I've searched all over these forums and tried everything that has been suggested-I'm hoping someone might have another idea... I recently bought a 2003 WR450 and it runs horribly. It will start up easily and idle smoothly. However when I twist the throttle it dies instantly. When I slowly roll on to the throttle the revs will increase up to about 1/3 to 1/2 throttle and then it starts to backfire before dying above 1/2 throttle. If I leave the choke on it will respond quite a bit better, but still is pretty rough above 1/2 throttle. When I do rev it high with the choke on, as soon as the revs come back down the engine will die regardless of what I do (give it more throttle, push in choke). Originally when I got it was jetted as follows: Main: AB168 Needle: OBDUT Clip: 3rd position Pilot: 50 Starter: 65 Leak: 70 Pilot Air Jet: 70 It is pretty much stock. I moved the clip up 2 positions and then down 2 without much change at all. I then put a 40 leak jet in and still didn't get any noticeable change. Now I'm sure that it is not jetting. After all the posts I have read my jetting is right in the range of most peoples. There is no way a small jetting difference could cause what I am experiencing. So then I took the carb apart and inspected all jets (all were unobstructed) and shot cleaner through all passages that I could find (fluid came out another area somewhere). I inspected the acv assembly and accelerator pump assembly for diaphram condition and proper assembly order. The float height is correct and I took the air filter off because it was gummed up. What could be causing this? Does the TPS have any effect on how it runs? Could it be the timing or electrical short? Exhaust leak? I was hoping that it was carb related, especially since it runs better with the choke on which was what my original thought was (too lean). But what could be causing such a major problem that it dies instantly with any throttle? Thank you in advance for any help you can offer!