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Camshaft & Lifter Wear

Camshaft & Lifter Wear

Corrosion attacks camshafts and lifters before most other bottom-end engine parts, especially when an aircraft doesn’t fly frequently. Learn how to prevent, identify and treat common camshaft and lifter problems before they cost you! 

Many folks think of engine wear and tear and overall condition as being represented by the amount of operating time since the last overhaul. The “time since major overhaul,” or TSMOH for short, is typically entered by mechanics into an aircraft’s logbooks at every major inspection and annual. One of the first questions that buyers ask when considering an aircraft for purchase is “How much time is on the engine?”

Cumulative engine operating time does matter. Moving parts, particularly in the high heat and high stress cylinder environment, do wear with use. However, one of the main reasons that engines have to get torn down and rebuilt isn’t due to wear from operating regularly, but from corrosion that occurs during prolonged periods of non-use. 

An engine’s camshaft and lifters are particularly susceptible to damage from corrosion. Corrosion and the resulting wear on the camshaft and lifter bodies are one of the primary reasons engines get torn down before reaching the manufacturer’s recommended time between overhauls (TBO).

Camshaft construction and function

Camshafts are a simple-looking part, but they play a major role in engine operation because they control and drive the entire valve train. The camshaft is gear-driven and typically turns at one-half the speed of the crankshaft. 

Camshafts are made of carbon steel and have varying numbers of “lobes” along their lengths. Each lobe has an elliptical shape and is put through a hardening process called “carburization” during its manufacture. 

The carburization process involves heating the camshaft in a specially-designed furnace or oven and exposing it to carbon monoxide gas as it is heated. This process causes the exposed surfaces on the camshaft (primarily the lobes) to absorb extra carbon, which makes the surface of the lobes very hard. The depth of the hardened layer on the lobe is about .015 inch (fifteen-thousandths of an inch). 

 

Lifter construction and function

The lifter consists of a cast-iron outer body that houses a hydraulic unit. There are a couple of Lycoming engines that still utilize solid lifters with no hydraulic unit, but the vast majority of engines have hydraulic lifters. Most mechanics use the term “lifter” when referring to both the lifter body and the hydraulic unit inside the body. 

Lifters are also sometimes referred to as “tappets.” The face of the lifter body is the part that is in continuous contact with the camshaft lobe that it rides on. The lifter face is made of chilled cast iron which gives it increased wear resistance. It is tough, but not nearly as hard as the carburized surface on the cam lobe. 

The lifter is designed to be a little softer than the lobe, so that the two are compatible with each other. If deterioration does start to occur, it generally occurs first on the lifter rather than the camshaft lobe. 

The hydraulic unit in the lifter body is designed to act as a solid unit as it opens the valve. It must also be able to expand or contract as needed to take up all extra clearance in the valve train. There are some variations in the exact design, but the basic function is the same. 

 

 

Valve operation

The cylinder valves are linked to the camshaft through the lifter, push rod and rocker arm. The camshaft rotates and the top of the lobe raises the lifter to its highest point. The push rod in turn raises the rocker arm end to which it is joined.

Since the rocker arm is mounted near its center on a steel shaft, it acts as a fulcrum and raising one end causes the other end to lower. The opposite end of the rocker arm that is in contact with the top of the valve stem then lowers the valve to its full open position. 

The time that the valve opens in relation to all other moving engine components, and the length of time that it stays open are both determined by the height and position of the camshaft lobe. 

The valve clearance, or valve “lash” as it is often called, is measured with both the exhaust and intake valves closed with the piston at top dead center (TDC) on its compression stroke. The clearance is measured with the hydraulic unit of the lifter “bled down,” or in its flattest position with no oil in the reservoir of the lifter. 

The clearance is checked between the valve stem and rocker arm with the push rod socket on the rocker arm pressed in toward the camshaft. The engine manufacturers set a minimum and maximum clearance for the valves. Too much or too little clearance can cause increased wear on the valve operating mechanism. 

The clearance is adjusted on engines with solid lifters by means of an adjustment screw on the rocker arm itself. On engines with hydraulic lifters, the adjustment is made by replacing the push rod with another of a different length. 

