section s, fuel system c marvel-schebler carbureter · marvel-schebler carbureter •...

GROUP II, ENGINE

SECTION S, FUEL SYSTEM

C Marvel-Schebler Carbureter

•

Marvel-Schebler Carbureters are used onthousands of tractor and industrial engines andhave been designed to provide many years oftrouble-free service, however, as in the case ofall mechanical devices, they do in time requireproper service and repairs. An understandingof 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 diag-nosing so-called carbureter complaints but willcreate customer satisfaction and a profitablebusiness for the progressive service shop.

To understand a carbureter it is necessary torealize that there is only one thing that a carbur-eter 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 func-tion of the engine to convert this mixture intopower.

There are three major factors in an enginewhich control the change of fuel and air intopower : 1—Compression. 2—Ignition. 3—Car-buretion.

Carburetion has been listed last because it isabsolutely necessary for the engine to have goodcompression and good ignition before it can havegood carburetion.

When the average person thinks of "carbure-tion" they immediately think of the carbureteras a unit. Carburetion is the combined functionof the carbureter, manifold, valves, piston andrings, combustion chamber, and cam shaft.

It can be readily seen that "carburetion" is afar deeper subject than consideration of the car-bureter 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 thecustomer.

It must be remembered that the function ofthe carbureter does not extend beyond deliver-ing the proper mixture of fuel and air to themanifold and the other factors which effect

power and economy cannot be changed or cor-rected by the carbureter. Inability to understandall the factors that effect engine operation is thereason many service mechanics change from fac-tory standards and attempt to improve on theengine set-up by their own methods or "stand-ards". All that any service mechanic should evertry to do is to make the particular engine he isworking on as good as the manufacturer in-tended it to be, but he can make it a lot worse.Far too many engines are running below theirstandard of performance in service today.

For the carbureter to accomplish its functionit must be able to vary the mixture strength de-pendent upon the engine demands. It mustsupply a mixture strength that will allow the en-gine to give maximum horsepower, whenever thethrottle is fully opened, while at part throttleconditions 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 posi-tion. The carbureter must also have an acceler-ating "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 mixtureproportion for any condition under which theengine is operating at any time.

In order to understand the function and opera-tion of the Marvel-Schebler Tractor and Indus-trial 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 SuctionEconomizer System, and The Choke System.

A 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 overhaul.

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GROUP II, ENGINE


Float SystemIt should be an estab-

lished policy that when-ever the carburetor is dis-assembled for whatevercause the service manmake following checks:

FLOAT VALVESEAT-4

FUEL BOWL-1

Fig. S- 13The float system controls the level and supply

of gasoline in the fuel bowl throughout the oper-ating range of the engine.

When the fuel bowl (1) is empty the floatand lever (2) and float valve (3) drop and fuelunder pressure from the fuel pump (or gravityfeed) is forced through the float valve seat (4)around the float valve (3) and into the fuelbowl (1). As the fuel in the bowl approachesthe correct operating level it raises the float andlever (2) with enough force to raise the floatvalve and cut off the flow of fuel into the bowl.As fuel feeds through the carburetor jets intothe engine the fuel level (5) drops, allowingadditional fuel to enter the fuel bowl.

Under actual operating conditions the fuellevel (5) and float and lever (2) automaticallyposition themselves so that the inward flow ofgasoline to the carburetor is equal to the out-ward flow of gasoline to the engine.

As 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 ex-cessive vibrations being transmitted to the

carburetor, float valve and float valve seat wear isaccelerated.

1. Examine float valve forany signs of wear. If itis not absolutely true oris grooved and hasn't aperfect taper, a newfloat valve and also anew float valve seatmust be used. Thesefloat valves and seatsare supplied in match-ed sets and are testedat the factory for leaks.Always use a new floatvalve seat gasket tomake sure of a perfectseal.

2. Examine float for any signs of failure. Totest metal float submerge float in pan ofhot water and if air bubbles are observedreplace with new float.

