Piston Engine Fuel Controls

Piston Engine Fuel ControlsBy Alan Jesmer

By now, the basic operating principles of the carburetor are well known. A float type carburetor consists of a main air passage through which the engine draws its supply of air, a mechanism to control the quantity of fuel discharged in relation to the flow of air, and a means of regulating the quantity of fuel/air mixture delivered to engine cylinders. This float carburetor widely used today dates to Marvel/Schebler Aircraft Carburetor company of the 1930s. In time, Marvel-Schebler became a division of Borg-Warner. Facet Aerospace then purchased the line from Borg-Warner and manufactured and supported this carburetor until Precision Airmotive Corp., Marysville, WA, bought the line in 1990 and became the current manufacturer of this carburetor.

The principle competition for Marvel-Schebler in the early days was the Bendix-Stromberg float carburetor. These fuel controls have a designated Model NA-XXX and range in size from the A-65 Continental engines up to the R-1340 Pratt & Whitney radial engines. They are still used today. Though they are no longer manufactured new, they are supported with replacement parts, maintenance and overhaul manuals, service information, and technical support.

Another type of fuel control introduced by Bendix-Stromberg were pressure carburetors. These fuel controls are distinctly different from float type carburetors of the same size and power range, as they do not incorporate a vented float chamber or suction pick up from a discharge nozzle. Instead they offer a pressure-fed fuel system that is closed from the engine fuel pump to the discharge nozzle. The venturi serves only to create pressure differentials for controlling the quantity of fuel in proportion to air flow to the engine. This type of fuel system offers the following advantages: Since the discharge nozzle is located downstream from the throttle valve, ice formation within the throttle body is eliminated. It maintains proper fuel/air ratios regardless of aircraft attitude. In some models, there is automatic compensation for temperature and altitude. Finally, with only minor modification, (mainly the location of the vapor vent outlet), they can be adapted for updraft, downdraft or horizontal operation.

Precision Engines, Everett, WA, is the current owner of these Bendix-Stromberg fuel controls. This line was obtained by Precision Engines in 1984. Although new units are not manufactured, they do manufacture all replacement parts, and supply all manuals, service information, (service bulletins, service information letters, etc.), and technical support.

Ultimately, Bendix developed a more refined system, the RS/RSA fuel injection systems. The earlier RS fuel injection system had a return to tank line similar to the pressure carburetors. To simplify the design even more, the RSA fuel injection system was introduced. This system eliminated the return to tank line and made for easier fuel management. With a simplified design, these injection systems are similar to pressure carburetors. Instead of discharging fuel through a discharge nozzle in the throttle body to the intake manifold, they meter fuel up to a single point injector just in front of a supercharger or to a flow divider. This distributes fuel to an air-bleed nozzle at each cylinder.

Both RS and RSA fuel injection systems are designed to meter fuel in direct ratio to the volume of air being consumed by the engine at any given time. This is accomplished by sensing venturi suction and impact air pressures in the throttle body. Opening or closing the throttle valve results in a change in the volume of air being drawn into the engine. This results in a change in the velocity of air passing across the impact tubes and through the venturi. When air velocity increases, the pressure at the impact tubes remains relatively constant, (depending upon the inlet duct configuration, etc.) and the pressure at the venturi throat decreases. This decrease creates a differential, (impact minus suction), which is used over the entire range of operation above idle, (approx. 1,200 rpm), which is a measurement of the volume of air consumption. Bendix Energy Controls division sold the RS/RSA fuel injection line to Precision Airmotive Corp. in 1988, the current manufacturer.

Please take note: Marvel-Schebler, Facet, Bendix-Stromberg, and Bendix do not exist anymore.

All fuel controls must be overhauled periodically according to FAA guidelines and Precision Airmotive Service Bulletins MSA-3, (for float carburetors), or PRS-97, (for RSA fuel injection). These service bulletins state that overhaul of a fuel control be accomplished at recommended engine TBO or 10 years calendar time, whichever occurs first. The engine manufacturers determined the number of hours between overhaul, while at the same time, Precision Airmotive decided the calendar time of 10 years between overhaul if the TBO hours had not been reached. This is because corrosion is the leading contributor to contamination, and many aircraft sit idle for extended periods of time. Also, if automotive gasoline is used, it will begin to deteriorate very quickly. Av-gas does have stabilizers added to help reduce rapid deteriorization but, with time, will break down as well. As these fuels deteriorate, they cause acids to develop, which are the primary ingredients in forming corrosion. Adding to this problem, extended periods of inactivity may cause rubber parts to become brittle, gaskets to become dried out, springs become weak, and jets may become plugged. Also, an unused fuel control is susceptible to the introduction of moisture, which is usually the beginning of another set of problems.

Running the engine, on the other hand, will help keep rubber parts moist, gaskets will stay pliable, jets are less likely to become plugged, and running frequently will help keep moisture out. If you are not going to fly for a while, especially more than a month or two, you may want to remove your fuel control and store it properly per the appropriate overhaul manual. One more important item to consider; in many installations, the fuel control is the lowest part of your fuel system. So where do you think any water is going to migrate? You're right, so it is important to drain your bowl, (of your float carburetor), at every annual.