The valves are held in the closed position by multiple strong valve springs. The camshaft and lifter must overcome the valve spring tension every time the valve is opened. This generates a large amount of pressure between the lifter face and the camshaft lobe. 

 

 

Lifter spalling

The camshaft and lifter bodies typically are lubricated in most engines by what is called “splash lubrication.” There are no pressurized oil ports on standard engines that deliver oil to the cam and lifter surface; they run only in the oil that is thrown off the crankshaft and rods as they rotate. 

At engine start-up, it typically takes a few revolutions of the crankshaft to begin getting any significant amount of oil to the camshaft. The lack of pressure lubrication along with the high pressure between the interface of the camshaft lobe and lifter face create an environment that is unforgiving of any defects in the surfaces of either the lobe or lifter. 

Engines that sit dormant for extended periods of time (between two to six months, depending on the environment) are subject to developing corrosion on both the lifters and on the cam lobes themselves. 

The camshaft on Lycoming engines is located in the top of the crankcase above the crankshaft. Its position in the top of the case means that oil drains off it first once the engine is shut down. On Continentals, the camshaft is located beneath the crankshaft, but it is still above the oil sump and residual oil eventually drains off it as well. 

The crankshaft is not as vulnerable to corrosion because its metal-to-metal contact surfaces are encased in bearings. The bearings typically hold a film of oil on the crankshaft surface. The camshaft has no bearings. It spins inside machined grooves in the crankcase. The lobes and lifters have a lot of their surfaces fully exposed to the air that drifts through the engine. 

Aircraft engine breathers are simply vents designed to prevent pressurization of the crankcase with the engine running. However, they also vent and expose the internal engine components to any humidity that may be present in the outside atmosphere as the airplane sits parked.

Condensation occasionally occurs inside the engine after shutdown as parts of the engine cool at varying rates. Condensation also occurs with different weather conditions during times of non-use. Moisture can cause small freckles of rust to form on the lifter face. As the corrosion progresses, the rust eats into the lifter surface. 

Once the engine is started again, the small patches of rusted material fall out as the lifter face is pressed against the camshaft lobe. After this process starts, the constant pressure between the cam lobe and lifter wears off more and more material even though the engine is being lubricated during operation. This process is referred to as “spalling” of the lifter. 

The removed material and the rough lifter face will eventually wear the camshaft lobe. If lifter spalling is caught early, the problem may be fixed by replacing the affected lifters. However, if any of the camshaft lobes have begun to wear, the entire camshaft—in addition to the damaged lifters—must be replaced. 

 

Lifter inspection on Continental engines

Most Continental engines use cylindrically-shaped lifter bodies that can be removed without splitting the crankcase. Lycoming engines use mushroom-shaped lifter bodies that have a larger diameter at the face of the lifter. These types of lifter bodies cannot be removed without splitting the case. 

On the larger Continentals (O-470 series, IO-520, etc.), lifter removal only requires removal of the affected cylinder valve cover, rocker arm, push rod and the push rod housing. The housing is removed by compressing the tension spring on the inboard part of the housing to release it. 

Once the housing is out of the way, the lifter can be removed by gently using a pick to grab it by the oil galley in the push rod socket and sliding it out. Engine manufacturers do not recommend using a mechanic’s magnet to remove a lifter because it could magnetize the ball in the check valve of the hydraulic unit and cause the valve to malfunction.

After the lifter has been removed, the camshaft lobe can be inspected using a bright light. Continental Service Information Directive SID 05-1B contains inspection criteria which should be used to determine if a camshaft is still airworthy. Continental recommends using a mechanic’s pick to probe any surface deformities. 

If the pick catches, the camshaft most likely will need to be replaced. If the camshaft lobe looks OK with no surface damage, the lifter itself can be replaced with a new one if it shows any sign of wear. 

Some mechanics are leery of replacing lifters without replacing the camshaft because the lifters and cam lobes do wear into each other slightly during the initial engine break in, but it is primarily the lifter, not the camshaft lobe that wears. 

