Check around the throttle shaft. It's a hard one to see, but if you spray a little carb cleaner right where the shaft exits the carb body on either side, the pitch of the motor might change. If it does, you've got a leak.
I have the same problem and one of these days I'm going to get it drilled out and put a bushing in. It runs fine above idle, though, so yours may be a different problem.
