I make up a witches brew of alcohol, acetone, break cleaner, carb cleaner then test it to make sure it doesn't pit the engine by using an old throw away LA .40. I strip the engine and put in in a sonic cleaner my wife better never find out I'm using. This is a really old MECOA .46. Don't sweat a little discolor but I make sure I get all the carbon off. If you have a lot of engines, maybe a club project then an old used crock pot and anti freeze is the best way to clean them. There are several threads on how to use the crock pot.