Like Rod said, a bad plug can still glow but will loose its umph once the igniter is take off. Compression is the best key to an engines shape, a good test is to hold the rear of the car off the ground by the pullstart, let the car go and it should either be held up by the pullstart or should fall slowly, it will fall a little then stop, fall a little then stop, if it is falling quickly then most likely the engine is shot, also do this when its cold not when the engine is warm. It sounds to me like you have a tuning issue.