I've experienced the same exact problem with my '94 KLX250. It had been running great for months and started this high idle problem suddenly. As far as I can tell, the culprit seems to be a worn carb slide bore wihich causes the slide to sometimes not bottom completely when the throttle is closed. It has the aftermarket Mikuni 33mm pumper carb and this seems to be a common problem. I won't know until I send the carb to Kientech for a plating of the bore. I really don't want to spring for a new carb if there is some other problem. I've already cleaned the carb jets and circuits completely, and made sure I had no air leaks when I reinstalled it. A lean condition could cause a high idle. This may turn out to be a difficult problem to solve, although I ran into this problem with my '00 YZ426 and it was due to an air leak where the air filter boot mated to the carb. Once I corrected this leak, all my carburetion problems vanished.