Two possible options:-
1. maybe the throttle cable has been adjusted so that the choke is on when on full throttle. If the cable is at its maximum then this is possible. It should not be necessary to run on full throttle try reducing it slightly.
2. It could be petrol starvation or water/moisture or old contaminated fuel in the carb. Try draining fuel/water off the bowl of the carb. by undoing small drain plug
Chris, July 2010