some carbs are built to handle more air flow than a standard motor will pull - if motor is running flat out at 1/2 throttle, then that's all you get until you get more flow (usually starting with better exhaust)
as with any carb, reading plug will tell if rich/lean
Make sure that the tube that holds the throttle cable is screwed in all the way, also, make sure that the slide is opening all the way. If that's okay, It sounds too rich to me, does it 4 stroke? If so,try a smaller jet.