Marty - the main metering jets screw in; but Carter metering is much more sophisticated than Holley.
Carter uses a metering rod that "floats" in the center of the fuel jet. Thus the metering area at any time is the area of the jet less the area of the rod. The rods are tapered and the height of the rod in the jet is controlled by engine vacuum pulling against a pre-tensioned spring. Different springs may be used for different values of vacuum (remember that the camshaft profile will have a great effect on the amount of engine vacuum).
Since circular area does not increase in a linear manner; the proper way to recalibrate any "metering rod" carburetor is to calculate the effective metering area (jet minus rod) at each vacuum step; calculate the percentage increase or decrease desired, and then calculate the proper rod diameters to provide the same percentage change at each vacuum step and then change the rod.
From a practical standpoint, this is difficult; as Carter no longer sells metering rods, and the enthusiast generally finds the correct rod to be non-available (or expensive). This is the reason I have suggested in many posts on this forum to start with the PROPER carburetor. If one needs to go only 1 or 2 thousanths, one may simply change the jet and the metering will be close. Starting with the proper carburetor will normally mean only a 1 or 2 thousanths change (unless there is a radical camshaft involved). If a radical camshaft is involved, it may be necessary to change the vacuum piston spring to one that is weaker. The idea is that the rod moves with changes in vacuum, thus permiting much better control over metering. With a radical camshaft, there may be insufficient vacuum to overcome the spring tension of the normal spring, thus keeping the rod on the power tip (much richer) all the time (and thus making one think one needs a smaller jet).
Options if more than 1 or 2 thousanths change is required would be either (A) use a rod and jet combination that is correct for most of your driving, and try to get the other steps rich; or (B) call that old geezer
in Missouri that custom cuts metering rods on a manual lathe at great expense.
Again, the easiest (and cheapest) method is to start with the correct carburetor. Take a look at the comment in my signature line about modifying carburetors.
Jon.