Aircraft owners who have large Continental engines that have been dormant for an extended period of time would be wise to inspect their lifters before they run the engine again. The process of doing so may very well catch a corroded lifter before it has a chance to wear a cam lobe. Six months would be considered an extended period of time under usual conditions; two months in humid or salty areas.

 

Clues that a camshaft may be wearing

Most camshaft and lifter wear goes undetected until fragments or small magnetic filings show up in the oil filter during an oil change. Even though amounts may be very small, the camshaft and lifter bodies are usually the culprits if an engine suddenly starts making magnetic material. Steel cylinder barrels or steel cylinder rings also are prone to rusting a little, but they generally stop shedding any rusted debris as soon as the airplane is flown a time or two. 

Regularly-occurring magnetic filings are most likely coming from the lifter bodies, and once lifters start shedding surface material, they continue to degrade and wear with subsequent use. The good thing is that other than possibly causing a tiny bit of engine roughness, cam lobe and lifter wear is not something that causes sudden stoppage of an engine. 

If small amounts of filings are found, the oil filter element and its contents should be sent to an aviation oil analysis lab to determine the source. An oil-only analysis will probably show nothing unusual in these cases because when the lifters start spalling, the flakes and filings they shed are so large that they get caught in the filter rather than being suspended in the oil itself. Most labs can examine the filter element itself to determine where the metal is coming from. 

Some mechanics that find very small amounts of magnetic material (less than enough to cover the end of a mechanic’s magnet) will recommend flying the plane for another 10 hours or so, then checking the filter again. The hope is that it will stop on its own, but an engine that is past the break-in period should not be making magnetic material. 

If the filings are coming from rusted cylinder barrels, an owner might get lucky and the oil filter might be clean on the next check, but it’s been my experience that almost every time I’ve found magnetic debris in the filter, it only gets worse and worse with use. 

 

 

Reground versus new camshafts

A camshaft or lifters that are worn badly enough to necessitate an engine teardown will require replacement. Many engine rebuilders in the field have no problems using a reground camshaft (one that was still within serviceable limits that has the lobes re-machined to a smooth finish.) Lifter body faces can also be reground to a smooth finish. 

If an owner chooses to use a reground camshaft or lifters, they should only get them from a high-quality aircraft machine shop experienced in the regrinding process. The carburized hardened layer on the camshaft lobes is only around .015 inch deep, and the layer’s depth can vary a little. 

If a machine shop accidentally gets below the carburized layer at any time during the regrinding process, the camshaft lobe will wear down rapidly once it is put back into service. The initial hardening process of carburization is completed only at manufacture. It can’t be duplicated in the field because it is almost impossible to keep the camshaft from warping during the carburization process. 

Additionally, the apex (or top) of the camshaft lobe is tapered slightly. One side of the lobe is slightly (around .003 inch) higher than the other side. This design causes the lifter body to spin as it contacts the lobe. This prevents the camshaft lobe from contacting the lifter face in the same place every time, and helps prevent wear on the lifter face. The exact taper may be hard for some shops to duplicate.

I personally would rather have a new camshaft and lifters if the engine is being torn down to the extent that they were accessible. However, most reputable engine builders that I spoke with had no problem using reground cams and lifters, depending on where they were done, and I know of more than a few engines that have easily made and even gone past TBO with reground parts.

 

 

 

Prevention

The best thing an owner can do to prevent camshaft and lifter problems is to regularly fly his or her plane—at least once a week in humid climates. Each flight should be long enough to get the oil temperature up to at least 180 degrees to evaporate any water in the oil system. Changing the oil regularly and often is a good preventative maintenance practice. The addition of an anti-corrosion additive also helps. 

The initial cost of prevention generally is greatly rewarded with a long-lasting healthy engine. 

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

 

RESOURCES >>>>>

TCM Service Information Directive 05-01B, “Inspection Guidelines for CM Camshafts and Hydraulic Lifters”
http://www.tcmlink.com/pdf2/SID05-1B.pdf

 

Anti-corrosion additives
AvBlend – PFA supporter

 

ASL Camguard
aslcamguard.com
Dissecting a Dry Air Pump

Dissecting a Dry Air Pump

A look inside your aircraft’s vacuum system. 

