he is running lean. anytime u do a mod that makes more air flow u gotta go up in the jets ( more air needs more fuel). do him a favor. tell him to take the air box lid off, throw some 280 mains and 27.5 pilots in the carbs, throw on a aftermarket clamp on air filter and maybe even get a timing plate and put it at +4 and call it a day. if he refuses to listen then let him run it the way he wants and watch his engine blow down the road