The main spring determines when the engine will cut-off at high RPM, it is only that it which should be hardened or shim, the intermediate spring is already completely compressed around 2000rpm ( that depend of the model of pump), if you put it harder the intermediate spring, not only that gives absolutely anything, the RPM will be hard to control at low RPM, the car will make shudder in first and second gear at 1/4 throttle, and the high RPM cut off will not be changed.