I had the same problem. What I did was pulled the throttle cable and it got cought on the mount where the metal end of the cable is connected to the carbs. Check to make sure the metal coated end of the throttle cable is sitting flush properly. If it is not the throttle cable is being pulled which is giving it throttle and the torrs is being activated. I did the same with my sled. I had to start it with starting fluid, the motor went up to around 3000 rpm if I hit the throttle it would pop and back fire. I seen the throttle cable was being pulled because the metal end of the cable where it is bent was not sitting correctly. I hope this helps.