Haven't been on here in awhile, but i recently got my bike, 2003 YZ450F, back after a WORLD of trouble. The original problem was that the bike had pretty much stopped running, I figured it was a clogged-carb or a valve adjustment issue. Tried to clean the carb myself, seemed to make it worse. Took it to a small shop, got the bike back, wouldn't start at all unless it was warm outside, and even then it was a pain to start. When I'd get it running, it had no power, and stalled when I'd stop. The shop said the carb was cleaned and valves adjusted, and that "it's just a COLD-NATURED bike...." BULL! I took it to a different, more experienced guy, a racer....turns out the "shop" left the half-moon shaped cir-clip off of the exhaust cam, allowing the cam to move back and forth laterally, causing the bike to jump out of time (I think). Anyways, got it back and it is ROCKIN, to say the least. Running better than ever, starts within 2 kicks, idles low and smooth, has monsterous power and literally "thumps" when it runs. I was really lucky that it didn't cause any significant damage to the internals. Point of the story is if you're unable to do the work yourself, find someone who has had DIRECT experience with your type of engine. From what I've heard, this cir-clip is a really easy piece to lose, as I'm sure a lot of small pieces are.