My '08 YZ450 runs great as long as throttle is being added or it is wide open. The problem is at part throttle when held steady...it bucks and sputters. The bike has stock header and can with the Pro Moto Billet end cap. It has head work. I haven't messed with the jetting since I've had it, but not sure what's in there as I bought it used. I need some advice on what I should be looking for to solve this problem. Thanks.