The air/fuel mixture screws adjust the fuel/air mixture at idle.
The idle speed screw on the linkage opens and closes the butterfly for optimum RPM.
To properly set the idle, you have to endure several iterations of going back and forth between these adjustments until you get it right. The air/fuel mixture screws are key to setting the idle. They must be set optimally. Then, the idle speed screw is used to adjust the RPM to specs.
The pitfall is that if the idle speed screw is turned in too much, the butterfly is opened to where the idle circuit is effectively bypassed. When that happens, the adjustment of air/fuel mixture screws doesn't affect the idle since the vacuum now feeds the engine through the main jets. You've seen posts on the forum where someone says they can turn the mixture screws all the way in and it still idles too fast. This is because the idle speed adjustment screw is in too far and opening the butterflies to where there is no idle circuit, and the fuel is coming through the primaries.
Hope this helps some. Good Luck!
"Get your facts straight first, then you can distort them as much as you wish"!!
1965 Fastback, Sunlit Gold, I6, T-5, Pertronix, CAA AC, SSBC frt discs