To adjust the idle mixture on most carburetors, begin with the idle mixture screw(s) turned out about one-and-a-half turns and the engine warmed up. Adjust each idle screw individually in order to achieve the highest manifold vacuum (which should also be the highest idle rpm). It is usually easier to get a more accurate vacuum reading as opposed to the idle rpm. In any event, a vacuum gauge and a handheld tach will allow you to set the idle adjustment properly. If you have to turn the idle screws open more than 3-1/2 turns, I would be concerned about some other problem.
Also, a lot of ignition issues wind up being blamed on the carburetor because iginition problems can also cause the type of symptoms that you are describing - especially with ignition systems originally designed for leaded fuel which of course is a completely different beast from what you are putting into the tank today. Make sure the timing is set correctly, that you have a strong coil, that you have good plug wires, no cracked distributor cap, good points and condenser, good rotor, etc, so that you are getting correct voltage to the plugs with the correct timing.
