With the long duration cams some carbs won't idle as clean without four corner mixture screws. If you open the throttle plates far enough to get an idle you are into the off idle circuit. Four corner idle mixture is used on carbs that use the secondary throttle blades to smoth the idle mixture out.
The only advantage to a four corner idle mixture is at an idle. " I don't know of any S/SS cars that leave at an idle".
Bill Beebe
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