The primer is about a simple a device as you can get.

The aircraft engine primer is a simple device with a piston/plunger, inlet and outlet and check valves in each port. The primer plunger is sealed by an "O" ring. On many models a needle valve seats when it is locked, preventing the flow of fuel from the gascolator to the intake manifold after priming is complete.

The Primer is mounted by a knurled bezel nut that seats on the instrument panel. It is double nutted against a large nut behind the instrument panel. (removal of the inside nut behind the panel protects instrument panel paint) Use protective tape, a strap wrench or leather strip to protect the knurled outer nut if turning that nut for primer removal.

The inlet line and outlet lines to the primer are affixed by two small ferrule nuts, usually 3/8" in size. (the lines are 1/8" tubing, usually copper or aluminum, but sometimes stainless steel)

On the pull stroke, the plunger pulls fuel through the inlet port and inlet check valve from the gascolator while the outlet check valve remains sealed.

On the push strok, the inlet port is sealed by the inlet check valve, and fuel is pushed out the outlet port past the outlet check valve to the intake spray nozzle(s) by the force of the piston.

If the primer does not work there are several likely problems.

1. The pull primer on my engine is now taking an extra pull (from 2 to 3) and seems to be sucking less:

Check the O ring for seal and condition. Unscrew the knurled nut on the front of the primer and you can pull the plunger out. The O-ring, is an MS29513-012. If it's in a homebuilt you could try an industrial Buna-N o-ring, preferably in a 50-Durometer rubber, which is much softer than the 70 they use in most O-rings. Some O-rings are as hard as 90, which make things sticky indeed. The use of lubrication is not recommended as it will wash out and clog the outlet port, outlet check valve, or the small spray nozzle at the engine end.

2. If the primer fails to retain prime then you have a needle valve or check valve problem. These cannot be worked upon in the airplane and primer removal is recommended.

The small screws on the back end of the primer retain tiny springs and check balls. Take the primer right out of the airplane if you want to look at them. Sometimes a bit of crud gets under a ball and the primer won't pump.

The plunger is just that. There are two little ball check valves at the back of the primer that are needed to make the pump pump. If the input check valve doesn't seat, it lets fuel back into the gascolator line and doesn't build up the needed pressure to spray fuel into the intake manifold. If the outlet check valve doesn't seat, it leaks back from the output and the primer doesn't pull fuel from the inlet line on the pull stroke.

3. Also a pressure test and blow out of the lines, (using compressed air) and lastly a blow out of the spray nozzles on the engine may be in order. These are very small and can clog with debris and carbon.

Doug Combs
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