2003 Piaggio NRG MC3, what's wrong with it?

My 50cc Piaggio is starting and running, but there is no power when trying to pull away. I have replaced the clutch and drivebelt as the belt was slipping and i thought that the clutch wasn't throwing out. I am quite a heavy lad at 16 stone. But when i take my weight of the bike it seems to accelerate fine. But surely it should be able to handle my weight. I am fed up of buying new parts and the problem still exists. I believe that the clutch isn't throwing out but it's not a clutch that is broken as it is brand new. Please Help!