 

 

 

When the earliest airplane gyroscopic instruments were introduced, the only available source for air pressure to spin them was an outside-mounted air venturi. The venturi accelerated the ram air pressure produced by forward flight through a narrowed opening. The instrument hoses were connected to the venturi at the point of lowest pressure, creating a vacuum that pulled a steady stream of air through the instruments. 

Although some VFR-only planes still use this arrangement, the trouble with this setup is that the amount of vacuum is low until certain airspeeds are reached, and the venturi can become ineffective due to ice buildup during inflight icing conditions. 

In the late 1930s, air pumps were developed that were engine-driven, creating air suction (or pressure) as soon as the engine was started. These early air pumps were lubricated with engine oil and would later be called “wet style” pumps. 

In the 1960s wet pumps were largely replaced with the “dry” pumps. Dry vacuum pumps are self-lubricating and have an oil-free exhaust flow that reduces belly deposits and provides a much cleaner source of air pressure on aircraft models that use the vacuum pump exhaust for inflating de-ice boots. The dry air pumps are also a little less expensive and weigh about half as much as the older wet style pumps.

 

 


How the pump works

The standard dry air vacuum pump consists of a rotating carbon vane assembly housed in an elliptically-shaped aluminum housing. The carbon vane assembly is powered by the engine accessory drive. The outer pump housing has an air inlet port on the front of the pump and an exhaust port on the rear. 

The rotor portion of the carbon vane assembly has slots that house the carbon vanes themselves. The vanes are free to slide inward and outward as the rotor spins. Centrifugal force keeps the vanes in contact with the inner wall of the pump housing. 

As the rotor spins, the vanes in the rotor slide in at the narrow section of the housing and slide outward to their maximum extension at the widest points of the elliptical housing they travel in. 

The intake air from the instrument system is routed through the pump fitting to ports in the forward section of the pump housing. The ports are open at the bottom and sides of the pump housing to allow air to flow in as the carbon vanes are beginning to move outward in the rotor slots. 

The air is then compressed as its compartment is compacted while the vanes rotate toward the narrow part of the housing. It is then accelerated out of exhaust ports located in the narrowest part of the ellipse. This all occurs through the first 180 degrees of rotation. 

As soon as the vanes move past the exhaust openings, they scoop in intake air from a second set of intake air openings—and the entire process is completed again in the second 180 degrees of rotation. 

  


How the vacuum system works

The airflow through a common single-engine aircraft vacuum system begins under the instrument panel. Air enters the system through a central pleated paper filter. The filter is located under the instrument panel. Ambient air is drawn into and through it solely due to the suction of the attached hoses going to the vacuum pump. It then flows through the attitude and heading indicators before reaching the system regulator. 

The system regulator combines additional air as needed to the intake of the pump so that the system suction stays within the parameters the regulator is adjusted to maintain. (The regulator has a slipover “sock” style filter to protect the pump from any particles that might be drawn in with the ambient air.) Airflow continues through the pump and then is exhausted into the engine compartment on most models. Aircraft with de-ice boots utilize the pump’s exhaust air to inflate the boots. 

The artificial horizon and directional gyro flight instruments are usually connected to the vacuum system in parallel with each having its own connections to the intake and vacuum air so that even if one instrument were to fail or become clogged, the other one still functions because it has its own connection to the air source. 

A suction gauge is connected in the system so that it measures the air pressure difference between the supply line from the central paper intake filter and the outlet of one of the instruments prior to reaching the system regulator. The pressure drop from the intake air (which is close to atmospheric pressure on nonpressurized planes) and the air being drawn into the regulator is measured in inches of suction. 

Most vacuum systems are designed to operate with around five inches of suction with the engine rpm at or near a cruise setting. If the system suction is too high, it can cause excessive wear in the gyros and the vacuum pump. If the vacuum system suction is too low, the instruments will not give reliable indications. 

