High idle, wide open, no load RPM's are probably 2520 to 2570 RPM...........2400 RPM is rated loaded HP speed. It has to run faster than 2400 under no load to be able to reach full fuel by the time you pull it down to 2400. Chances are, if you actually LOOK at the throttle lever on the injection pump to see to it that the lever is fully open and against the stop bolt, RPM's being too low isn't your problem. But, there are many throttle cables from the cab to the engine that sometimes need replacement because they are stiff and do not achieve full stroke/RPM's.