Most of the early Ford distributors have an adjustment within the port. You remove the outer vacuum nipple fitting and can reach inside ( I believe its an allen screw) and adjust the vacuum. I think they went to the wayside when emmision standards got stricter. There was a short periood of time when the mfg. were mandated ( I love that word) to make engines "tamper proof". I can remember having to pull/drill restrictor caps to adjust idle mixtures enough to allow a good idle. It was usually after the engine got some miles on it and the circuits were no longer pristine. I bet I have a thousand restrictor caps from when I worked at the Honda (motorcycle) dealer. Hmmmm...may be needed in some "restoration"...sounds like e-bay material.
