For IVFlyer
Sorry for the delay ... been out in the breeze earning a crust or two.
Not sure where you get your 25 rpm from. (rotor rpm?) Your flight manual will advise the E1AD engine as fitted to the standard C model should idle at 1400 t0 1500 rpm. At that speed in reasonably still air, the blades should not turn unless the belt is incorrectly adjusted, but please refer the problem to the factory maintenance manual or a qualified with type experience.
We type experienced pilots will always be happy to help but the proper source for information is the factory publications.
Best wishes. Dennis Kenyon.