My 2.5R was doing the same damn thing, brand new at that even. Anyhow, it seems after bashing around, the tranny case hit the rear of the carb, at the time I didnt know. check that cps carb for a crack or 2. Then ten bucks down to hobby shop, get new fuel line and a filter, ofcouse if the carb has a crack, get a new one, OS make good replacment ones.
Sounds like a air leak of some kind.