Hi all.. Recently bought a 2000 yz426, relatively clean bike, but not perfect. Stock exhaust, previous owner put a 450 carb on it with the hot start lever. First 4-5 times I rode it, it ran fine. Throttle response, power, was all perfect. Fun bike. The other day, I went out to start it to ride in the woods behind me. First or second kick it started as normal. When I went to shut the choke off it died. It's never done that before. Usually I can shut the choke off instantly and start riding! Like I said, before it carburated perfectly!! Restarted, did the same thing. Restarted and noted some things. WIth the choke on, it will stay running but will not respond to throttle. If I shut the choke off it will either die or won't respond to throttle and die. It's like the throttle is a kill switch. Pulled the carb apart and clean the jets, as I've read on the internet that this is a classic symptom of a plugged pilot jet. Everything in the carb looked clean. While the pilot jet hole seemed small to me (I couldn't fit a small staple through it) it didn't appear to be clogged. Put everything back together. Same thing, started 1 or 2 kicks on choke, sounded ok, but would literally die if I gave it any gas or shut the choke off. Wierd, cause this literally happened as the bike sat! Ran perfect when I shut it off. Runs like crap when I restarted it. Changed the plug.. color looked good, maybe a tad rich or dark but not that bad. Other thoughts? Supposedly the guy before me adjusted the valve recently. Motor sounds tight. Thanks all! Should add that I looked pretty hard for misc air leaks as well, I don't see any.