sounds like a possible air leak, usually it is at the carb where it mounts to the intake. 2nd culprit is intake gasket not sealing. I've heard ppl find the slide is not going all the way down in the carb, usually from cable bind or you can adjust the cable at the throttle (if that's the case) tske the air filter off and see if it returns to closed. shoot some wd40 or white lithium grease down the cable while you're at it.