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FUEL SYSTEM FAQ

The following are typical of the questions received and answers given at Kelly Aerospace over the years. The answers given are by no means the only or definitive answers but they are designed to lead you in the direction to remedy a given problem. The goal is to share our experience and to save labor time in troubleshooting. Any actual repair activity must be done in accordance with the appropriate maintenance or service manual from the applicable source. (manufacturer/airframe and/or engine to latest revision).

Frequently Asked Questions: Carburetors

Frequently Asked Questions: Servos

Frequently Asked Questions: Fuel Pumps

Frequently Asked Questions: TCM Fuel Injection System

Q: I received a carburetor from one of your distributors. Upon installing the carburetor, I have noticed fuel leaking from the throat with the mixture control in the ICO position. How can that happen on an overhauled unit?
Our largest warranty claim on carburetors revolves around shipping and handling issues. There is no way the float can be protected or isolated from the rigors of shipping. So unfortunately, the float, needle valve, and seat may become misaligned or even damaged if the carburetor is dropped or handled roughly. A misalignment of the float or needle can effectively change the fuel level in the bowl, allowing it to overflow the discharge nozzle and drain overboard though the airbox and onto the wheel-pants or hanger floor.

Q: Why can't I find the exact replacement "10" dash number for my Lycoming engine in the type certificate data sheets?
When Lycoming certified their engines, they certified a series of carburetors for an engine group. As an example, the O-320 series of engines were certified with MA-4SPA carburetors. This allowed Lycoming to choose the carburetor that best served the engine requirements on the test stand. If you check in the engine overhaul manual, it will usually indicate a Lycoming part number for the carburetor used. This number can then be crossed to a "10" dash number. In some cases, there is more than one carburetor part number eligible for a specific engine application. In this case, you may wish to consult the appropriate airframe type certificate data sheet to find the actual approved engine and then determine which carburetor applies to that airframe/engine combination.

Q: What are the carburetor model number designations?
The MA-3, -4, -5 etc. designates the mounting flange and size where the carburetor mounts to the sump. The "S" indicates a larger throttle bore dimension. The "P" designation indicates the carburetor is equipped with an accelerator pump and an "A" designation indicates the carburetor has an altitude mixture control which the pilot can use to manually lean the engine in flight.

Q: How can I tell if my carburetor has been modified with a one-piece venturi?
The carburetor data tag provides valuable information. There are four additional designations normally stamped on the data tag. A "V" stamp indicates a one-piece venturi is installed. A "MF" stamp indicates a metal float is installed. A "92" indicates the recall on metal floats that occurred in 1992 has been complied with. An "A1" designation indicates the drain plug has been removed and inspected for excessive thread lubricant and the carburetor bowl has been flushed.

Q: Can I use auto fuel in my engine with one of your carburetors installed?
All of our carburetors are overhauled in accordance with all current information using many of our own PMA approved parts. All are expected to be installed on certificated engine/aircraft combinations. If another entity provides an STC Approval for using auto gas in a particular installation, it becomes the STC applicantís responsibility to provide all the necessary documentation for proving there will be no ill affects to the engine or associated accessories and parts. In some instances, Auto Fuel has a propensity to adhere to the inner induction walls. At shut down, this remaining residual fuel may run down the throttle bore of the carburetor and drip on the hanger floor while the airplane sits static.

Q: I bought a carburetor for my TCM O-200 engine and installed it in my homebuilt. It doesn't seem to run correctly. What is the problem?
We work with homebuilders on a regular basis regarding this issue. Many homebuilt aircraft do not use the same air intake system as was installed on the original certificated aircraft/engine combinations. Some homebuilts rely on creative cowling configurations, changes to the routing of intake air, and even changes to the airbox itself in order to compensate for varying design criteria. These and other changes can alter the dynamic of the airflow into the carburetor and, consequently, the fuel/air mixture into the engine. Unsteady or turbulent airflow can directly affect the amount of fuel metered through the discharge nozzle. The quality of the air-stream directly influences the metering of fuel. All float-type carburetors flow fuel according to the volume and mass of air flowing through the carburetor. In many cases, we are able to compensate for changes in airflow dynamics by working with the aircraft owner and tailoring the fuel flow by re-setting the carburetor to a leaner or richer fuel flow using our specialized flow bench which, by the way, is one of only two in the country used for carburetor overhauls.

Q: Is the carburetor idle mixture pre-set at overhaul?
The idle mixture is set to a nominal position when delivered from the shop to the distributor. This setting is approximately 1 1/2 turns from the full rich position. All carburetors should be set-up in accordance with the engine/airframe manufactures recommendations after the carburetor is installed and prior to the first flight. Proper idle characteristics are a combination of speed and mixture and can only be set on an engine running at normal operating temperatures. Mixture settings may require changing due to seasonal variations or changes in the density altitude at the airfield the aircraft is operating from.

