From reading i think he has a different pump or something. And if the carburator was changed for the 300 the carburator and pump will not move in the original ratio, thus maybe having free play or hitting something in wide open. the bike will smoke during a cold start, that is very normal. And if the pump is runing rich it will smoke a lot during cold start.
It might also be wise to check if your oil is runing past the ring and into the cranks.
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feel the wind