1st try Re-centering the servo. With electronics off, remove the horn from the servo (just remove the screw in the center of the horn and pull the horn off), then turn on the electronics and set the radio servo trim to 0. With the electronics still on, re-install the horn on the servo while holding the throttle valve in the carb closed with the horn and linkage. You can then adjust the trim if needed to completely close/open the carb throttle valve.
If it does it again after you center the servo, you may have stripped a gear in the servo...