Twin-engine aircraft with two vacuum pumps also utilize various check valves so that a failure of either pump doesn’t cause the system to lose vacuum. 

 

 

 


Vacuum pump failure

Vacuum pumps are built to run for several hundred hours—but one of the biggest downfalls of dry air vacuum pumps is that when they do fail, it is usually suddenly and without warning. 

The carbon vanes, by design, will wear down over time as the pump operates. Eventually, the vanes can become so short that they will either hang up, or come completely out of their slots as they rotate through the wide part of the ellipse and cause the sudden stoppage of the rotor assembly in the pump. 

Also, the inner wall of the aluminum housing is prone to developing indentions and slight deformities as the vanes slide in and out against it. These indentions can cause one or more of the vanes to hang up and break apart. 

Some vacuum pump manufacturers have incorporated a wear indicator port on the side (or on some models, the rear) of the pump. The ports allow access to check the length of the vanes, which can help catch an impending failure. (The pumps are designed with a nylon drive coupling that shears in two if the pump does lock up, so that the gears in the engine accessory case are not damaged when a pump fails.) 

Contamination within the vacuum pump can also cause a sudden failure. The hoses used in the vacuum system can become dried and brittle over time. If internal pieces of hose begin to flake off, or if any contaminants get into the vacuum system downstream of the central paper filter, they go straight through the pump. The small sock filter on the regulator only filters the ambient air that is added to the flow as the pressure is regulated—not the already-filtered air from the instruments. 

Some mechanics use Teflon tape or some type of sealing compound on the pipe-threaded instrument and pump fittings in the vacuum system. Teflon tape and other sealants are not recommended for use at all in the instrument system, because pieces of the tape or sealant can make their way into the system as the fittings are threaded into place and these may eventually get sucked into the pump. 

The filters themselves can become sources of contamination over time if they aren’t regularly replaced. The sock filter in particular can become so dried-out and brittle that pieces of it may be ingested into the airflow. Most vacuum pump manufacturers require replacement of all filters at the time of installation in order for the pump warranty to be valid. 

Solvents used to wash down the engine during maintenance are very damaging to vacuum pumps. If any of the solvent material gets into the pump, it causes the graphite powder—which is always present from normal wear—to turn into a paste that gums up the inside of the pump. Pump manufacturers recommend completely covering the pump with a resealable plastic bag and tie wraps before washing down the engine.

Oil contamination is also a big culprit in premature vacuum pump failures. One of the biggest sources of oil contamination typically comes from a leaking oil seal on the engine accessory case adapter drive. The adapter drive gear in the accessory case is made with a splined hollow shaft that spins the vacuum pump drive coupling. 

There is an oil seal that the vacuum pump drive gear is inserted through. It naturally wears out over time because the shaft is spinning inside of it. Once the oil seal begins to degrade, it allows oil—under pressure—to head straight for the pump drive coupling and into the pump itself. This excess oil causes a gummy paste to form that eventually binds the pump. 

Kinked fittings or hoses can cause excess wear on a pump by forcing it to work harder than it should to maintain vacuum suction. (If suction levels begin to degrade, lots of mechanics simply increase the suction by adjusting the regulator—instead of determining the exact cause of the suction loss. If a system starts to become sluggish, the root cause should be determined before simply cranking up the regulator.) 

 


Vacuum pump replacement

Vacuum pumps are typically straightforward to replace.

The hoses and fittings on the old pump should be removed before the mounting nuts are removed, so the pump is held tightly in place as the hoses are pulled off the fittings. 

Most vacuum pumps are mounted on four studs and secured with plain nuts and lock washers. Typically one or more of the nuts are difficult to access with a normal type of wrench, and vacuum pump manufacturers make a specially-curved wrench that helps gain a little access. 

The old mechanic’s trick for breaking loose the nuts that are in a tight place involves using a long flat blade screwdriver placed on the loosening side of the nut. The screwdriver is then gently tapped with a small hammer to break the nut loose. 

A new oil seal should be installed anytime a new vacuum pump is installed—whether the old oil seal is leaking or not. The seals wear out over time and require periodic replacement. They are also reasonably priced (around three dollars each), so cost is not a consideration. 

