Fuel foaming is one of the difficulties we face when using model engines in model airplanes. The vibration of the engine is transmitted through the model's structure and to the fuel tank. This vibration then agitates air into the fuel that's in the tank. The added air leans the mixture.

The leaned mixture then can cause the engine to speed up, if it's been set on the rich side, or overheat and sag if it's set at or very close to peak. If the engine overheats, it can become ruined.

One key is the inability of the engine to hold a needle setting. The engine's running well, and then starts to go lean. It can go very lean. With the too-lean running, the RPM will start to sag. It can sag enough that the fuel foaming stops. The engine richens and picks up. The process starts all over.

On the ground, a typical sympton of fuel foaming is very lean running, and it's not cured until you open the high-speed needle valve a lot. Then the engine goes suddenly very, very rich.

You then lean the engine to get it back to a good setting when it suddenly goes very lean and the needle doesn't have any effect until you've turned it back out a lot.

To cure fuel foaming, you have to examine the entire fuel system, from tank to engine, including how the engine's mounted, along with propeller and spinner balance. Anything to smooth out the vibration must be done.

Sometimes, there's not enough foam rubber around the tank, and sometimes there's too much. Too little lets vibration get into the tank, and too much compresses the foam to uselessness. It's a balancing act.

The airframe can also be a cause. Many modern models have very light structure that doesn't absorb the engine's vibrations very well. The vibration has to go somewhere, and some goes to the tank. The rest goes into attacking the glue joints of the model. In cases with the light airframe, a soft mount may be the only solution.

Article after article has discussed fuel foaming and its causes and cures. There may be no simple solution. It's always a "search and destroy" mission.

We hope you find this information helpful. Should you have any further questions or concerns, please do not hesitate to contact us at this email address. (Please be sure to copy all previous emails into any future questions.)

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Bill Baxter
Hobby Services
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