4 barell rochesters used 2 adjustment screws. With the engine off adjust the screws fully inward then turn them out 2 full revolutions. Start the engine and continue turing each screw by a quarter turn in sync with each other until the idle seems correct. If you are advanced enough you can also use a vacuum gauge attached to a manifold vacuum source and watch the vacuum readings while turing the screws. If the vacuum drops too much then you leaned it out too much and you need to go back a quarter turn or until suitable. Hope this helped.
The real answer: First the assumptions. I assume that it is in fact adjustable because you give no date of the carb. If the carb pre-dates 1972 then it is. And I assume the intake manifold is a dual plane intake. And I assume that the car starts and runs but needs the A/F ratio tweaked. If it doesn't start, then set the screws at a base setting of two turns "out". A dual plane intake separates totally the intake runners between cylinder 1,3,5,7 and 2,4,6,8. Therefore 1/2 of the intake manifold serves fuel to the right bank and and the other half to the left bank. Therefore, the adjustment screws need not and should not be turned "in sync". If using a vacuum gauge as the writer above suggests, turn each screw independently for the highest vacuum. Then back-off screw 1/4 turn.
If using a tachometer, the suggested way. Then turn one screw clock-wise for highest RPM (rich condition) and then turn the screw counter-clockwise (lean condition) until engine just stumbles; turn screw clockwise 1/4 turn. Do the same for the other adjustment screw.
P.S. The previous writer is an idiot.
68 Impala convertible
67 GTO
Charlie Linkowich
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