The 'regulator spring' is not responsible for returning the rack to idle speed. It is the force of the weights which returns the rack to idle position where it is limited by an adjusting screw. Only the start spring applies any force against throttle but it is minor. Is it possible that the fuel limiter has been "adjusted" and is interfering with the governor? If not it seems reasonable to assume that either the governor weights are worn or the rack in the high pressure pump is sticking and should be removed and serviced. At least remove the pump and feel the rack movement with your own hands. It should move completely effortlessly. As for the weights, you need a pretty good eye to see if they are flatspotted. Replacing them might be a last resort.