I bought a 2002 WR426 in Ohio and it is now in Grand Junction, Colorado at just @ under 5000' elevation. It is running really ratty. I changed the main jet to the next lower size, put a new spark plug in, cleaned the air filter. When I took it out to ride it ran GREAT for the first 20 min. or so, but it was back firing when I released the gas. I stop the engine and after that it ran doggy the rest of the day. It seemed like the spark plug was beginning to foul and it was running almost at half the performance, but less frequent backfiring. Anybody know the tricks to higher elevation performance? Do the WR450's have these same problems? Thank-you so much for your replies!