said by ctggzg:
A couple of people mentioned Sea Foam. I've actually been adding a little of that to the gas every since I bought the mower.
As I mentionned before, try to run PURE seafoam through the engine, even if it won't keep running, force it. It will make a ton of smoke! but it will clean whatever is along the way.
I'd try to pass 1-2 ounces of pure seafoam.