If you live at altitude, think long and hard before buying a YZ426. You might draw the short straw. After six trips to the dealer, my 2001 still won't run more than 10 or 20 minutes without fouling the plug. My first hare scrambles on the bike consisted of ride 1 lap, foul plug, pull tank, change plug, ride one lap, foul plug, etc. (My 1999 ran like a top, always started, carberated perfectly, wish I had never sold it). Colorado Powersports has tried a 40, 42, 45 pilot, all needle clip positions, 152, 158, and 162 main, and various modifications to the accelerator pump. All with the same results. The colder the weather, the quicker the plug fouls. Ignition, float level, and TPS check out fine. Diagnosis with an Exhaust Gas Analyzer says everything is OK. Its not a starting problem. It fires first kick with a fresh plug. Although most 426s run fine in Colorado, I know of two other bikes like mine. My friend had a 2000 that after six months of this, with various dealers trying to fix it, made his dealer give him a 250 instead. I am out of luck, I bought mail order. Yamaha is sending out a different needle to try. They seem clueless. If the needle doesn't work, I am going down to the KTM dealer. Maybe I should wait for the new Honda. Anybody at sea level want to buy a used 426?