Examine cork float for bare places orcracks in coating. If either are found, or iffloat shows evidence of having been soggy,replace with new one. (Do not attempt torecover float with shellac or varnish.)

3. Set float height to the proper specificationfor the particular model carburetor beingserviced. Make certain that the entireassembly works free and that there is nobinding.

4. Wash fuel strainer assembly in gasolineand clean screen with air under pressure.If the screen, or the threads on the strainerare not in good condition, install a newassembly. When re-installing fuel strainerassembly always use a new strainer gasketif a gasket is used to obtain a seal.

It has been proven, with few exceptions,that with a float system in good order,

carburetor flooding only occurs when dirtor foreign matter becomes lodged betweenthe float valve (3) and float valve seat (4).

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THROTTLEVALVE-6

PRIMARY IDLEORIFICE-7

riIDLE ADJUSTINGNEEDLE SEAT-12

FUEL BOWLCHAMBER-9

IDLE JET-11

i,I • :/• 15-r , . •SECONDARY

IDLE ORIFICE-8

IDLE ADJUSTINGNEEDLE-13

•: FUEL BOWL-

IDLE FUEL PASSAGE-10

GROUP II, ENGINE

SECTION 5, FUEL SYSTEM

The Idle System

Fig. 5-14

The 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 function.

When the throttle valve (6) is in the idle posi-tion the edge of the valve is between the primaryidle orifice (7) and the secondary idle orifice(8). With the valve in this position the air pres-sure (manifold vacuum) at the primary idleorifice (7) is lower than the air pressure in thefuel bowl chamber (9) and fuel is forced fromthe fuel bowl (1) into the idle fuel passage (10).As the fuel travels through the idle fuel passage(10) it passes through the metering orifice ofthe idle jet (11) to the point where it is combinedwith air entering through the idle adjustingneedle seat (12). The mixing of air with gasolinehelps to atomize the fuel and this process isrepeated at the secondary idle orifice (8) as thefuel travels through the idle fuel passage (10).As this rich mixture of fuel and air emerges fromthe primary idle orifice (7) it is reduced to cor-rect proportions by the air which passes around

the throttle valve (6) sincethis valve must be slightlyopen to permit the engineto idle. The resultant mix-ture is correct for operat-ing engine at idle speed,provided the idle adjust-ing needle (13) is prop-erly adjusted.

As the throttle valve (6)is slowly opened from theslow idle position it gradu-ally subjects the second-ary idle orifice (8) to in-take manifold vacuum,and the secondary idleorifice (8) no longer bleedsair to the idle fuel passage(10) but feeds an addi-tional quantity of fuel into

the 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 engine.

As the throttle valve (6) is opened still wider,the idle fuel delivery begins to fade out, how-ever, the throttle valve at this point is far enoughopen for the power fuel feed system to beginfunctioning.

The 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 clean-ing. 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 carburetor.

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GROUP II, ENGINE


Power Fuel Feed System

NOZZLE AIR BLEEDS-15

ACCELERATING WELL-16

NOZZLE-14

VENTURI-17 :r..,

\\V. ,:f N>■

‘ • — b.: \1I...• .• \ ■N

''■ 14111 111 %

111 1 11, 1 1 1r 4 ...11--of- ''•

NOZZLE AIR VENT-21

THROTTLE VALVE-6

•• FUEL BOWL-

FUEL BOWLCHAMBER-9

I

POWER JET-20

venturi (17). At the sametime, the fuel that is storedin the accelerating well(16) is also forced throughthe nozzle air bleeds (15)into the nozzle (14). But,because the size of thepower jet (20) and theposition of the power ad-justing needle (18) restrictthe amount of fuel whichcan enter the nozzle (14),the fuel in the accelerat-ing well (16) will soon beexhausted and air willthen enter through thenozzle air bleeds (15) tomix with the fuel passingthrough the nozzle (14).The amount of air that canenter into the nozzle (14)is limited by the size ofthe nozzle air vent (21).

