07 WR250F over night suddenly stumbles on top end(super fat sooty plug). I took it to the guy I bought it from(also has a shop) and started messing with the main jet. It now will rev through the range but not with good power! Its also starting to stumble a little over the whole rev range. Its not guttless just lost its punch. It starts faster than I can press the button, has very good compression. Im down to a 160 on the main jet and still sooting up the plug. Mods= exhaust restrictor removed, snorkel removed and Boyseen Quick Shot 2. I have the biggest ride of this summer coming up next weekend. Please HELP! Im in central MN.