Varnish is a major issue if you are usnig plain castor in that overtime it goes black and the engine looks unsightly. Once in a while, I will clean the engine externally with some soap water and a toothbrush. This I follow-up with thorough drying with a hair-drier. I dont use a heat gun as it is too hot. Usually, when I am done, the engine is as hot as it has just come back from flying. Havent have had to do this since I changed over to Klotz though.

For internal cleaning, I keep immersed in plain alcohol overnight or so and then re assemble and oil thoroughly.

Ameyam