check to make sure its not running to rich or too lean or that you don't have a fuel or air leak somewhere in the exhaust system (your exhaust system works kind of like a fuel pump) if theres a whole or leak in the exhaust system it can cause the engine to run too lean on fuel. check your fuel lines to make sure theres no air leaks. the reason you lengthen the fuel line from the tank to the carb is to keep from getting a lean or fuel bog when your racing, as the tank gets lower on fuel you have a better chance of getting points as you take turns and jumps, this way you have plenty of fuel in the fuel lines to help keep that from happening.