I had similar problems I chased. Check the contact point of the round throttle return spring on the throttle. Mine was rubbing against the aluminum of the manifold, digging into it after so many years, making the closing position of the butterfly different each time I let off the gas. I ended up removing it and relying on the secondary spring to snap the throttle closed. Fixed the problem and made my throttle a lot lighter to open.Make sure your secondary spring is in good shape and well anchored if you go this route.