For years I have used boiling in water with any kind of mild surfactant and normally use dishwasher power. This is primarily for cosmetic cleaning. However the very best initial approach in the case of an engine that is stiff but still can be turned a bit with a prop mounted is simply soaking in glow fuel old, new whatever. This is the reason you should never discard any old contaminated fuel that does not run well anymore, away.


In an extreme cases heat can be the answer and this can be done by boiling or very careful use of a heat source with a prop mounted and gentle rocking of the prop while hot.