I bought my 01 426 new, put an e-series pipe fatter main jet and the bike has run perfect. Last weekend out at the Ca. desert 3000 feet it would not idle and backfired when the throttle was let off. I cleaned the carb to no avail. I don't think it's jetting, weather, or altitude as it's run great over the last 3+ years. It has been VERY well maintained so it's not valves, or piston slop. I changed the plug (mainly to look at it's color) and obviously that didn't solve it. Changed Gas thinking that was a possibility and still nothing.