http://www.airfieldmodels.com/engines.htm

That will show you how to REALLY get an engine clean. But I'm talking about when it is covered with burnt oil, etc.

For post flight cleaning, I just use an air compressor and a soft bristled brush. I don't use any kinds of solvents or such.

FYI - if you wipe the crud from your engine after each flight, you'll have much less burnt crud on it. Usually burnt oil is a sign of an engine that is over-heating if it's on the engine and not just the muffler.