I have a McCauley prop governor on my IO-550 Legacy with Hartzell three blade prop.  We discussed the problem of Hartzell governors at the Continental maintenance clinic in Fairhope.  Out of curiosity, during my pre-takeoff engine runup (contrary to our Continental instructor's advice), the McCauley governor would still regulate the RPM at least as low as 1400 RPM. 

 

Dennis

Legacy, 700 hours