Besides periodic overhauls, there are other conditions that may require maintenance and, or, overhaul:
ADs - Airworthiness Directives. These are mandatory FAA publications that must be complied with.
SBs - Service Bulletins (mandatory). If the factory feels it is a safety of flight issue, they will issue a mandatory service bulletin and per the factory, it should be complied with. There are other severe situations that apply for overhaul such as but not limited to: engine fire, external or prolonged air intake manifold fire, contaminated fuel such as water, rust, sand, etc. You see, there are more conditions than you can imagine, so take a good look at what could be affecting you. Read all bulletins carefully before removing a unit for compliance. Pay particular attention to the Index of manuals, bulletins, and service information letters, P/N 15-895. This manual identifies the units affected by the bulletin, part number, and serial number, if listed. This publication is a quick reference for float carburetors and RS/RSA fuel injection systems and shows which service bulletins are required for each model of fuel control. This manual is updated annually and is always available at no charge, so request it every year direct from the factory, (Precision Airmotive). It is embarrassing to spend an hour removing a unit, then not be able to find the part you are supposed to be inspecting because it does not apply to the bulletin. It is also expensive to send a unit to a repair facility for bulletin compliance so be sure the bulletin does apply. If in doubt, call and ask product support for help.

Repair kits
Since float carburetors can be maintained, repaired, and overhauled in the field, there are a number of repair kits that can be used in your fuel control depending on TBO periods, ADs, SBs, etc. It is always recommended to install a minor repair kit if you pull the halves of a float carburetor apart for any reason. Here are a few recommendations.

Major repair kits are recommended when engine TBO or 10 years calendar time has been reached per Precision SB MSA-3. Since most people do not have a flow bench to check the operation of the carburetor, it is recommended to always install all the parts from the Precision Airmotive repair kit to ensure the carburetor is back to original new factory specifications on all critical components. Additionally, there is no sure way to know what someone has done to the unit before you received it, such as illegally modified jets, idle tubes, incorrect screws, washers, O-rings, etc. There are other kits out there that don't include parts that the factory recommends, so know what you are getting and use the parts list to verify. There are certain components such as floats and venturis that are not included because they may be re-used if they are within overhaul tolerances.

Minor repair kits are recommended when a carburetor is disassembled for any reason. Again, it is highly recommended to install all parts in this kit. There are too many variables, such as amount of hours on the carburetor, treatment of unit, and maintenance practices to decide what parts of the kit to put in. Keep in mind good maintenance practices make for longevity of a carburetor. The minor repair kit contains parts such as gasket kit, needle and seat assembly, pump plunger assembly, and assorted screws and washers. These parts are vital to install for proper operation. The gasket kit included is also vitally important to install. Believe it or not, there is a science to gaskets and their development. Wear of gaskets can consist of compressed gaskets from overtorque of body to bowl screws, brittle or dried-out gaskets caused by prolonged sitting or just plain old which need to be replaced. Many times, a carburetor will not perform as well as it did prior to disassembly if old parts are used to reassemble a carburetor. This is probably the best reason to use all kit parts. Be safe, put them all in.

Throttle shaft kits are very important. Certain conditions may cause the throttle shaft and bushings to wear out prior to TBO such as flight training or rental aircraft where rough treatment of the throttle may cause premature wear on these components. These kits are available in addition to the minor repair kits for those carburetors that require service prior to TBO and which a service technician has determined a major kit is not required. These kits are listed in the full service manual. The minimum of a minor repair kit is required to complete the installation of the throttle shaft kit for those carburetors that need to have the halves apart. Refer to Precision SIL 2-25-92 for correct kit part number.

666-XXX Kits: There are several of these kits and they are used primarily for modifications done in the field per service bulletins or service information letters. Unless these kits are mandatory service bulletins, most of them do not need to be accomplished until service is required, next annual, improved performance is desired, or TBO has been reached. To reference these kits, check your full service manual and your engine OEM for all the information you need. It is up to the maintenance technician to determine prior to TBO, which kit is needed for the carburetor in question if a major overhaul is not necessary.

No matter what kit you use, always refer to the appropriate maintenance manual, in this case, the full service manual. It contains all the information for maintenance, repairs, and overhauls for all supported Precision Airmotive float carburetors. Another tool to use, the MSA overhaul interactive CD-ROM comes in handy. This CD is a 28-minute step-by-step demonstration of disassembly, inspection, repair, reassembly, and test of Precision Airmotive float carburetors.

Troubleshooting aids

The art of troubleshooting
With proper troubleshooting techniques, and there are troubleshooting manuals available from Precision, problems can be isolated to the components affected. These manuals also cover topics such as understanding basic principles of operation, preventative maintenance, and inspecting your fuel injection servo, flow divider, and nozzles.

The art of effective troubleshooting is what separates the professionals from the amateurs. To be able to pinpoint the problem or problems causing the malfunction is effective troubleshooting. Removing and replacing components on a trial-and-error basis is troubleshooting at its worst form. Removing and replacing is extremely costly for the owner and it teaches the maintenance technician nothing. Pretty soon, the maintenance technician isn't looking so good either.