POWER ADJUSTING NEEDLE SEAT-19

POWER ADJUSTING NEEDLE 18

Fig. S- 15With the throttle valve (6) in slow or just off

slow idle position, fuel rises up through thenozzle (14) and out the nozzle air bleeds (15)to fill the accelerating well (16) to approxi-mately the height of the fuel level in the fuelbowl (1).

As the engine speed is increased from theslow idle position the air flow through the venturi(17) is gradually increased, and as the idlesystem begins to diminish the velocity throughthe venturi (17) is high enough to create a pres-sure at the tip of the nozzle (14) slightly lessthan the pressure in the fuel bowl chamber (9)and the accelerating well (16). Fuel, therefore,feeds Horn the fuel bowl (1) through the open-ing between the power (load) adjusting needle(18) and the power adjusting needle seat (19),through the power jet (20) and out the nozzle(14) to be discharged into the air stream at the

The result of air bleed-ing into the nozzle (14) is,to help atomize or breakup the fuel into finer par-ticles, to regulate the quan-tity and the rate of dis-

charge of the fuel fed from the acceleratingwell (1'6), during acceleration, and to providethe correct mixture proportions for full throttleoperation.

As the throttle valve is opened toward thewide open position the velocity through the ven-turi (17) continues to increase, lowering the airpressure at the nozzle (14) and resulting inadditional fuel being supplied to the engine asthe speed is increased.

When the throttle valve (6) is opened sud-denly from slow or just off slow idle position,the fuel stored in the accelerating well (16) isforced out through the nozzle air bleeds (15)very rapidly and serves to provide the extrarichness required by the engine to meet thesudden load. When the throttle valve (6) isclosed fuel again fills the accelerating well (16),ready for the next acceleration.

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THROTTLEVALVE-6

GROUP II, ENGINE


Back Suction Economizer Systemamount of air that is drawn out of the fuelbowl chamber (9) is controlled by the sizeof the economizer jet (23), the economizer

ECONOMIZER ORIFICE-24 orifice (24) and the position of the throttlevalve (6) as its position determines the

ECONOMIZER JET-23 manifold vacuum or suction on the econo-

mizer orifice (24). As the throttle valve (6)is opened from the fast idle position the

BOWL VENT-22 economizer orifice (24) is gradually ex-

\ \\\X•,%\\%%N.N from the fuel bowl chamber (9), throughposed to manifold suction, and air flows

the economizer jet (23) and out the econo-mizer orifice (24). This air must be re-placed by air entering through the bowlvent (22) but as the size of the bowl vent(22) restricts the amount of air that canenter, the resultant pressure in the fuelbowl chamber (9) will be lowered, reduc-ing the difference in air pressure betweenthe nozzle (14) and the fuel bowl chamber(9). The flow of fuel will therefore be re-tarded so that the exact economy mixtureratio will be delivered to the engine at thisparticular throttle opening. Opening thethrottle valve (6) further exposes the entireeconomizer orifice (24) to manifold suction,resulting in additional air being removed

from the fuel bowl chamber (9), again leaningout the mixture ratio to the correct proportionsfor this new throttle position. After the econo-mizer orifice (24) is fully exposed to manifoldsuction the amount of air that is drawn out ofthe fuel bowl chamber (9) is controlled by themanifold vacuum or suction at any given throttlevalve (6) position and as this suction decreasesas the throttle approaches wide open position,less 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 isnecessary.

The 'Back Suction Economizer System" as-sures the proper metering of fuel to the enginethroughout the service life of the carburetor asthere are no moving parts to wear out or adjust-ments to get out of order. It is essential, how-ever, that the system remain free of dirt andforeign matter because any foreign substancein the system will restrict the flow of air therebycreating improper pressures in the fuel bowlchamber and resulting improper fuel deliveryto the engine.

