Guys and gals,
My recently rebuilt Mc21 has been riding awesome but yesterday when I got to the parking garage, the RPMs sh*t up to almost 10k without moving the throttle. Then I would flare the throttle in hopes it would idle down and it did. Today, I started the bike (on the kick stand so leaning over a bit) and once it warmed up it did it again but would not idle down after I flared the throttle. So, I of course hit the kill switch, motor still running... what?!?? Then I turned the key off, motor still running... WHAT!?!?

To be clear, under normal conditions the bike will NOT run with the key off or kill switch enables. My thought is that the motor was causing detonation from the high RPM and higher compression ratio but nonetheless, what is causing the bike to surge to 10k without moving the throttle??? Even if the throttle position sensor faulted, the cabs need to open up to provide air/fuel to feed it at 10k, right??? If not, then it must be running lean as hell and that scares me.

So I finally got it to stop by sitting on it and standing it upright, off the kick stand and it useless down. Also, it also idled at 3k until I sat on it and stood it up and then it dropped to 1.2k.

This is anything but a simple two stroke motor

Decided to take the bike to work (cautiously) and found the following new information...

When I rock the bike back and fourth to move it around (100 point turns as the steering dampner is so much fun when trying to move the bike around under no power) with no hand on the throttle, the bike starts to surge. Sometime moving the bike forward makes it surge, sometimes moving it back does, and either motion will also cause it to drop idle too (when it is obviously surging). Strange, does this sound like fuel splashing through the carbs into the crankcase?

Never seen this before. BTW - when riding, it just runs perfectly. Thanks guys

I would check that the throttle cables are not snagging or pulling tight when you turn the handlebars,as you say it seems alright when riding, presumably with the bars in a straight line. Another thing that could make the revs rise is a weak mixture caused by an air leak, might be worth checking the rubber inlet manifold for splits or perishes, personally wouldn't have thought that would make it rev to 10k, but worth a check.
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Proud Father of , 05 ktm 400exc supermoto 2018 honda crf rx supermoto

Lesviffer750, you were spot on and yes I should have assumed this but perhaps was in too much panic. It turns out the that the locking nut where the throttle cable meets the hand throttle was loose (came completely unthreaded) and turning the bars was pinching the cable thus pulling the cable and causing the engine to surge. Thanks again.