Then what is the fuel pressure? The IP has a gear-type pump on it which should produce 30 PSI at idle, measured at the fuel filters. No more than 75 PSI at 2800 RPM. The gear pump is easy to replace but a lot of times, it is the pressure regulator on this pump. You can find it, trace the lines.
If the engine will not reach 2800 RPM with no load, check the collar actuator. This is under the engine stop control cover (timing cover, 2 screws on side of IP). Pull the 2 screws and the cover. Move the rod/lever, it needs to move freely. If not, this needs cleaning.
It control collar actuator is fine and fuel pressures correct, then this is governor adjustment or fuel delivery. This is more complex.