If an owner is having a shop replace the pump, it is best to specifically request that the oil seal be replaced in addition to the pump, because some mechanics don’t replace them unless they are leaking. 

There are two gaskets that require replacement: the one between the vacuum pump and its drive housing, and the one between the drive housing and the engine accessory case. 

Be sure to check the aircraft maintenance manual to be sure the new vacuum pump being installed is the correct model number. Pumps rotate either counter-clockwise or clockwise as viewed from the rear of the pump and case. (The rotation is specified as “CC” or “CW” in the part number.) Putting the wrong pump on will cause it to spin opposite the direction the rotor slots and vanes are designed for, and the pump will fail in short order—if not immediately. 

The vacuum system hoses and system regulator that are just upstream from the vacuum pump must be checked for contamination whenever a failed pump is being replaced. Pieces of the old pump vanes often get sucked backward into the suction hose—against the normal direction of airflow—as the pump fails because the system still retains a lower pressure for a few seconds even though the pump has stopped. 

If the hose isn’t cleaned or replaced after a sudden pump failure, carbon and vane parts will be sucked into the new pump upon startup.

If compressed air is used to blow out the lines, be sure all the instruments are disconnected so they don’t get blasted with excessive pressure or contaminated by unfiltered particles. Also, as the manufacturers specify, all of the vacuum system filters should be replaced at each pump replacement.

Owners that fly a lot of hard IFR might consider periodically replacing a vacuum pump based on time in use alone, even if that pump is operating properly. 

Many new aircraft are shifting toward a glass panel configuration, but the benefit of the vacuum pump system is that it will still power the pneumatic instruments even in the event of a total electrical failure. A little preventive maintenance and upkeep on the vacuum system can help owners be assured that the indications on the gauges can be trusted in the clouds.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting preventive maintenance tasks.

 

 

 

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or
comments to .

The Right Mix: An Aircraft Carburetor Overview

The Right Mix: An Aircraft Carburetor Overview

 

Many Piper aircraft depend on a carburetor. Piper Flyer contributing editor and A&P Jacqueline Shipe explains the operation of this fairly simple—and very reliable—invention.

One of the most recognized carburetor manufacturers for the GA fleet is Marvel-Schebler. The company has been around a long time, having its beginnings in the early 1900s when George Schebler and his friend Burt Pierce worked together to design the first carburetor using a tin can with a flap to regulate airflow. 

They both went on to patent their designs, with Pierce calling his carburetor the “Marvel.” Both the Marvel and the Schebler designs were successful and used on a variety of engine types. 

In the early days of General Motors, the two merged and became known as Marvel-Schebler Carburetor Co. (Author’s note: Burt Pierce also designed the still-popular Marvel Mystery Oil through Marvel Oil Co., which he founded in 1923.) In the beginning, the Marvel-Schebler Carburetor Co. made carburetors for cars, boats, tractors and airplanes. 

The company has since changed hands several times, being purchased and resold by Facet Aerospace Products, Zenith Fuel Systems, Precision Airmotive and the Tempest Group (who called it Volare Carburetors until it acquired the Marvel-Schebler trademark in 2010). Today, Marvel-Schebler Aircraft Carburetors LLC produces a complete line of aviation carburetors and parts.

Although Marvel-Schebler is the most recognized brand for aviation carburetors, there are other FAA approved manufacturers, including AVStar Fuel Systems in Florida. 

AVStar was formed in 2007 and has gone on to become the supplier for Lycoming Engines as well as numerous individual customers. AVStar manufactures a line of carburetors as well as kits and parts for use in almost all carburetor models in the General Aviation fleet.

 

 

 

How a carburetor functions

Aircraft engines rely on a steady source of fuel to provide the energy needed to support combustion. Liquid fuel must be vaporized and mixed with the proper amount of air in order to burn properly in the cylinders. 

Many General Aviation planes depend on a carburetor to provide a continuous, reliable source of properly mixed fuel and air to each cylinder. The aircraft carburetor has a relatively simple design and is typically very reliable.