To be effective, keep these several points in mind in developing an effective troubleshooting procedure for engine, its systems and components. Precision Airmotive has a suggested method which, with minor changes, can be adapted to almost any engine, and consists of five basic steps.

  1. Study the symptoms.
  2. Isolate the system affected.
  3. Determine the probable cause.
  4. Checking and repairing.
  5. Testing and documenting your actions.

Remember, where a symptom may be present may not be what is causing the problem.

The troubleshooting manual (p/n 15-810), training manual (p/n 15-812), and operations and service manual (p/n 15-338), are designed to aid the maintenance technician in effective maintenance and troubleshooting of the RS/RSA fuel injection system. There is basic maintenance that can be performed on the fuel injection system such as inspecting inlet filters, replacing the fuel inlet fitting O-rings, inlet strainer O-rings and cleaning air bleed nozzles. Some repairs may be done in the field such as lapping the lower idle valve and mixture valve, if it is the old style brass valves. Newer style hard coat anodized valves may not be lapped. You may replace clevis components which are external parts, if they are worn. With proper maintenance and troubleshooting techniques, your fuel injection system will reach TBO with few or no interruptions along the way.

Carburetor: Repair or replace
When any fuel control has reached its TBO, there are several options available. Of course, all repairs and maintenance must be performed in accordance with all FARs. You may have a carburetor overhauled by a licensed maintenance technician or an overhaul repair facility, and there are plenty of reputable shops out there. If you don't like the last two options, you can just replace your fuel control with a new, rebuilt, or overhauled unit.

If you decide to replace a carburetor, there are some options. Factory new and factory-rebuilt carburetors are available only from Precision Airmotive. Both of these carburetors are built and flowed to factory new specifications and bench checked on an air-box flow bench to OEM original specifications and drawings. The major difference between these two fuel controls is the price. An overhauled carburetor may be purchased from Precision Airmotive or an overhaul/repair shop. The factory allows an overhauled carburetor to have components worn beyond original factory new tolerances. All Precision overhauled carburetors are bench checked on an air-box flow bench and must meet factory new flow specifications. Another important item to consider is the warranty that comes with each unit. Warranties vary greatly from company to company.

Fuel injection systems
The same options that apply to carburetors also apply to new or rebuilt fuel injection servos. Precision Airmotive fuel injection systems require a flow bench to properly adjust units after repairs and or overhauls have been accomplished to meet OEM original specifications and drawings. Again, the major difference between new and rebuilt is the price. Another option is to contact reputable repair facilities that maintain flow benches to repair or overhaul systems. As you recall, in reference to fuel injection systems, only proper maintenance and effective troubleshooting can be accomplished out in the field. When repairs or overhauls are required, the unit must be sent to a repair facility that has flow benches to properly adjust units for proper operation.

New flow dividers are available only from Precision Airmotive. These units are built and flowed to factory new specifications and bench checked to OEM original specifications and drawings. A maintenance technician may overhaul flow dividers but there must be provisions to pressure test the unit for proper operation. Flow dividers may be disassembled for cleaning per the instructions in the flow divider manual, p/n 15-540, flow dividers, only if it is necessary. This must be done on a clean workbench. The clearance between the piston and flow divider body is only .0004 inch so keep it clean. Flow dividers normally don't need maintenance, but should be overhauled at recommended TBO periods.

Factory new fuel injection nozzles are available only from Precision Airmotive. It is important to keep the nozzles clean for optimum operation and they should be cleaned every 50 hours, check your manual, P/N 15-541, air-bleed nozzles.

Photo by Precision AirmotiveWatch for changes
You can help yourself considerably by getting to know your fuel control and how it works. Always look for indication of operational changes or variances. Watch for changes in your fuel consumption. Regularly do leaning tests looking for abnormal rpm rise or drops. For float carburetors, pull carburetor heat and take notice of what is normal and look for major changes in rpm or operation. Check rpm rise to peak at different settings and record what's happening. Watch for abnormal EGT readings, know what is normal and what isn't. Keep up on all service bulletins and service information letters, remember the 15-895, Index of manuals.

Of course, if there are any abnormal changes in operation, start troubleshooting right away. There are a number of sources to get troubleshooting information: Try the web at: precisionairmotive.com, click on support. While there, check the many different topics offered.

As you become familiar with your fuel control take note of subtle changes, such as rpm variations when you lean at altitude, pull carburetor heat for landing, or at idle cut-off. Be aware of anything that is different from your last flight and write it down for later comparison. Fuel control symptoms will typically increase over time, so early detection and troubleshooting can prevent a bigger problem. Remember, Precision Airmotive, has a substantial amount of knowledge and information to help guide you in isolating a fuel control problem so don't hesitate to call as soon as a problem develops. With proper maintenance and effective troubleshooting your fuel control will take you where you want to go for many years to come.

Additional ReSource
Alan Jesmer - product support
Precision Airmotive Corp.
14800 40th Ave NE
Marysville, WA 98271
(360) 651-8282