At the end of each flying session, when the engine is cooled down, I clean the engine using an old toothbrush and a mixture of Alcohol and Ammonia. I spray the engine with a mixture of about 75%( 70% Isopropy Rubbing Alcohol) and 25% Ammonia. I have been doing this for years, and the fellows in the local flying club comment on how new my engines look. This same solution of 75/25% Alcohol & Ammonia is also used to clean the plane residue from 50/50 castor & synthetic fuel.