Sounds like an air leak as someone else said. This series of saw has that crappy intake boot clamp made of plastic. (until the updated metal one came out) I've seen a lot of them toasted because of it. also where the carb mates to the intake boot leaks especially if someone has tightened the carb bolts too tight. And the bearing cup where it mates the plastic case. Thats where I'd check for your problem if your carb diaphragms are good.