my 1/10 buggy was doing that and it turned out that the rubber boot around the LSN had crap built up behind it. It wasn't allowing it to fully close and always kept the RPM's up, check that. Also with the car off, when you open/close the carb is it smooth or does it feel rough? If that feels fine and there is nothing holding it back, look at your servo. Disconnect the servo from the carb and turn you electronics on. Hit the throttle a couple times and check to see if the throttle servo goes back to it's normal position. Other than that, I dont know. May need a new Carb. How much fuel did they "say" was ran through it?