I am trying to figure out a problem with a friend's '99 XR650L. Basically, the bike starts fine and runs fine and until you really gun it hard, say around the top of second gear, it starts to stutter, almost like it's hitting a rev limiter but I know the motor can rev higher than that. Bike is totally stock. Have already replaced spark plug, cleaned air filter, checked wiring and sprayed carb cleaner through it. Appreciate any insight.