Sounds like the carb has gummed up or has gotten some debris in it. Also, this model is jetted extremely lean from the factory, and is only made worse when the air intake is opened up by removing the baffel. If the grey wire on the ignition circuit is also cut, things get even worse still. I have an 04 and an 07 model WR450F, and both of them from the factory were dismal at best out of the box. They both came 165 main jets. I have put in the JD Jetting
kits in both bikes and performed all the free mods(grey wire, baffel, throttle stop. With the JD jetting
, the bikes both use 170 mains, which is five steps above the stock baseline setting. Both bike rip and run perfectly with absolutly no problems. I have had the 04 for about 2 years and did these mods soon after I got it. I liked it so well, that I gave it to my son, and got me an 07.
I would try getting me a jet kit, pulling the carb off and cleaning it, and installing the kit. It comes with easy instructions. Good luck.