I have an '02 CRF 450 which is stock apart from stainless valves and a Yoshimura pipe. It has a very annoying habbit of stalling from about mid-revs when the throttle is suddenly closed and opened again, for example when doing a tight turn around a tree. The bike makes a loud cough and just dies. Then it is a bitch to start (5 minutes of kicking). Too much or too little fuel getting through? I have had the valve clearances checked and turned the air screw out to spec.