Something really odd happened to me recently. I have a 1977 Yamaha DT250D 2 Smoke. It has a Mikuni VM28 carb with the standard carb mounted lever enrichiner setup. The carb doesn't have a accessible drain and I have been in the habit of turing off the gas and running all the gas out of it when parked for a while. This and non ethanol gas has kept me out if carb problems for some time. The last time I did this, I turned on the enrichiner as I generally do, but opened the throttle as I thought it was sucking up the last bit of fuel. At this point, the engine revved to 7000 and after releasing the throttle it hung there. I turned off the key and it stayed there for several seconds. Finally, turing off the enrichiner killed it. After cooling off I started it back up and it seemed completely normal. The engine was hot when I ran the gas out and I'm guessing it was running with heat generated from the lean mixture and ambient heat like the model airplane engines I played with as a kid. I'm positive there is no sticking throttle and the switch grounds the magneto ignition. Anybody have other ideas?