Reprinted with Permission, Aircraft Maintenace Technology, October 2000

The Marvel(ous) Schebler carburetorBy Randy Knuteson

E verything is simpler than you think and at the same time,more complex than you can imagine. These words werespoken some 200 years ago by a philosopher — not an

aircraft technician straining to understand what went wrong withhis carbureted engine.

A quick glance at a Marvel Schebler Carburetor might leadsome to believe that these units are far too simplistic for today’smodern aircraft. The obvious advantages associated with fuelinjection (even fuel/air distribution, no carb icing concerns, etc.)seem to overshadow the value of the much-maligned carburetor.However, a closer, more scrutinizing look reveals an intricate rela-tionship between various sub-systems within these carburetors.The details that comprise these systems vary from model to model.It is only when the functions of these systems are fully understoodthat one gains a renewed appreciation for their elegantly simpledesigns. These units have continued to serve us well through thepassing of time.

Principles of operationIn the most simple of terms, an aircraft carburetor is a device

for introducing and mixing a metered amount of fuel to the cylin-ders. Unlike direct injection that provides a precise and uniformdelivery of the charge directly into the intake port of each cylin-der, the atomized fuel from the carburetor seeks the path of least

resistance as it travels through the induction tubes. Fuel distribu-tion is far from exact as indicated by split CHT and EGT readings.According to Lycoming, a variance of 150 degrees is not at allunusual. The job of the carburetor is to perform two very basicfunctions:

1. It measures out an appropriate amount of incoming air 2. It mixes that air with fuel to assure that a proper charge

enters the cylinders under all operating conditions.Fuel from the wing or header tanks is fed either by gravity or

by a low-pressure pump to the inlet of the carburetor. The actualpressure available from a gravity feed system is about one PSI foreach forty inches of head of fuel (as measured in distance from thesurface of the fuel in the tank to the point of discharge into thecarburetor). Low-wing aircraft or “Turbo-normalized” aircraftrequires as much as six to nine PSI of pressure to perform at alti-tude. These installations would require a gravity feed of approxi-mately some 240 inches of head pressure. Such distance rendersthe gravity-feed method impractical if not impossible.

As fuel rises in the float bowl chamber, it lifts a float that ishinged to the throttle body. The float fulcrum lever carries a nee-dle valve, the point of which extends into a seat at the inlet of thecarburetor. When the fuel level rises far enough in the bowl, theneedle valve begins to partially restrict or close off fuel flow. Theheight of the fuel in the discharge nozzle is controlled by the posi-tion of the float and the needle valve in the float chamber. As fuel

October 2000 • Aircraft Maintenance Technology • www.AMTonline.com2

is discharged from the carburetor, the floatlowers and allows more fuel to fill thebowl. In this manner, optimum fuel level isalways maintained so long as the float levelhas been properly set at overhaul.

A partial vacuum created by the pistonduring the intake stroke draws air throughthe carburetor. The air passages in boththe carburetor and the manifold aredesigned to admit a sufficient amount ofair to fill the cylinders by the end of theintake stroke. The throttle plates’ functionis to regulate the admission of air to thecylinders, thereby controlling the poweroutput of the engine.

Basic BernoulliTwo pressures work together to dis-

charge fuel from the carburetor bowl. Theatmospheric pressures in the bowl cham-ber exert a downward force on the fuelwithin the bowl. And there is also a dropin pressure (a vacuum) at the neck of thedischarge nozzle caused by the action ofthe venturi in the throat of the carburetor.The resulting pressure-differential works tocreate a push/pull action on the fuel.

Proper fuel metering is accomplishedby the strategic placement of the discharge

nozzle in the venturi tube.The venturi is situated inthe intake airstream at thepoint of mean velocityimmediately upstream ofthe throttle valve.

Imagine an aircraftwing rolled into a cylinder,dramatically reduced insize, then placed into thebore of a carburetor. Inessence, you have formedthe aircraft wing into a ven-turi. The basic law ofphysics discovered byBernoulli is as active in the

throat of a carburetor as it is across the sur-face of an aircraft wing. A venturi tube cor-rectly positioned in the throat of a carbu-retor causes air to move at a much fasterrate as it passes through the constriction(See diagram,right). As air velocity increas-es, a reduction in static pressure (pressuredrop) causes a suction force to draw fuelup the discharge nozzle. The amount offuel drawn up the discharge nozzle isdependent upon the speed and conditionof the air sweeping through the venturi.The greater the velocity, the greater thesuction on the fuel in the discharge nozzle.