Most aircraft carburetors are fairly straightforward in construction. A top part, called a throttle body, houses the throttle valve, mixture control and venturi; a lower bowl section, called a reservoir, holds a consistent volume of fuel. 

Almost all aviation carburetors are float-style carburetors. This means that a float mechanism regulates the fuel level in the reservoir (i.e., bowl). 

 


The float mechanism

The float is hinged on the rear, allowing it to pivot up and down. A pencil tip-shaped float valve is attached to the top rear of the float. 

Fuel enters the carburetor through the inlet screen, flows down through the float valve and its seat, and into the carburetor bowl. As the fuel level rises, the float and the attached float valve also rise until the float valve is implanted in the seat, shutting off the fuel flow. 

As the fuel level in the bowl drops, the float and float valve also descend, allowing fuel to once again flow into the bowl.

The float travel from full-up to full-down is relatively short; it is stopped on the descent by a tab on the rear hinge. The level to which it rises up is stopped by the attached float valve and seat. 

 


Adjusting fuel level

It is important to maintain a correct fuel level in the bowl. If the fuel level is too low, the engine will run too lean; if it is too high, the engine will run rich and fuel may leak continuously from the discharge nozzle. 

The fuel level is adjustable by adding or removing washers under the float valve seat to extend or lower it, or by bending a tab on the float itself at the point of contact with the float valve to extend or lower the valve.

 

 

 

Airflow

Airflow through the carburetor throat begins at the aircraft air filter and proceeds through the airbox into the throat of the carburetor. 

A venturi in the carburetor throat narrows the airflow opening, increasing the speed of the air, thereby lowering its pressure. (This is based on Bernoulli’s principle of airspeed and pressure being inversely proportionate; the same principle explains how an airfoil generates lift.) The outlet for the fuel discharge nozzle from the bowl is placed in the center of this low-pressure area. 

The air chamber on top of the fuel in the carburetor bowl is vented to atmospheric pressure. The pressure difference from the atmospheric pressure on top of the fuel in the bowl versus the low pressure on the fuel discharge nozzle causes fuel to flow out the fuel discharge nozzle. 

A throttle valve (i.e., a butterfly valve) located just downstream of the venturi controls mass airflow through the carburetor throat. As airflow increases, the suction effect on the fuel discharge nozzle also increases proportionately, allowing more fuel to flow. 

 


Fuel flow

Before fuel flows from the bowl out the fuel discharge nozzle, it is routed through the mixture control valve. The mixture control valve is attached to the mixture control arm. 

The mixture control valve on most models contains a shaft (also called a stem). The bottom of this shaft is shaped like a half-cylinder. It rotates in a cylindrically-shaped sleeve with an opening on the side. 

When the mixture is set at full rich, the open part of the shaft/stem is aligned with the opening in the sleeve, allowing full fuel flow through the valve and out of the nozzle. As the mixture control is pulled back to leaner settings, the opening becomes more and more narrow until it is completely closed at cutoff.

When the mixture control valve is open, fuel flows from the mixture sleeve through the main metering jet (this is a fixed orifice that controls the maximum amount of fuel allowed to exit the main discharge nozzle once the mixture control is set to full rich) and into the discharge nozzle well, where it begins to be mixed with air from bleed holes in the nozzle. From there, it flows up and out the main discharge nozzle and into the intake pipes for the cylinders. 

At low throttle settings with the throttle valve nearly closed, there is not enough suction on the main discharge nozzle to cause fuel to flow out of it, but there is a slight amount of airflow between the edge of the throttle valve and the wall of the throttle body. 

This small area of airflow around the edges of the throttle valve acts as a venturi, forcing airflow to speed up as it passes between the edges of the throttle valve and the carburetor throat and lowering the air pressure. 

In order to provide adequate fuel for idling, small openings are made in the throttle body in this area of low pressure. Ports connect the openings with the inner section of the main fuel nozzle and draw fuel from the nozzle at low throttle settings. This arrangement provides an adequate fuel supply for idle speeds. 

