Ray - the pre-1968 carburetors were less responsive to adjustments in the idle mixture screw(s).

With the beginning of Federal smog emission requirements in 1968 (California was 1966), most of the carburetor manufacturers changed the taper on the idle mixture screw. Take a look at a screw from the 1970's, and compare the taper to the 1950's. The longer taper smog screws are much more responsive, and one can obtain a more consistent idle with these longer tapered screws. Of course, the manufacturer changed the taper for easier adjustment for smog emissions.

Jon.

Good carburetion is fuelish hot air
The most expensive carburetor is the wrong one you attempt to modify.
If you truly believe "one size fits all," try walking a mile in your spouse's shoes!
The Carburetor Shop