Unimpeded AirflowThe onrush of air through the throttle

body serves to mix and atomize the fuel asit makes its way to the cylinder intakes.This mixing of fuel and air in the throat ofthe carburetor helps to convert much ofthe liquid fuel into a gaseous state. Enginespeed, efficiency and power are greatlyinfluenced by the quantity and nature ofthis homogenous charge. Because of this,its extremely important that airflow be un-impeded by sharp bends in the inductionor gasket material protruding into theairstream. Such obstacles can produce

unsteady or turbulent airflows directlyaffecting the fuel metered through the dis-charge nozzle. The quality of the airstreamdirectly influences the metering of fuel. Asmall piece of gasket material, a damagedor restricted air filter, a loose venturi, orany foreign object lodged in the carburetorthroat can ruin the metering ability of thecarburetor.

Mixture ControlFuel is always metered in relation to

the weight, not the volume of air passingthrough the carburetor. As the aircraftascends in altitude it passes through atmos-phere that is constantly changing. Pressure,temperature, and density steadily declines.Since thinner air is less dense, each poundof air occupies a greater volume of space.So, as the airplane gains altitude, the vol-ume of air passing through the carburetorwill continue to remain proportional tosuction in the manifold, but the weight ofthe air will decrease as ambient air densitydecreases. Since the air is less dense, a nat-

.05 .06 .07 .08 .09 .10 .11 .12LEAN FUEL/AIR RATIO RICH

HO

RSE

POW

ER O

UTP

UT

HORSEPOWER VARIATIONWITH CHANGE IN MIXTURE RATIO

(@ CONSTANT RPM)LEANBEST POWER BEST POWER

RICH BEST POWER

This curve shows the relationship between power output andmixture ratios (based on weight, not volume).Combustion canoccur with mixtures as rich as .125 (1 to 8); and as lean as .062(1 to 16). Lean best power (economy) setting is 0.075, while rich

best power is 0.087.Best performance is approximately 0.080.The exact shape of this curve varies from engine to engine.

Left: Brass discharge nozzles. Top nozzle is of the pepper-boxdesign. This atomizing design is frequently installed in the small-

er TCM carbs to correct erratic fuel flows associated with theinstallation of the single-piece venturi (Precision S.B.s MSA-7,

MSA-8, MSA9).

Right: Kelly Aerospace Power Systems manufactures dischargenozzles from a single piece of stock (top). Older discharge noz-

zles are welded where the neck meets the wrenching flats. Heavyporosity or a side load on the neck may compromise these welds.

Visually inspect the integrity of the nozzles to assure that theneck will not separate from the body. The neck of the nozzle

would be ingested into the engine if such a separation occurred.

The rate of fuelflow from the dis-charge nozzle is proportional to

the amount of airpassing through

the venturi.

THROTTLE

PLATE

28 inches

26 inches

30 inches

Pressure dropacross venturi.

(Inches ofMercury)

MainVenturi

PrimaryVenturi

DischargeNozzle

FLOW FLOW

FLOW

Atomizing

Non-Atomizing

www.AMTonline.com • Aircraft Maintenance Technology • October 2000 3

ural richening of the fuel/air ratio occurs. Amanual mixture control enables the pilot toalter the ratio of fuel to air. At any giventhrottle setting, the mixture may requiresome adjustment to compensate for ever-changing conditions from sea level to alti-tude. The mixture control also provides asecondary function of completely closingoff fuel flow at ICO.

Idle CircuitAt low rpms, the idling system is

independent of the main metering jet. Atidle speeds and up to approximately 1,000to 1,300 rpm the main jet has little or nofuel passing through it. This is because thethrottle valve is almost closed and there islittle air swept past the discharge nozzle. Atthis point, fuel is drawn up the idle bleedtube in the bowl and then through the idleemulsion channel within the throttle cast-ing to ports adjacent to the upper edge of

the throttle valve. As the throttle valve isopened, suction at the idle mixture portdecreases and the main jet takes overentirely. Fly-holes in line with the mixtureport aid in the smooth transition from idleto full power. As the throttle opens it pro-gressively unveils these secondary and ter-tiary bleed ports. Any sudden opening ofthe throttle results in a lag between thetime the idling circuit stops functioning andthe main jet takes over. This is becausethere isn’t sufficient air flowing through thecarburetor throat to draw fuel from themain discharge nozzle. An acceleratorpump is used to compensate for this delayand eliminate sudden flat spots created bytemporary lean mixtures. The acceleratorpump is mechanically linked to the throttleand discharges fuel through a tube adjoin-ing the main nozzle. The accelerator tubeprotrudes into the airflow just inside theprimary venturi in the smaller carbs andinside the main venturi in the larger carbs.

