Don't take the mount off yet. There are two levers, one which moves with the aperture ring, and one which provides the auto aperture function. It's the second one that has to be free, set the lens at minimum aperure and see if the second lever moves freely, and the spring returns it smartly. Then, if it's sticky, you can go on to find out why. I say leave the mount on, because it could be rubbing against the mount.
Originally Posted by EdSawyer