Don`t rev the poor wee bike in neutral to warm the engine. It isn`t good for the motor at all. Wait about ten secs at fast idle, set of and push choke in after about a minute of normal riding. If he has no other problems then this is the answer. Lots of small bikes hate having loads of revs when cold and on choke.
My wee commuter, mastiff 663 cc is just the same and will cough and fart if you rev her. Just ride and she is fine.
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