Check your pipe where it is mounted to the back of the motor and check it at the joint gasket where the pipe and header meet and see if it wet underneath it or around it. If it is wet around it or dripping off at those spots onto the pullstart or chassis at the joint coupling you could have a air leak and when you give it throttle you are not getting enough pressure to flow fuel to the carb. I just had this problem with my protos and it would idle fine and keep running sitting still but when I touched the throttle it would die. I had to different air leaks on my pipe and it was wet their. Make sure everything is sealed with not leaks and that your carb is tight and not moving.