicatorsofacorrectfloatsetting.Adjustsothat:(1)fuelstopsflowingasthefloatcomestoalevelposition,(2)thereis3/8inchbetweenfloatandbodycasting,and/or(3)thebowlwillfilltoapoint3/4inchdownfromitstop,andthenstabilize.Thesefloatscanspringleaksandlosetheirfloatation.Inspectfloatforcracksorunsolderedjoints.Totestforleaks,immersethefloatinhotwater.Appearanceofbubbleswillindicateleakage.Donotattempttorepairadamagedfloat.Theoriginalfloatsweremadeofnaturalcorkandweresealedwithaspecialvarnishcoating.However,thevarnishwouldeventuallydeteriorateandfuelwouldsaturatethecork,makingituselessasafloat.Hollowmetalfloatswereintroducedin1946(FSB153S,61546).Itshighlyunlikelythatacorkfloatwouldhavesurviveduntiltoday!Replacecorkfloatsiffound.Overalongperiodoftime,thefloatneedlevalveitselfbecomeswornbythefuelflowandthusfailstoshutoffcompletely.Thiswillcauseexcessivefuelflowandpoorengineoperation.Conversely,asthefuelevaporatesfromthefloatbowl,itleavessediment,residue,andvarnishesbehind,whichclogthepassagesandcaninterferewiththefloatoperations.Thisisparticularlyaprobleminantiquetractorsoperatedforonlyapartoftheyearandlefttostandwithfullfloatchambersformonthsatatime;commercialfuelstabilizeradditivesareavailabletoreducethisproblem.

Diagnosing Faulting Carburetors PartialQuotefromRobertsCarburetorRepairWebpageAlthoughmanythingscancauseyourtractortoidleincorrectly,therearethreepassagesinthecarburetorthatmostoftenwillnotletyourtractoridlewell.Threepassagesinthecarburetoridlesystemare90%ofthecarburetoridleproblemsthatshowuponmybench.Thesepassagescannotbecleanedoutwithawireorsoaked.Wearedealingwithrust,notoldfuel.A6inchlong#40aircraftdrillbitisneededtorunthroughtwoofthepassages.The#53drillbitisneededforthethird.Thesedrillbitswilltakethepassagesbacktotheiroriginalsize.Youwillneedtoremovethedrillplugstodothisprocedure.Removeallthedrillplugs,andcheckallpassageswhenoverhaulingyourcarburetor.ExpandingRobertsDataSomeThe#1passage(IDLEAIRFUELMIX)islocatedthroughthefrontofthecarburetor(manifoldside)andisthetop10x32drillplug(#40drillbit).Thelowerdrillplugofthetwoisa12x32.The#2passageislocatedinfrontof,andparalleltotheidleadjustscrew(alsoIDLEFUELAIRMIX)(#40drillbit).The#3SecretPassageislocatedinthebottomofthecarburetorstemandacrossfromtheidleneedletip(thelowerlegoftheIDLEFUELAIRMIX)(#53drillbit).Twoother

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smaller,yetalsoimportantpassagesintheIDLESYSTEMarethefinalupperlegsoftheIDLEpath;oneforthefuelairmix(PRIMARYIDLEORIFICE),theotherforidleair(THESECONDARYIDLEORIFICE).Thesearelocatedonthethrottlevalveendofthecarburetor,andcleanedwitha#60aircraftdrillbit(PRIMARY)anda#57bit(SECONDARY)respectively.ThenthereistheIDLEAIRINpath,asmall(#53)holeontheventurisridgeat3:00oclockasviewedfromthechokeendofthecarburetor.Forabsolutedefinitivedrillingdataandbitsizes(bit#versusdia.)seeCLEANINGintheOverhaulsection.ProtectingthecarburetorOrificesinanycarburetorarespecificallydesignedforthefunctionstheyperform.Theirprimaryfunctionistometer(tometeout)fuelintherightquantitiessoastoobtaindesiredengineperformance;thisbeingtruewhetherstarting,idling,orunderaload.DrillinganyoftheorificeswithlargerbitsthanarecalledforwillpermanentlyaltermeteringfunctionandmaypossiblyrenderthecarburetorLOST!Inordertoensurethatthiswillnotoccur,operatethesedrillbitsintocarburetorpassagewaysbyholdingtheminthehandonlyitsokaytomountasmallhandleonthebit.Donotchuckthemupinapowereddrill!Itisassumedthatbyhandyouwillnotbeabletodrillanypassagewayororificelargerthanitwasdesignedtobe,thusprotectingthecarburetor.Infact,JohnDeereServiceManualSM2024(Carburetors)hasaverybluntwarningaboutthis:CAUTION:Nevercleanholesorpassagewayswithsmalldrillsorstiffwire,asaslightenlargementorburringoftheseholeswillchangetheperformanceofthecarburetoraswellasremoverustpreventiveonpassageways.Thereisnomethodofcleaningotherthansolvent.However,ifafter60plusyearsrusthasoverwhelmedthefactoryappliedrustpreventive,thereisnochoicebuttousethespecifieddrillbits.Butuseextremecare!Troubleshooting -- Starting Problems are Most Common Mytractorcannothandstart.Pullitdownthestreetanditwillstart,andwillrunaslongastheenginespeedisup.Iboughtarecentlyrestored1940Handhavenotbeenabletogetitstarted.Thesellersaysitwasrunningbeforehesoldit.Thecarburetorwasdamagedintransitandreplacedbyarebuiltone.Mytractorwontstart.Magnetosnapisniceandcrisp;itthrowsabluesparkacross1/4inchgap,anditisfiringattherighttimeforeachcylinder.Compressionissupernear80PSIoneachside,andtocrankitsapsmystrengthinnotime!IfIremovetheplugs,squirtsomegasolineintoeachcylinder,andreinstalltheplugs,Icangetbothcylinderstofire,butwillnotstartandrun.

