This is the best stuff I ever used: https://www.net32.com/ec/sharpertek...erm=4579465933818439&utm_content=All Products But it is so expensive I only used the sample that came with the machine. I also like Simple Green Extreme, Wood Bleach(Oxalic Acid) and Lestoil. Personally, I have never tried anything that is flammable because I never thought that heating a flammable liquid would contribute to a long life without pain.
A karting friend recommended Awesome Cleaner. Found at Dollar General. I dilute 1 gallon to 1 bottle of water. Cleans carbs and carbon from heads and doesn't discolor the finish. Around $3 a gallon.