if your engine won't shut off, it is sucking air from somewhere, most likely through the carb casket. you need to tighten the set screw just below the carb. if that doesn't help, get a new seal. the problem with the check everything before you start building is that you don't notice a lot of the stuff until you build. for instance, my engine thrust line was an inch and a half too low. i never would have noticed that.