~~~~~~~~~~~~~~~~~ThesearetypicalcomplaintsheardfromthecommunityofJohnDeereowners.Thereareothers.Giventhreeessentialproperties,asimpleinternalcombustionengineasintwocylindertractorswillusuallystart:(1)Enginecompression,(2)Ignitionatthecorrecttime,and(3)Fuelinamoderatelycorrectquantity.

CompressionThereisadirectrelationshipbetweencompressionandmanifoldvacuum.Lowcompressionbringsdecreasedmanifoldvacuum,which(inturn)decreasesthedrawofidlefuelairmixduringstarting.Evenifenoughfuelisdrawnwithhelpfromthechoke,reallylowcompressionresultsintoolittlekickofafiringcylindertostarttheengine.Seecompressiontesting,Chapter6.

Ingeneral,runningacompressiontestisalwaysagoodpracticetoeliminatevariables,andfortheHtractor,anenginewithcompressionreadingsof50PSIormoreshouldstart;evenaslowas40PSItractorsareouttherebeingstartedandareworking.Anewengineisratedat80PSI.Forahandstarttractor,one

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maynotbeabletospintheengineveryfast.Thus,compressionreadingsarelikelytobelower.Perhapsequallyimportantisthatthereadingsbewithin10%ofeachother.

SparkVerifysufficientignitionatthepropertime.Youshouldhaveasharp,distinctivesnapsoundoftheimpulse,andanicebluesparkthatwilljumpatleast1/4inch!The#1ignitionsparkshouldoccurastheflywheelL.H.IMPULSEpassesitsreferencemark,and#2shouldoccur1/2revolution(oftheflywheel)later;aseachcylinderisonitscompressionstroke.Makesureyouhavestrandedmetalliccorewires.SeeSparktestinginChapter7.

FuelToolittlefuelmakesthetractorstarthard,andtoomuchfloodstheengine,causingittosmokeandhaveverylittlepower.Fueltankandlinesdeliverfueltothecarburetor,andthecarburetoristhemeteringdeviceforfueltotheengine.Ensureyouhavefuelflowtothecarburetor;thenbelookingforsignsofeitheradry,orafloodedengine.Thelistherebeginswithdebrisinthefuelsystemcausingastoppage(Besuretochecktoseethatthefuelseparator/fuelfilterhasascreeninit),afloatsettoolowtoprovidefueltothecarburetorsidleneedleatthepointwherestartingfuelentersthecarburetionprocess,andfinallyblockageoftheidlepathwaypreventingthefuelairmixfromreachingtheintakemanifold.LET'S BREAK A MYTH:CarburetorIDLEandLOADneedlesettings[exceptforbeingallthewayin(closed)]arenotcriticalinenginestarting.Thus,asettingoffrom11/4to13/4turnsoutforIDLEandfrom3/4to1turnoutforLOADneedleisperfectlyfineforanenginestart.So,untilyouhaveyourenginerunning,setthemlikethisandforgetaboutthem!Infact,whenyourcarburetorissetupcorrectly,aserviceableenginewillstartwithnohelpfromLOADfuelatall.TheLOADneedlecouldbecompletelyclosed!Ageneralpatternoftroubleshootingbeginstoensurewehavecleanfuel.Ensureyourfuelfilterhasafunctionalscreen,andifthereisvisibledebrisinthefuelsedimentbowl,doacleaningjob.Placeasuitablecollectorunderthecarburetorandobservefuelflowasyouopenthecarburetorsdraincock.Lettheentirebowldrain,andobservetheflowrateafterthatpoint.Itshouldcontinuetobeasteadystream.