Hello... We are working on a 2011 yz450f with 90 hours on it. The valves were checked about 15 hours ago. The air filter is fresh. The oil and filter have been changed. We just replaced the clutch basket and hub. After all the maintenance listed above the bike ran fine for 10 minutes. Now it starts and idles fine, but stumbles badly when it hits the mid to upper rpm's. It never quite hits the rev limiter.. though it feels like it. It sneezes and will through a small fireball on deceleration occasionally. Last night we pulled the spark plug and found it was pretty black.. almost like the choke was left on. When the choke is pulled it does change.. so it doesn't appear to be in the choke circuit. We replaced it and the same stumble occurs. We also connected the tuner and didn't see any failures. The same fuel can was used.. that was also used a few days earlier.. with the same fuel in it. Any help or direction would be greatly appreciated. Thanks!!!