Fig. S-16

The amount of fuel supplied to an engine iscontrolled by the size of the power jet, the po-sition 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 addi-tionally during part throttle operation to obtainmaximum economy. To provide this leanermixture Marvel-Schebler Tractor and IndustrialCarbureters make use of the "Back SuctionEconomizer System. With this method of meter-ing fuel, the air pressure in the fuel bowlchamber is regulated and controlled accordingto load conditions by a combination of bowlvent and economizer passages communicatingwith the throttle bore of the carburetor.Through regulations of the air pressure in thefuel bowl chamber the fuel flow through thecarburetor can be controlled to provide the prop-er mixture proportions for the engine.

All the air that enters the fuel bowl chamber(9) must first pass through the air cleaner andthe bowl vent (22). The size of the bowl vent(22) controls or limits the amount of air thatcan enter the fuel bowl chamber (9). The

FUEL BOWL CHAMBER-9V' p.e

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GROUP II, ENGINE


Choke System

iuumuuu0A‘AwFig. S-17

The choke system is used during cold starting valve (25) position use is made of a springloaded relief valve (26) in many applications.This valve opens automatically with enginespeed and load and eliminates a great deal ofmanipulation of the choke on the part of theoperator.

When the engine has obtained normal oper-ating temperature the choke valve (25) must befully 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 ex-haust system. The remainder of the raw gasolineis forced between the pistons and cylinder walls,washing away the protective oil film and increas-ing engine wear, and enters the crankcase whereit dilutes the engine oil.

Any adjustments that are necessary on thecarburetor should never be attempted until the

engine has obtained its normal operating tem-perature and the choke valve (25) has beenplaced in the wide open position.

and the warm-up period. Under these cold con-ditions 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 tempera-tures 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 com-bustible mixture for starting the engine.

The function of the choke valve (25) is torestrict the amount of air that can enter thecarburetor and to increase the suction on the nozzle(14) so 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 ac-complished by the operator positioning the chokevalve to provide the proper mixture. However,a few degrees movement of the choke valve (25)will make a big change in the mixture strengthand to help reduce the sensitivity of the choke

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IDLE ADJUSTING NEEDLE (FUEL)

IDLE ADJUSTING NEEDLE (AIR)

VENTURI

NOZZLE

POWER JET

BOWL DRAIN IPOWER ADJUSTING NEEDLE

ECONOMIZERJET

ECONOMIZERORIFICE

BOWL VENT

Fig. S- 19 Back Suction Economizer System

GROUP II, ENGINE


Model TSX Carburetertive hole plugs, and alsoto prevent vapor lock or"percolation" of the fuelwhen the carburetor isoperated under ex-tremely hot conditions,resulting in hard start-ing or erratic engine op-eration.

The Model TSXcarburetor is completely seal-ed against dust or dirt.All air entering the fuelbowl of the carburetormust first pass throughthe air cleaner. Thethrottle shaft bearingsand choke shaft bear-ings are sealed to elimi-nate dust and dirt enter-ing at these points.

The back suction econ-omizer system Fig. S-19 is provided with are movable economizerjet. The size of this jethas been carefully es-tablished by engineer-ing tests to provide theexact fuel requirements

for maximum economy at part throttle opera-tion. Always use the economizer jet specified inthe individual carburetor service parts list toassure proper engine operation. On some carbu-

Fig. S- 18 Idle and Power Fuel Feed Systems

The Marvel-Schebler Model TSXCarbureter is manufactured in threeS.A.E. nominal sizes: 7/8 inch, 1 inch,and 1 1/4 inch. In addition to these vari-ations in size, there are also variationsnecessitated by the specific require-ment of the engines on which the car-bureters are used. Many engines, forinstance, require special throttle andchoke operating levers, and for pur-pose of calibration, they may have dif-ferent size jets, nozzles, venturii, etc.For this reason when ordering parts,refer to the individual carburetor serv-ice parts list for the engine on whichthe carburetor is installed.

