Idle mixture screws on the secondary’s and not primary is normal on some of the governor carbs, not all, but some.
Curb idle is set with the engine warmed up, air conditioner off and sometimes in drive.
There is more than one way to adjust the idle mixture. By ear, which I have done for years, a vacuum meter, probably the most accurate and with a tachometer.
To ensure optimal performance from your engine, always confirm it has reached its peak operating temperature, with the choke fully open. Any attempt to adjust the idle mixture with the choke partially engaged is futile. Further, ensure the idle speed screw isn’t in contact with the choke's fast idle speed cam. To make this a seamless process, you'll need a vacuum gauge linked to the manifold vacuum.
Mastering the art of measuring idle quality is paramount. Every internal combustion engine relies on a certain degree of idle manifold vacuum, stemming from the cylinders' demand against a closed throttle. While there are several methods to measure engine vacuum, the most prevalent reads the vacuum as negative pressure, depicted in inches of mercury (“Hg). Often, vacuum/pressure gauges are shown in either “Hg or centimeters of mercury (cmHg).
Generally, production and mild-performance engines idle between 12 to 18 “Hg. A stable gauge needle is what you're aiming for. Beware: an erratic reading might signal a significant issue, like a leaky intake valve, which can disrupt your engine's harmony. Thus, always ensure your engine's health before tweaking the idle mixture.
Prioritize checking the position of each idle mixture screw before you ignite the engine. To do this, rotate every idle mixture screw clockwise, noting the turns until it lightly sits. This typically ranges from 1 to 2 turns out from full seating. Exercise caution to avoid causing damage. Ensure all screws are adjusted uniformly to guarantee a balanced output from the carburetor's idle circuit. A starting point is typically 1 to 1½ turns out.
Once your engine is idling (preferably in Park with the emergency brake on), set the idle speed, aiming for about 850 rpm. Adjust using the idle speed screw located on the primary throttle linkage on the carburetor's driver side.
Your focus should then be on the driver-side idle mixture screw, adjusting it clockwise about 1/8th of a turn and keeping an eye on the vacuum gauge. If there's an increase in engine vacuum or speed, adjust the opposite side by the same measure, and evaluate. If the vacuum reduces after the first adjustment, revert to the base setting and turn the screw out by around 1/8th of a turn.
This preliminary sequence aims to optimize the idle mixture screws on the carburetor for the best possible idle vacuum. In the event the idle speed surpasses the intended rpm post-adjustment, revert to the initial engine speed by adjusting the idle speed screw counterclockwise. This might slightly lower the idle vacuum, but it's an expected part of the process.
Idle mixture screws aren't making any difference in RPM?