benelli 49x revs die off completely at 40mph ????????

hi there i just bought a benelli 49x. it has a polini 70 kit, leo vince tt pipe, and a sports carb on it. it gets to about 40 MPH but then the revs just die off and wont rev again until the back wheel as completely stopped. if anyone could tell me what it is or how to sort it out i would be very grateful :beg: . thanks