I have a 1990 DR 250S that idles erratically. If I turn the idle screw in, the idle stays the same no matter how far it turn it, until the throttle is blipped, then it settles at 4000 rpm. Then if I turn it out slowly, it'll finally drop to about 1200 again but it tends to stall easily. In adition, if I don't turn it down far enough, it will stay reved up when shifting gears. There is no in between. The bike runs great otherwise. Any ideas?