This sucks! I have an '03 YZ450 that suddenly quit running right, half way through my last ride (it was making a snapping sound on hard acceleration). I posted a description of my problems here at TT, but no one was able to come up with any causes that I hadn't already looked into, so I took it into a dealer to have them take a look. We talked over all of the things that I had tried to troubleshoot as possibilities (bad gas, needing a new sparkplug, cleaning the carb, etc.), and I decided to have them do a leak-down test on it. They said I had 15% flow through the exhaust valve (uh-oh!). So they tore into it, and I just found out that it is going to need $1100 worth of repairs! They are telling me that the exhaust valves had excess carbon built up on them, keeping them from seating correctly. This caused more damage to the engine, and they are recommending that I replace the valves and get a complete new cylinder (with piston and rings, of course). OK, here is the pisser – nobody can tell me what went wrong! I could understand if I did something dumb, but they can't give me a reason for the damage! I bought the bike brand new, have ridden it less than 15 times (estimating 30 hours on the engine). I had the valves checked two rides ago, and they were well within tolerance (it was still running fine then). I only run super unleaded gas (sometimes mixing 50-50 with 110 octane race gas). I never go more than three rides without an oil change, and I use Yamalube. I clean the air filter every time I change the oil (more if it needs it). I am NOT flaming the 450 here, I still love the bike and was planning on getting an '06 YZ450 (well, I'll probably have to wait until '07 after this bill … ouch!), although I do have to say that my confidence has really been shaken on the reliability of the bike. Anybody have any insight on what might have gone wrong? Other than somehow I just got a lemon?