Hi all Ive finally got around to rebuilding my 01 wr 426 top end, its had all the valves done, new cam chain, high compression je piston kit and a hot cam exhaust cam. which has all been set up buy a friend of mine who is a mechanic. Now the problem ive got is at quarter to half throttle it stutters and pops and when it gets past half throttle it clears and takes off like scaled cat.making it quite hard to ride when trail riding. it also sometimes pops and spits back on the over run. So any help to sort this problem out would be great. its also a stock u s exhaust muffler.