You might want to check your hot start. I have a 2001 yz426 with the hot start right on the carb and havn't had any of those problems. My 2003 yz250f had the thumb hot start with the insert into the carb being threaded plastic. I had a similar carb problem with my 2003 yz250f and it was these plastic threads on the insert that threaded into the carb that were worn and messing with my the airflow and carbueration of the motor. I had my idle screw turned up so high that I could literaly idle up a hill in first, but when I tried to turn the idle down, I couldn't keep the bike running. I found out that the worn plastic threads on the hot start insert into the carb were allowing more air into the carb and causing the problem. Replacing the hot start plastic/threaded piece solved my problem. I don't know what style of hot start you have on your 02 426, but you might want to check and maybe replace that piece. Or.... Buy and new WR 450 and call it a day!