Easy way to clean is to just pour in some alcohol (rubbing, denatured, Bacardi 151 , etc) and turn it over some. The alcohol loosens the old congealed oil quite well. Once it's turning freely and the alcohol is coming out clear, put the engine in the oven at 250-300 degrees for about 10 minutes to evaporate the alcohol. Take it out (use a pot holder, oven mitt, etc) and oil it up. Note that you should also check that the needle and spray bar assembly aren't clogged up.