The Model TSX Carburetor consistsof only two major castings:

1. The throttle body casting whichforms the cover for the fuel bowl.

2. The fuel bowl casting which con-tains the air inlet.

Cast iron material is used for rug-gedness. It will be noticed (Fig S-18)that all passages, whenever possible,are drilled from the top face of thefuel bowl casting to prevent any fuelleaks to the outside of the carburetor,because of shrunken gaskets or defec-

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FLOAT VALVE

FLOAT VALVE

CHOKE VALVE

GASOLINE DRAIN STRAINER

GROUP II, ENGINE


Model TSX Carbureter

Fig. S-20 Float and Choke Systems

reter 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 sup-plied to the engine. In addition, there are engineand carburetor combinations that do not requirethe back suction economizer system. In thesecarbureters the economizer orifice has not beenmachined in the throttle body casting.

To provide additional economy, in addition tothe back suction economizer system, some carbu-reters are provided with two adjusting needles,the 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 engine.

There are two variations in carbureters hav-ing the power adjusting needle, commonly calledthe adjustable jet type carburetor. In Fig. S-18is shown these two arrangements. The adjust-ment of either type is accomplished in the samemanner.

A large percentage of the Model TSX Carbu-reters are provided with an idle adjusting needlewhich alters the fuel and air proportions of themixture which enters the carburetor bore fromthe idle passage. This is known as an

air adjusting idle needle. The upperinset in Fig. S-18 shows an idle adjust-ing needle which alters the amount offuel and air mixture which enters the

carburetor bore from the idle passage.This is commonly known as a fuel ad-justing 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, orcounter-clockwise to enrich the idlemixture.

A dual float mechanism Fig. S-20is used in a fuel bowl that almost com-pletely surrounds the nozzle. This de-sign and construction is such that thetractor, or engine, can be operated atany angle up to 45 degrees withoutseriously affecting the fuel and airratio and without flooding because themean level at the nozzle tip is practi-cally constant at any angle of opera-tion.

Some carbureters are equipped with a spring-loaded governor control lever to permit manualclosing of the throttle to an idle position forengines equipped with certain type governors.An example of this type lever is shown in Figure4, however, there are other variations of thistype dependent upon the particular application.

While there are many variations produced bycombining the different types and sizes into aspecific application, all Model TSX carburetersincorporate the same engineering principles andare alike from a functional standpoint.

Fig. S-21 Spring-Loaded Governor Lever

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GROUP II, ENGINE


Service Instructions for Model TSX Carbureters

Type A—Fixed Jet Type B—Adjustable Jet Type C—Adjustable Jet

The 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 permitexamination of external parts for damage. For type carburetor being serviced see illustrations above.Instructions apply to all types unless specified otherwise.

1— 5—

Remove Power AdjustingNeedle Assembly.

Type B.

2

Remove Bowl CoverScrews and Lock Washers

Separate Castings.

3

Remove Float Valve,Bowl Gasket, and Venturi.

If Valve is grooved ordamaged, replace Valveand Float Valve Seat.

4

Remove Float Valve Seatand Gasket.

Remove Economizer Jet.

NOTE. Not required in allcarbureters. Check serviceparts list or repair kit ofcarburetor being serviced.

6

Remove Idle Jet

NOTE. Not required in allcarbureters. Check serviceparts list or repair kit ofcarburetor being serviced.

7

Removing Idle AdjustingNeedle and Spring

Replace with new Needleif grooved or damaged

8

Remove Throttle ValveScrews, Valve, andThrottle Shaft andLever Assembly

Replace with new shaftand lever assembly if ex-cessive looseness betweenshaft and throttle body.

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9—

Remove Throttle ShaftPacking Retainerand Packing

Force out Retainerwith small screwdriveror punch

10

Remove Main Nozzleand Gasket.

