Fuel foaming can usually be determined by the fact that the engine's running very lean. You can open the high-speed needle a LOT before the engine will go rich...and then it will go rich with a vengeance, becoming extremely rich. You'll then start to lean the engine and then it will suddenly go very lean, and you'll have to open the needle a lot again to get it rich. No smooth needling.

This happens because the engine builds up to the RPM where the air starts to get agitated into the fuel. At that point, the engine goes lean at high speed. Then it stays at that high speed until the needle's so far open that the engine just falls to almost choking rich.

Fuel foaming can happen in the air but not on the ground. The plane's free to vibrate in all directions in the air, but is restrained by the ground and the modeler's hand when not flying. The engine will also unload in the air, and increase in RPM.

You can have too little or too much foam padding around the tank. Too little, of course, provides no vibration isolation, but too much can result if you've packed in the foam rubber to help hold the tank in place. In that case, you may just as well have used unpadded wood to hold the tank.

Fuel foaming can be a "bear" to track down and eliminate. Sometimes, you wind up having to replace the entire fuel system with a different type of tank to cure it. Sometimes, the model is such that you just can't totally get rid of it.

bax