my yfz450 is sick and it has been sick for a very long time it starts to stutter when it reaches high rpms and eventialy dies. the people at the yamaha dealership were i live dont even know whats rong with it. ive jetted it from 140 to 185 and it still stutters it has an hp glass pack exhaust and K&N air filter

 

From what its sounds like to me is that your YFZ450 is running out of fuel on top end. Check the fuel line for any kinks, disconnect your fuel line from the tank and then turn on the fuel to see if you get a steady flow of fuel coming out. Your pingle could be cloged up inside the tank as well.

 

The carb could be slightly clogged to. Check them both out.

 

Ive tired that and it still dosent work

 

This Might sound Stupid, but give it a try [img]i/expressions/face-icon-small-smile.gif[/img].......Check your Gas Cap and Vent hose......Mine was plugged and wouldn't allow air into the tank. As the motor used fuel the tank developed a vacuum and stopped flowing fuel into the carb......It would STOP running......I'd let it sit for 2 or 3 minutes it would fire back up run fine then it would do the same thing all over again.......[img]i/expressions/face-icon-small-shocked.gif[/img]

 

Thats not stupid thats one of the laws of physics.