Taken from someone else's post:

"Contrary to popular belief, the mixture needle does not control the idle circuit air/fuel ratio. Although this may simply be a matter of semantics, there is a difference between the mixture in the idle circuit itself and the mixture that ultimately reaches the intake manifold. The idle circuit calibration is determined by the diameters of the idle feed restriction and the idle air bleed. A 'feed restriction' is nothing moere than a metering jet for the idle system and the air bleed serves as an airflow regulating orifice. As the air/fuel emulsion exits the idle port it is mixed with small amounts of intake air that flows past the throttle plates. Varying the throttle plate opening (with the idle speed screw) increases or decreases both the amount of air that is allowed to flow into the intake manifold and the amount of pre-mixed air/fuel emulsion discharged at the idle port. Since these two idle mixture components are affected by degree of throttle opening, the ratio of intake air to idle mixture air/fuel is not drastically altered by changes in idle speed. Turning the idle mixture screw varies the volume of air/fuel emulsion discharged into the manifold, not the ratio of air to fuel in the emulsion. Therefore when a satisfactory idle cannot be achieved by tweaking the mixture screws richer (more idle air/fuel to intake air) or leaner (less), it can be assumed that the carburetor idle calibration is not within the range demanded by that particular engine.

The use of two four-barrel carburetors is an excellent example of such incompatability. With such an installation, the volume of air/fuel emulsion is effectively double that available from a single four-barrel mounted on the same engine. Even when the mixture screws are adjusted toward lean, there may simply be too much emulsified idle fuel flowing to allow a clean idle. In such cases the idle feed restrictions of both carburetors must be reduced, in cross section size,to lean the idle mixture to an acceptable level. These types of alterations are confined almost exclusively to highly specialized race applications. Satisfactory idle performance can, for the most part, be achieved by varying the settings of the idle mixture screws, and the idle speed screw."

In other words, if you are running way too rich at idle then it is very likely you need to look at the air bleeds and idle feed restriction to change the A/F ratio at idle. Or easier yet, change to a different carburetor that more closely matches your application.