Any fuel-borne contamination or cor-rosion latently within the carburetor canmigrate to these idle passages and create alean condition at low power settings. If thecontamination is significant, it will notallow for proper fuel/ air ratios at idle. Atsea-level, mixture rise as indicated on thetach should be approximately 25 to 50rpm. At higher field elevations of 5000-ft.or more, the mixture rise will be more inthe neighborhood of 75 to 100 rpm. Anidle adjustment screw that requires morethan four turns out to achieve a propermixture rise at ICO is a good indication ofcontamination in the idle circuit.

Don’t touch that jet!A restriction at the base of the dis-

charge nozzle serves as the main jet in MAseries carburetors. In HA-6 (horizontalside-draft) series carburetors, the power jetin the base of the bowl performs the samefunction. Some well-meaning technicalwriters have instructed mechanics to honethese jets in an effort to cool cylinder heador exhaust gas temperatures. Apparentlythey believe this to be a “cure-all” for leanrunning engines. A mechanic would be ill-advised to follow such instructions sinceno process approval exists for openingthese jets in the field. Furthermore, tam-pering with the jet size could mask otherserious problems. An induction leak, anincorrectly sized economizer jet, an incor-rect float setting, or perhaps even thewrong choice of carburetor could all con-tribute to a “lean” running engine. Anotherproblem with this mode of attack is thatthe jets vary in design. Some jets arestraight, while others are contoured orstepped. A mechanic who takes a drill bit,a ream, or sandpaper to the entrance of astepped opening may soon regret thatdecision. Fuel flows could exponentiallyincrease by merely breaking the steppededge and thereby creating a venturientrance. By contouring a stepped jetwe’ve managed to decrease the pressureand increase the velocity of the fuelthrough the jet.

When you start boring out nozzles,several things can happen. As you begin tosee appreciable results – the tendency is totake out more and more material. “If a lit-tle bit is good, a lot may be better.” Wrong!

Carburetor icing remains a serious problem for lightaircraft as evidenced by the number of incidences report-ed annually. Charles Lindbergh experienced icing whilecrossing the continental divide in The Spirit of St. Louis ashe prepared for his historic flight across the Atlantic. Tocorrect this problem, Lindbergh’s mechanics equipped theplane with a carburetor air heater.

The formation of ice on the throttle shaft and plateis a natural byproduct of the pressure drop across the ven-turi when certain atmospheric conditions exist. The typi-cal temperature drop within the throat of the carburetor is40 to 60 degrees. This temperature drop causes the dewpoint to drop and the air to become increasingly dense.Moisture in the air forms into water droplets that adhereto the cold surfaces in the throat of the carburetor. Carbicing can readily occur when outside air temps are as highas 90 degrees. In fact, it is often at these OATs that carb

icing is most menacing because warm air is capable ofsustaining more moisture.

A 1971 report by the National Research Council(NRC) of Canada revealed that Carburetor ice could bereduced by the use of gasoline soluble inhibitors. Thestudy selected temperatures, humidity and throttle platesettings that would cause the most severe icing condi-tions. It was found that the positioning of the throttle plateat approximately 40 deg (70 percent of max opening, orapprox. cruise configuration) produced the optimum build-up of ice. Manifold vacuum readings were used as ameans of assessing ice build-up. Visual observations pro-vided a secondary form of measurement. The study deter-mined that “ice formation occurred preferentially on theedges of the throttle plate and spread progressively acrossthe plate face.” The use of ethylene glycol monoethylether at 0.15 percent proved an effective deterrent to ice

build-up. Of more interest however, was the discovery thatcoating the throttle plate and shaft with 0.00125 layer ofTeflon “produced a marked reduction in ice formation.”“The throttle plate was virtually clear.” What is not clearis why in the ensuing years the manufacturers never uti-lized this data.

