More than likely your throttle brake linkage is not adjusted correctly. I would reset the linkage and retune the engine to run with the throttle (carb) completely closed then go from there. Usually what happens is the engine is tune with the throttle slightly opened then when the brake is applied it pulls the throttle completely closed thus cutting the engine. I have seen many people struggle with this issue and it can be aggravating to say the least.

I hope this helps.