did you check to make sure the throttle trim is not turned all the way? What I would do is:
1. set throttle trim to the middle which will give you adjustment both turning it left or right
2. turn on the buggy and controller.
3. if your throttle arm is not centered remove the screw and take off the arm and replace it so that it does not look as though it is NOT giving the buggy gas or the brake.
4. check the gap opening inside the carb.
5. when the brake is applied make sure that the gap does not change..what I do is apply the brakes and hold the brake and adjust my gap that way.

Hope this helps.