Type AType B

Remove Choke ValveScrews, Valve, ChokeShaft & Lever Assembly,Choke Return Spring,and Choke Bracket.

11--

Remove Power Jet.

Type AType B

12— 17—

Remove Power Adjusting Install Throttle ShaftNeedle Assembly. Packing and Retainer.Type C Assemble new retainer

and packing on throttleCarbureters not having shaft. Insert shaft inadjustable needle remove carburetor and tap lightlypower jet. until retainer is flush with

casting face.

13— 18

Remove Main Nozzle Install Throttle Valveand Gasket. and. Screws.

Type C Install valve with angleidentification mark facingflange face of carburetor.Tap valve lightly to cen-ter in throttle bore. Tight-en screws securely.

14Remove Retainer Plugand Gasoline DrainStrainer.Strainer can only be re-placed on carbureters hav-ing a curled hair or felttype strainer. Only re-place when impossible toclean with gasoline andcompressed air. Porousmetal type strainer can-not be replaced. Cleanonly.

15—

16—

Remove Choke ShaftPacking Retainerand Packing.

Force out retainer withsmall screwdriveror punch.

ASSEMBLE

GROUP II, ENGINE


Before 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 gaskets.

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L. To I32 4

pj I momlay

GROUP II, ENGINE


19

Install Economizer Jet.

20

Install Idle Jet.

21

Install Idle AdjustingNeedle and Spring.

Set approximately oneturn from seat forpreliminary setting.

22

Install Float Valve Seatand Gasket.

Use new Float Valve andSeat Assembly.

23—

Assemble Bowl CoverGasket and Venturiin Casting.

Install Float and LeverAssembly andFloat Lever Pin.

Set floats 1/4" from gasketface to nearest edge offloat, keeping edge of floatparallel with gasket. Ad-just by using bending tool^M-8.

25

Install Choke ShaftPacking Retainer andPacking.

Install retainer as shownin illustrations below.Note: On some carburetormodels the packing is re-tained by choke bracket inplace of packing retainer.

25A

On carbureters counter-bored Vs" to h" deep in-stall retainer with cupfacing towards casting.Tap lightly until flush withcasting face.

25B

On carbureters counter-bored to 1/4" deep in-stall retainer with cupfacing away from casting.Tap lightly until flushwith casting face.

26—

Install Choke Bracket,Choke Return Spring,Choke Shaft and LeverAssembly, Valve, andScrews.

Center valve in castingbefore tightening screws.

27

Install Power Jet.

Type AType B

Install Main Nozzleand Gasket.

Type AType B

Use new gasket.

Install float valve.

24— 28—

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29—

Install Main Nozzleand Gasket.

Type C

Use new gasket.

30Install Power AdjustingNeedle Assembly.

Type C

Use new gasket.


31

Install Gasoline DrainStrainer & Retainer Plug.

Stake retainer plug inplace with center punch toinsure secure locking.

32

Assemble Castings.

Invert throttle body andlower fuel bowl over floatstaking precaution thatventuri guides bodies intoposition.

33

Install Bowl Cover Screwsand Lock Washers.

Tighten screws graduallyuntil all are tight.

34—Install Power AdjustingNeedle Assembly.

Type A


GROUP II, ENGINE


Adjustment Instructions

PRELIMINARY ADJUSTMENTS

Set 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 position.

LOW SPEED OR IDLE ADJUSTMENT

Set 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 run-inengine.) Turn idle adjusting needle* until engine beginsto falter or roll from richness, then turn needle in theopposite direction until the engine runs smoothly.

NOTE: It is better that this adjustment be slightly toorich than too lean.

'IDLE ADJUSTING NEEDLE—AIR ADJUSTING

To richen the idle mixture turn the idle adjusting needleto the right or clockwise.

`IDLE ADJUSTING NEEDLE—FUEL ADJUSTING

To richen the idle mixture turn the idle adjusting needleto the left or counter-clockwise.

