Did the topend on my 87 warrior about a year ago. Runs great but has a slight hesitation at high rpm. I've been getting a backfire through the carb every now and then while starting the motor. Any idea what causes a carb backfire? I have not done anything yet such as re-clean the carb or adjust the valves. Since I did a complete valve job, I know I need to get in there and readjust them.