You were curious about your plugs......Well, there a little harder to read with the various pump blends that are out there. But I can see with a 10% ethanol blend, which a lot of the country runs, that one number up may be in the ballpark for you on that 600. If you post the list# off the air horn (choke housing), I can look up the jetting/secd. plate, on your exact carb to give you a idea of where to go next?