(Thisistoverifyopenfuellinesandfilters.)Sometimesapartiallycloggedlineorfilterwillpassenoughfueltopermitstartingandidling,butwillstarvetheengineifunderaload.Next,wewanttogetafeelforthefloatsetting.Asmalldevicecalledafloatleveltesterishelpfulhere;installahosebarbinplaceofthecarburetorbowldraincock,attacha10to12inchsectionofclearvinyltubeandtie(orhold)theopenendupalongsidethecarburetor.Turnonthefuelandwatchtheclearhose;giveittime.Theleveloffuelshowingshouldbeapproximately3/4inchbelowthetopofthebowl.Twootherquickandeasycheckshere;(1)BackouteithertheIdleorLoadneedleandchecktheendforwetting.Thereshouldbeenoughfuelinthebowltowettheneedlesend.Thisobservationismoresuccessfulinthemorningathighesthumiditywhichslowsevaporationoffuelfromtheneedletaper!(2)Thisone,validonlyfortheHtractor,isavolumetest.Closethedraincockandopenthefuelvalvestoallowfuelflowintothecarburetorbowl.Leaveitopenforafullminute.Thenclosethefuelvalve(s),andmeasuretheamountoffuelyoucandrainfromthebowl.Youshouldcollectbetween4and5fluidounces!Floatsettingcriteriaifthecarburetorsbowlwereremoved,thislevelwouldbeachievedifthefloatisdeadlevelasitclosestheneedlevalve;apointwherethefloatisbetween3/8and1/2inchbelowthecarburetorscastingastheneedleisclosed.ForHtractors,thisdistanceis3/8inch.

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Finally,oneturnstoblockageintheIDLESYSTEMofthecarburetorthesourceoffuelairmixforstarting.ThisisaverycommonproblemwithJDcastironcarburetorsnowsome5of6decadesold!Seetheparagraphset(below)forashort,nowrenchtestyoucanperformtodeterminewhether(ornot)youridleandloadsystemsareopen.Imightaddherethatcleaningthecarburetorsinternalpassagewaysispartofanoverhaulprocedure,andifoverhaulingyourcarburetoriswhatyoudecidetodo,amusthaveiseitherourHCarburetorOverhaulProcedure(inBookorCDform).Andtoorder,gotowebpagewww.jdhpubs.com,andproceedfromthere.ThisprocedurehasbeenincorporatedintoChapter8ofthe3rdEditionoftheJDHRestorationGuide.Forothertractorcarburetors,youareencouragedtoobtaintheDVDCarb2byRobertBeaverofRobertsCarbRepair.TheGreenMagazineBookStorehasbeencarryingtheDVD.DrillBitSetsFromtimetotime,www.jdhpubs.comwillalsocarrythissetoffourbits(forJDHtractors).SeeHomepagetoorder.

No-Wrench Testing -- LOAD and IDLE Pathways AtestisconductedtodoublecheckyourcarburetorsIDLEandLOADpathwaysbymanipulatingtheIDLEandLOADneedles.Hereishowitgoes:NOTE:ThistestinvolvesradicaladjustmentofboththeLOADandIDLEneedles.Beforeyoustarttheengine,observethepositionofeachneedlelever,andcountthenumberofturnsOUTthateachofthetwoneedlesisatsothatyoucanputthembackwheretheywerebeforethistestisrun.Warning:WhenturningaLOADorIDLEneedleallthewayin,takeitgentlyastheneedlebottomsout.Goingupagainstthestopwithtoomuchforcewilldamagetheneedle.(1)WiththetractorrunningatFastIdle(fullthrottlenoload),turntheLOADneedleallthewayin.TheenginewillcontinuetoruniftheIDLESYSTEMisopen,andifitstopsrunning,theidlesystemISNOTopen!Assumingthistestisapass,resettheLOADneedlewhereiswasandgoontotesttwobelow.(KnownExceptionThistestmaynotbereliableforDLTX51Carburetors!)(2)WiththeenginerunningatFastIdle,turntheIDLEneedleallthewayin.TheenginewillcontinuetorunprovidedtheLOADFUELsystemisopen,however,inthismode,engineoperationwillbeathrottlingpattern.Asenginespeeddrops,thegovernorwillopenthethrottletodrawLOADFUELbecausetheidlepathisclosed.Thispattern,however,indicatestheLOADFUELchannelisopenandworkingasitshould.Iftheenginestopsrunning,theLOADFUELchannelisobstructed.++++

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Diagnosing Faulting CarburetorsNo-Wrench Testing -- LOAD and IDLE Pathways