The reason the throttle plate position varies from P to S is due to the throttle blades not fitting correctly , flip the carb over & take a look how bad they fit , loosen the throttle plate screws & 'jiggle' the blades in the bores , refit & see how different it is , even had to file the blades on a couple of em.

These carbs are speced on 'timing figures' , lean em out how they should be & BGs timing figures go out the window , is it just sales patter?