Start with the easy/obvious. Disconnect the throttle at the engine and see if manually moving the lever you can get full RPM. Could just be an adjustment of the cable.
I have done this. The throttle control at the engine is at the maximum postition, and engine still below max rpm.
I would think the engine should always have some return fuel no matter what the throttle position, but maybe I am wrong. I do not have a good understanding the relationship between the injector pump and the fuel injectors.