Not needle and seat but second stage as it acts differently on gas. The fact that it gets better as it warms up points to that. I had the same problem. i learned how to quickly get the second stage to activate. When it was cold and couldn't get high revs, I pulled over, popped the bonnet and activated the butterfly. The carby would then perform properly while still cold.
But if it flooded, you may have some crud stuck in the needle and seat.
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