Model airplane fuel is around 70% or more methanol which is highly hygroscopic (absorbs moisture). You can demonstrate this to yourself on a hot humid day by pouring some of it into a saucer. In short order, the surface will turn cloudy which is the methanol sucking the moisture out of the air. To keep your fuel free of moisture, keep your fuel jug tightly closed and use the red plug that it comes with.

If you're concerned about rust, take the backplate off the engine and peer inside. If you see any red spots, that's rust forming. You don't need to disassemble the engine to stop it. Just hit it with some 3-in-1 oil.

Before you depart the field, to prevent problems with moisture and rust in your engine, pull off the fuel line with your engine running to remove any fuel. You don't need to add any after run oil, which doesn't do any good if any fuel is left in the engine, since the fuel contains lubricant and a residue is left behind.

Finally, one of the dumber idea is to subject your engine to a crockpot bath of antifreeze. I remember when the field geezers came up with this idea it was only to clean the outside of the engine. Now they have gone a step further and taken it apart to let all the pieces stew in this mess (350 degrees for 4 hours?). The inside of your engine isn't the same as the inside of a radiator. Depending on temperature and time, the aluminum parts may discolor. Also all the lubricant will be washed off the bearings and other parts. Mechanical parts need to be treated with respect and clean in special solvents and quickly re-oiled to prevent rust.

To keep your engine clean, cover the carb and exhaust outlet (Tower has rubber covers), and give it a shot of Wal-Mart's Mean Green followed by a hot water bath. Next towel it off and finish with a hair dryer. There is also a product called Demon Clean that works quite well:

http://www.cooperfuels.com/demonclean.htm

Ciao,

Mr Akimoto