The idle mixture screw (the small screw in the side of the casting just under the bolt flange) is actually an air bleed adjustment.
Turning it IN restricts the air bleed to the idle circuit, which enriches the mix.
Turning it OUT leans the mix.
Start with the screw about 1/2 to 1 turn off the seat, fine tune it from there.
It is not like a modern carb that the screw adjusts the fuel flow. That said, it is near impossible for the idle circuit to be so rich it won't run. If it is blubbering black smoke at idle, chances are the float level is too high, the float is sunk or bent, or there is a problem with the needle/seat, loose, bad seal, trash holding the valve open.
But, you need to get the engine speed down in the idle circuit range for the idle circuit to be in use. Once the throttle is partially open, the idle circuit stops drawing and it's trying to idle on the main circuit, which won't work very well.
Be sure when the dash lever is in the idle position, the throttle stop screw is held firmly against the stop. If there is slack in the governor link, the idle will be erratic, never return to the same speed.
Other causes of carb idle problems can be a worn throttle shaft, throttle plate loose, bent, installed backward, or not properly centered in the bore.
Another common problem, warped housing where the bowl screws to the upper housing. There is a small hole in the gasket that aligns with a passage from the bowl to the upper housing. That is the fuel supply for the idle circuit. If the upper housing is warped (tightened down with the venturi installed upside down or not seated), or the hole in the gasket is in the wrong place, or the passage is blocked down by the main well, it will not draw up fuel for the idle circuit.
But...
Many times an innocent carb takes the blame for a multitude of problems!
A sick engine will not idle well. Have you checked the compression, set the lifters, checked the distributor for side play in the shaft, checked the centrifugal advance and timing, adjusted/replaced the points, checked/replaced the plugs, wires, checked the air cleaner for free flow (mouse, bird, mud dobber nests)?
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