 

 

Idle adjustment

The idle speed and mixture are adjustable, and are the only two adjustments that can be made on most carburetors. Most planes should idle at speeds of 600 to 650 rpm. The idle speed adjustment is simply a stop screw that limits the rear travel of the throttle arm. (It screws in to increase idle speed; moving the screw counterclockwise decreases idle speed.)

The idle mixture adjustment is a large screw on the top rear of the carburetor that screws a needle closer to or further from its seat, which allows more or less fuel to flow through the idle passageways. 

The idle mixture is made leaner as the screw is turned in and richer as it is backed out. It should be adjusted so that there is a 25 to 50 rpm rise in engine speed when the mixture control is pulled all the way back to shut down the engine. 

If there is no rise when the mixture is pulled back to cutoff, the idle mixture is too lean. If there is a rise of more than 50 rpm, it is too rich. 

There have been instances where the idle mixture screw has vibrated loose and fallen out. If this happens, the engine won’t idle at all, but will try to shut down when the throttle is reduced to idle settings.


Basic maintenance and troubleshooting

Aircraft carburetors are generally reliable and seldom require much attention. The internal parts of a carburetor rarely need maintenance if the airplane is flown regularly and clean gas is used. 

An inlet screen that the fuel supply line attaches to can be removed for cleaning. Generally it stays pretty clean, because most debris gets caught in the aircraft fuel strainer before it has a chance to enter the carburetor. 

Over time, the throttle shaft bushings wear, especially on training aircraft that endure several power changes and throttle movements every hour. Worn bushings can allow a slight intake leak and cause an overly lean mixture. 

Most carburetors have an accelerator pump that squirts a stream of extra fuel into the intake air as the throttle is advanced so the sudden burst of extra intake air doesn’t create a lean condition and cause the engine to stumble, especially if the throttle is opened suddenly. The accelerator pump has a plunger that gets worn with use and periodically requires replacement.

Any leaks coming from a carburetor are cause for concern. A carburetor that leaks when sitting with the engine off most likely just has a tiny bit of debris trapped between the float valve and seat. Draining the fuel from the carburetor bowl and then flushing it by allowing it to refill and draining it again will most likely clear it up. 

 


Long-term storage of an aircraft

A carburetor on a plane that has sat with the aircraft fuel shut off may not allow fuel to enter the bowl when the fuel is turned on due to a stuck float valve. Gently tapping the side of the bowl with a small rubber mallet sometimes jars it loose and allows fuel to re-enter the bowl. 

If a stuck valve is suspected, momentarily crack open the supply line with the fuel turned on to be sure gas is getting to the carburetor, then re-tighten. Next, slowly remove the drain plug to see if there is fuel in the bowl. An empty bowl indicates a stuck valve or an obstruction in the inlet.

For folks that have an auto gas STC, it is best to never leave a plane with auto fuel sitting in the tanks, lines or carburetor for extended periods. Auto fuel causes deposits of varnish to form on the inner surfaces of the fuel system and often seizes the mixture control valve in place. 

If a plane is left sitting for a season, it will be far better for it to sit with Avgas in it. (Better yet, you may wish to “pickle” the aircraft. For more information, take a look at Steve Ells’ 2015 article “Flying, Interrupted: Modern Engine Preservation” in the archives at PiperFlyer.org.) 

Aviation carburetors are some of the most reliable inventions ever made. Their simple design and quality construction offer years of trouble-free service as long as they are flown regularly and proper steps are taken to ensure a clean fuel supply.

Know your FAR/AIM and check with your mechanic before starting any work. Always get instruction from an A&P prior to attempting any aircraft maintenance tasks.

Jacqueline Shipe grew up in an aviation home; her dad was a flight instructor. She soloed at age 16 and went on to get her CFII and ATP certificate. Shipe also attended Kentucky Tech and obtained an airframe and powerplant license. She has worked as a mechanic for the airlines and on a variety of General Aviation planes. She’s also logged over 5,000 hours of flight instruction time. Send question or comments to .

RESOURCES >>>>>

Avstar Fuel Systems Inc.
 
Marvel-Schebler Aircraft Carburetors, LLC
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