Tuning is everything with any size and type of gas engine. I don't know what you're running through the engine at the moment but if you're using a "break in" oil, and especially Lawnboy, get rid of it. In any case, it sounds like your engine needs tuned.

There are some excellent pointers on how to tune a gas engine in the newbies to gas engines sticky at the beginning of this forum. Just because it says it's for newbies does not mean people with some previous gas engine experience can't take advantage of the lessons taught in it. If you are not well experienced with gas engine tuning I'll suggest you take a little time to read what's already there rather than write so much of it all over again.