if it picks up after you have let the throttle go it means its weak the reason it picks up again after you have let the throttle go is that the air is shut off with the throttle slide and so the mixture is ok again until you re open it and it goes weak again.the reason is unclear if the main jet is the right size and not blocked up ,it could also be that there is water in the fuel or that someone has diluted the fuel with oil or diesel ?i would reccomend emptieng the tank and refilling it with fresh petrol and also i would raise the needle position on the throttle slide by moving the circlip to the slot nearest the pointed end of the needle which will allow the fuel to enter the venturi in the carb earlier and also lift out of the main jet further.ok ps i am a pro bike mechanic for a living.
the ped med., August 2009