This is for regular carburetors. Backing it off 2 turns or so is a good "base" measure just to get the car started.

To make it run right, you can easily adjust idle mixture by ear:

- Make sure engine is fully warm, and the choke is fully open (shine a flashlight down the carby if necessary to see)
- Turn the screw half turn at a time in (clockwise)
- When you hear the engine slow down, back it out half turn until you hear it speed up (listen carefully)
Do this several times until you get the feel for it. Turn it too far in, you're leaning it out and will eventually stop the engine.
When you turn it out, you are richening the mixture, and it will start to run "rough" and kinda blubbery (hard to describe, but try it)
If you kill the engine, back the screw out again and start it back up
- Once you figure the point where turning it just begins to slow down, turn it in 1/4 turn at a time to fine tune it
- Continue this until the engine is running at the fastest, smoothest setting as far in as the screw will go. For best power, you can leave it here ... but ...
- For best economy and emissions, turn the screw in about 1/8 of a turn more. You'll hear the engine slow down just a little bit. This is the factory setting.

This will work well based on a couple of important foundations:

1. Set the points exactly (new points with a feeler gauge, used points need a dwell meter)
2. After dwell is correctly set, you can time the engine carefully. Use a timing light is best ... check any book for process or ask here for more information
3. Make sure all vacuum hoses are tight and there are no leaks -- these will affect idle mixture and running mixture. Notorious leaks are in old vacuum housings on the air cleaner (the bimetal is supposed to leak a little, but not the round flap valve servo). and all the round vacuum actuators on the carb itself. Better to plug a hose with a screw than to let it leak.
4. When the above is right, set idle speed: 650-700 rpm for a stock or mild camshaft. Do this before setting the idle mixture.