I can only assume that: A. You have a new diffuser gasket. B. Your rebuild did not include the throttle. C. You can only have the choke wide open at the high RPM.
The high RPM is indicative of a vacuum leak. A. The mounting nuts need to be tightened to 30-35 pound-feet of torque to properly crush the copper rings and seal the gasket. B. It is possible that you have worn-out throttle bores and a worn-out shaft. Bushings and a new shaft would be called for in this instance. C. Do you have a vacuum gauge?