EGME is currently available under the name “Prist®

PF205 Low-Flow”, and is said to be a practical anti-iceadditive. A device called a carb temp probe is also avail-able. The probe enables the pilot to monitor temps withinthe carburetor and to detect when conditions are mostconducive to icing. For additional information regardingcarb-ice formation refer to Pilot Precautions andProcedures to be Taken in Preventing AircraftReciprocating Engine Induction System and Fuel SystemIcing Problems (FAA AC-20-112) and consult your POH.

Carb Icing

Looking up the throat of the MA-4SPACarburetor. This vantage point reveals the

proper placement of the pump discharge tube.The tube must be positioned just within the

center ring of the primary venturi.

Sure, your high-end fuel flows begin tolook better and so do your CHTs andEGTs. But unfortunately, mid-range fuelflows may become exceedingly rich. Thisrich condition is especially apparent as youtransition between idle and full power. Anengine that bogs down rich may surprise apilot forced to do a go-around when onshort final. Actuating the throttle causes

the pump plunger to spray an additionalslug of fuel into the carb throat.Thoughtful engineers have carefully tai-lored the idle circuit, the accelerator pump,and main jet for optimum performance. Amodified nozzle jet may cause the air/fuelratio to be overly rich. There is a point ofdiminishing returns, where high-end fuelflows no longer increase but mid-rangefuel/air ratios become too rich. And oncethis occurs, your wallet becomes increas-ingly lean as you shell out additional dol-lars for a replacement nozzle!

Choosing the correct carburetor

A common misunderstanding throughthe years has been that an engine/carburetorcombination that works well in one airplanewill function as well in another style air-

frame. Additionally, it is believed that if thesystem performs well in a test cell, then itwill perform equally well in flight.Unfortunately, to the chagrin of many ahomebuilder, this is not always the case.Airframe performance, cowling configura-tions, baffling, and choice of air-box, all con-tribute dramatically to fuel/air ratios and toengine cooling.

When selecting a carburetor for yourengine, always refer to both the engine andairframe type certification listings. Many havemade the critical mistake of selecting a car-buretor based solely on the part number(s)listed in the engine manufacturers parts andoverhaul manuals. Some have selected a car-buretor based on the part number currentlyinstalled in their airplane or on an airplane ofthe same model tied down on their airfield.Others have chosen a carb based on theOEM’s application guide. All these methodswill narrow down your search, but they willnot with any certainty guarantee that youhave selected the right unit for your particu-lar airplane. Incomplete engine logs, theswapping of engines to airframes, and theuncertain history of some engines mandatesa reliance on type certification data whendeciding on the appropriate carburetor foryour particular application.

To some, the theory and mechanicalfunctions of carburetors may seem rather for-midable. Yet, when each subsystem isviewed separately, it becomes readily appar-ent that these systems are easy to under-stand. In spite of all the intricacies associatedwith them, Marvel Schebler (Precision) car-buretors remain the same efficient, reliableunits our fathers and grandfathers flewbehind with confidence for decades. AMT

Photo shows acollapsed brass

float. Neverintroduce shopair to the inlet

fitting, dis-charge nozzle,or fly holes in

the throat of thecarburetor —the high pres-

sure might col-lapse the float.

Venturis. The two-piece venturi (above left),is subject to annual and 100-hr. inspections

as per AD 98-01-06. Single piece venturi(above right) requires no recurrent inspec-

tions. Carburetor data tags with ‘V’ stampedon them indicate that the single piece venturi

has been installed.

Randy Knuteson is Director of ProductSupport for Kelly Aerospace Power

Systems in Montgomery, AL.

Pump Plunger

Pump DischargeCheck Valve

Pump InletCheck Valve

Bowl DrainFitting

Fuel Inlet

Mixture Lever

Throttle/PumpLinkage

Idle EmulsionChannel

Idle MixtureAdjustment

Throttle Plate

Idle BleedTube

MainDischargeNozzle

Nozzle AirBleed Holes

Plunger InletCheckValve/ScreenMain Jet

Mixture ValveMetering Sleeve

Float Fulcrum

Needle/Seat

Venturi

Secondary & Tertiary Idle Delivery

& Air Vents

Photos of cut-away views of

MA-4SPACarburetor.

Model shown istypically installed

in TextronLycoming 0-320series engines.

October 2000 • Aircraft Maintenance Technology • www.AMTonline.com4 KA031003