if its not stalling it sounds like the idle is set too high, the sign of a bad or broken spring is stalling when you hit the brake and your not able to start it on the ground. try lowering the trim on the radio, if it stalls raise the idle using the screw on the carb, the one with the spring on it. clockwise raises the idle. slowly lower the trim and see if you can lower the idle and stop it from moving then adjust the screw as needed.