Q: I purchased a Bendix servo from one of your distributors and I'm having trouble adjusting the idle speed and mixture. I have turned the thumb-wheel to its maximum amount of travel and still need more adjustment. Can I remove the clevis and reposition the thumb wheel to the center to obtain additional travel?
In most cases, idle mixture adjustments should be able to be accomplished with the clevis set in the position as received, or with just a few clicks of the thumbwheel. However, in certain instances, it might be necessary to re-center the clevis thumb wheel for more travel in a certain direction. Follow these steps when re-centering the Mixture Clevis assembly:

Note: if no mixture rise is observed (too lean), it will be necessary to move thumb-wheel to the Rich mixture block (as indicated by the "R" and arrow etched on the clevis block). If the mixture is in excess of 50 RPM (too rich) it will be necessary to turn the thumb-wheel in the opposite direction of the arrow. Obviously , all work must be accomplished using approved documentation

Fuel Fig.1

Q: I bought a fuel pump from one of your distributors, installed the fittings and then installed the pump on my engine. I find it works fine at higher RPM's but won't support the engine at idle speeds without the boost pump on. What's going on?
Our biggest problem with low pressure at idle speeds is pump contamination. Debris lodged between the valve and seat of the relief valve assembly is the single-most cause of pump failure. The smallest speck of contamination can effectively reduce fuel output pressures or cause the pump to operate in an erratic fashion at lower RPMs. Sediment in the form of rust, scale, lint, sealant, aluminum compounds, have all been known to cause pumps to lose their pressure. It is strongly recommended that fuel lines be flushed prior to installing them to the pump. Furthermore, installation of pump fittings can introduce unwanted TeflonÆ paste, tape, or material pulled from the fitting or housing itself. Be certain to clean the fittings and pump threads thoroughly before installing the fittings to the pump. Avoid using a vise to hold the pump when installing or repositioning fittings, as this can permanently damage the assembly. If possible, mount the pump to the engine prior to making any fitting changes.

On occasion, fuel-borne contaminants can actually be purged from the relief valve seat. The procedure is simple. First, remove the safety wire that secures the 9/16-in. jamb-nut to the valve housing and turn it one-half turn to loosen it. Next, using a straight-bladed screwdriver, turn the relief valve adjusting screw counter-clockwise until it no longer turns. The screw has an internal stop that prevents it from falling out. It's important that you count the amount of turns required to fully release tension on the relief valve spring. Now, with the throttle in the wide-open position and the mixture at full rich, engage the boost pump for a few moments. Before attempting to restart the engine, be certain to return the adjustment screw to its original setting and safety-wire the nut. About 30 percent of the time this effectively flushes the contaminant from the seat. The odds aren't great, but it's worth trying before removing the pump and sending it out to a reputable Repair Station to be repaired.

Proper fitting installation procedures, including proper use of thread sealants is critical to proper pump operation. In addition, many operators who receive pumps with fittings installed move those fittings to accommodate their installations. In some cases, the loosening and re-tightening of those fittings creates a sliver of metal cut from the cast pump housing threads by the steel fitting or pipe sealant or delaminated plating from the fittings to migrate to the relief valve seat within the pump. Extreme caution and care must be taken when installing fittings and fuel pumps on engines in the field.

Q: I received a KAPS overhauled TCM fuel injection system on my overhauled engine and have now installed the engine on the aircraft. Was the fuel injection system set-up properly by the engine shop during the engine test run?
Not necessarily! All TCM systems are required to be set-up when the engine is installed on the airframe. The correct TCM document for reference is SID97-3A. This document provides all the information necessary to properly set-up an injected TCM system on the aircraft.

Some shops make the erroneous assumption that a freshly overhauled fuel system need only to be bolted to the engine and flown away. They say "after all, wasn't the system overhauled and calibrated to factory specs?" Yes ó however, variations in induction systems, supply systems, and operating environments mandate a final "tweaking" of the installed components. Still, some tenaciously hold to the mind set that "it should be set up for my airplane out of the box." For this reason, many Repair Stations have difficulty persuading technicians to use calibrated hand-held gauges while setting up their Continental systems on the airframe. While TCM strongly recommends the usage of externally plumbed gauges, some technicians still consider this procedure nothing more than a mere suggestion. Failure to follow this "suggestion" often results in lackluster performance, frustration, and dissatisfied customers. Always use these gauges if you expect to appreciate the systemís full-performance capabilities. Teledyne's Service Information Directive 97-3A provides clear instructions for final adjustments once fuel components are installed on the airplane.