POWER OR LOAD ADJUSTMENT (TYPE B, TYPE C)

With the engine running at governed speed under load,turn power adjusting needle to the right, or clockwise, alittle at a time until the power drops appreciably. Thenturn the needle to the left, or counter-clockwise, 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 obtained.

NOTE: Carbureters TSX-107, TSX-330, TSX-339, TSX-355, TSX-385 and TSX-398 use the fuel ad-

justing type idle needle. All other Model TSX Carbureters use the air adjusting type idle needle.

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GROUP II, ENGINE


Service ComplaintsIDLE—UNEVEN IN OPERATION

The idle construction used in Marvel-ScheblerTractor 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 mind,however, that there are times when these smallholes may become plugged with particles of dirtor lint, but very seldom. If idle trouble is experi-enced, 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 hour,and a slight leak will result in a very erratic orrough idling engine.

Other causes for a rough idling engine are :uneven compression, caused by sticky or leakingvalves; leaking valve seats; tappets with im-

proper clearances; leakage past pistons andrings; cylinder head gasket leaking ; weakspark, or spark plug points not spaced correctly ;ignition cable covering cracked and thus ground-ing spark, and cable not assembled properly inthe distributor cap which causes corrosion andweak spark.

The spark timing of the engine is most im-portant, and should also be checked very care-fully and set exactly on the mark as called forin factory standard specifications. In fact, allof the above items must be checked very care-fully to factory standards, and not just given acasual inspection with the common expression"Everything looks O.K." You can KNOW defi-nitely that the tractor is up to the standards setby the manufacturer.

POWER AND ECONOMY—LOW

Complaints are received from the field thatthe engine will not pull or develop its maximumhorsepower, 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 difficulty.

It must be clearly understood by all service-men 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 depart-ment develops the new engine to pull the re-quired horsepower. In the design there are cer-tain fixed dimensions that never change. Forinstance, the bore and stroke, the displacement,compression ratio, diameter of valves, lift ofvalves, diameter of intake passage. The

carburetor engineer works out the diameter of throttle

bore, v,enturi size, and provides for means ofadjusting and regulating the power fuel mixtureratio, as well as the idle. Now, in service, con-sider, 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 maximumhorsepower is the fuel mixture ratio.

If compression, ignition, and timing, whichare variable, are first properly checked by aservice man and set to factory specifications,very little difficulty will be experienced in ad-justing the carburetor to give the maximumhorsepower and economy.

A great deal has been said regarding the im-portance of engine tune-up and the reasons forservice men being exact in their service work onengines. The reasons why a carburetor may not

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Carbureter ModelFactorySetting Where to Measure

"TSX" 7/8 ", 1 - , 1 'Ai 14 From the gasket to the nearest surface of thefloat.

GROUP II, ENGINE


function properly when everything else has beenchecked and set to factory standards will nowbe covered.

With the present type carburetor constructionused on Marvel-Schebler tractor and industrialcarbureters, not very much can go wrong withthe possible exception that it may foul with dirt.There are only two places that are subject towear—the throttle shaft and bushings and thefloat valve and seat. The wear on the throttleshaft and bushings, and resultant air leak there-from 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 floodingtrouble. Both faults can be easily observed bythe service man, and corrected by replacingworn parts with new ones. The proper function-

ing 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 workedcorrectly at first, when passed by the inspectorsat the tractor factory, it will always function thesame, provided these passages are all free fromdirt.

On a carburetor complaint from the field, theonly thing a service man can do to the

carburetor is to disassemble it. BE SURE that thepassages are open and free from dirt, that thereis no wear on the throttle shaft and bushings,that float valve and seat are O.K., that the floatheight is correct, and that a good air-tight sealexists around the bowl gasket. If such carburetorservice does not correct the complaints, a com-plete check of the engine must again be made.

To 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 seated.

NOTE: 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 operation.

Printed with permission of:

Marvel-Schebler Products Division, Borg-Warner Corporation

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