Can you try one thing.....run the bike and when you accelerate and it loses power put the choke on. If it then momentarily tries to pull harder then it is a weak (very) fuel mixture.
To me it sounds as if you are drawing in big gulps of air and not fuel when you put the motor under load....i.e. Big air leak?

Just my view but worth a try.

Mike
_________________
I don't mind dying...I just don't want to be there when it happens!