I bought a '99 with 26000 miles in southern AZ, around 2000 ft, and I'm riding in northern AZ, 5500 ft. The bike ran fine for a while, got pretty good mileage in town, but terrible mileage on the highway, 28-32 mpg. So I figured it was a bit rich on the top end. I replaced the plug, it was fouled, cleaned the K&N air filter, and put a leaner main jet in. From a 144 to a 140 main. I put the carb back together as per the maintenance manual. Fired it up. Well crap I made about 6 miles from my house and had fouled the plug completely and needed a ride home. Problems I found: too much K&N oil on the filter restricting air flow making it run rich. I cleaned the filter and tried again, same thing happened. Ive messed with the pilot air screw a bit based on what I've read in forums, and ran it in a range from 1/2 turn out to 2 1/2 turns out, doesn't seen to change much. Heres what the old girl is doing. Between 2000 and 3000 rpm it lags and fades, blowing black smoke, though my oil level hasn't changed. In the top end it runs pretty good, pops a little bit but holds the rpm pretty well, some black smoke. When I decelerate it backfires when it gets down around 3500 to 2000 rpm. Any ideas what is wrong with it. I'm taking it in next week to get the valves adjusted. I don't think its been done yet.