Have you got a race pipe on it now? Sounds to me that it might be running too rich! Does it smoke loads when she`s under load?? if so drop the jetting. pull you plug out and see what colour it is, it might be black. If its white then too lean, bigger jet! You will always need to play with jetting with every after market pipe.
I have thougt about starting to tune mopeds as theres loads nipping around the streets and with all these kids wanting to go faster