Fuel works good. I have a liter jug of fuel that I use specifically for engine cleaning (20% nitro, 16% oil) The nitro will free all the gummed up parts and old/built up oil and carbon in the engine and the oil in the fuel will coat all the parts and ensure no rust happens. You can get in there with some q-tips and pipe cleaners to get the really gummed up stuff off. Just dont do any scraping or rubbing to the piston skirt or the inner part of the sleeve.