My 1990 Honda Super Cub has a problem that seems to occur mostly after it gets wet. Sitting in neutral, if I slowly open the gas it will reach full revs easily, but if I twist straight to full throttle it will lose power and even stall.
It's an intermittent issue anyway, so it's hard to say, but I think it tends to go away when the engine warms up.
This answer suggests that this is a fairly well-known carburettor problem. What is that problem? The carb looks like the newest part of the bike, and a mechanic recently claimed to have cleaned it.