Any 2 stroke (especially of the age of an mc16) is not going to pull perfectly at those sort of revs and most start to cough and splutter a bit as the RC valves try to make the best of a bad job. It's also not going to pull very strongly either, I've not ridden an MC16 but Wayne's stock MC18 is a bit woolly at best.

Even my mc28 on the o.e. Honda card though superbly tractable sounds like its "missing" around the 5000 rpm mark at part throttle, try to get a go on another reasonably hot 250, it may give you a better idea of if you really have a problem or not. But it's never going to pull cleanly right from tickover at any revs and any throttle setting like a 4 stroke will.
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