there is a screw on the bottom of the carb. it is your high speed adjustment. start your blower and run it athigh speed and turn the screw clockwise until the engine starts to slow down, then screw it ccw untill it runs at it best. there is a screw on the side of the carb that is your low speed adjustment. when your engine is idling do the same as with the high speed. the low speed should be adjusted first. the backfiring could be from a bad muffler. if it is replace it.hope this helps.
jon, January 2009
