Here is a review of a product that is supposed to be great:

"Pictured is my 20+ year old Fox .35 control line engine. Note that is now clean and shinny thanks to the Dawn Power Dissolver. It was covered with the dark brown varnish crap that comes as a by-product from burning caster oil glow fuel. That varnish is something that will not come off easily. Well, after reading about this stuff in Model Airplane News™ in a tip by Randy Byers from Bloomington, Indiana, I went to Kroger and bought a bottle. I had my doubts, but all I did was spray it on, wait a little bit, and then started brushing it with an old tooth brush. That brown gook came rolling off. After I cleaned the engine, I rinsed it in hot water; compressed air dried it, and then applied my usual after-run oil. (Marvels Mystery Oil) So if you have a varnished up engine and muffler, give it a try. It works!"