If he's having problems with it running, there are several things I would go after. Be sure to take the carb out and clean it thoroughly. Be sure the jet and carb body passages are clear. Make sure the gas is fairly fresh. If unsure, drain it and get some new stuff. Set the pilot screw at 1.25-1.5 turns out to start with. If it's backfiring on decell, it's lean. Hmmm, that's about all I can think of offhand. Oh, check the air filter. The big four-strokes suck a lot of air. Be sure not to over oil the filter or excess oil can get sucked into the intake and cause all kinds of squirrelly problems with the carb. I think, but I'm not positive, that the stock jetting on this is a 45 pilot and a 160 main. You might pop those out just to see if you're in the ballpark. Back to your original question, a fresh plug won't hurt, either. FWIW, I have seen plugs cause some crazy problems also.