According to my shop manual the maximum no-load speed is coded into the "setting code" on the injection pump.
The manual says:
The setting code on my engine is 2643H058BE/1/5150 which would indicate the the correct no-load maximum speed is 5150 RPM.
The code is stamped into a label on the side of the injection pump... like the (hopefully) attached photo.
If yours is the same, I am not sure I see the problem yet..
Whenever you are checking the rotational speed of the engine for critical adjustments, be sure you do not trust the installed tachometer. They can be wildly inaccurate.