Im guessing youre running premix? Its very common on dynos for a 2 stroke to seize on the shut off if youre not used to doing them.

Its also common for MX bikes to do it if going down hill on a closed throttle.

With an oil pump on the bike even if you shut off at high speed oil is still feed into the engine. Of course with premix if you shut off not only do you get no fuel, you dont get any oil either.

I think i can see where dave is going with his question tho.
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If I have to take the carbs off once more...