I have the same carb on my 68 Mustang. I can't really give you any answers to your questions, but thought it might help to know that my 68 is the same. Mine has a rubber circle attached to the arm that covers the hole. I can't tell from the picture if yours has the rubber piece. I've never seen this lever move when I have someone push on the throttle or if I do it from the engine compartment, so I wouldn't mind knowing what it's for and how to properly adjust it.