engine sputtring at 8500 rpm

A high rev sputter is usually valve float caused by either bad valve springs or a lack of back pressure from the exhaust. If you have a leak in your exhaust or have changed your muffler. This could be your cause.Check the gaskets around connectors and at the head.Even a small leak can cause a lack of back pressure.And if you changed your muffler did you get the right one for the bike.Mufflers are tuned for proper back pressure so a wrong muffler is also a possible cause. Hope this helps