Turn the air-idle screw (the big one) clockwise (in) to reduce air.  This will have the secondary effect of a richer mixture, but its primary effect is to reduce the amount of air that passes into the manifold, and the engine slows.  Turning counter-clockwise (out) allows more air, and the engine speeds up.  Once you have the desired idle speed, you trim the mixture with the fuel screw – the small one.  This will affect the idle speed, so repeat both steps in smaller increments until both are in perfect balance.

It should be obvious why the air-idle screw is so much larger than the fuel screw; they approximate the air/fuel ratio.  Relatively minute quantities of fuel are required compared to the amount of air.

It is a bit confusing scrolling up and down through main text and comments.  At this time, I am closing comments on this post, pending a re-write that will aggregate everything into one article with all illustrations in place.

“Next set the volume of gas available at idle speeds. This is done using the Volume Control Screw. Please note that the Volume Control Screw controls the AIR volume, not the fuel volume. Screwing it in reduces the air and makes the fuel/air mixture richer. And of course turning the Volume Control Screw out increases the concentration of air and makes the mixture leaner.
Note: The Volume Control Screw is the smaller of the two adjusting screws, located on the left side of the carburetor just above the Idle Cutoff solenoid (which has a black wire from the positive side of the coil attached to it). The Volume Control Screw is NOT used to set the idle speed – that’s the job of the Bypass Screw
”
How is correct?

Amazon.com
Muir, John, How to keep your Volkswagen alive, A Manual of Step-by-Step Procedures for the Compleat Idiot.

Looks like the new edition is paperback instead of spiral bound, but otherwise the same.  Also think about getting the Bentley shop manual, which is excellent but written for mechanics.

I am not a mechanic, but enjoy tinkering. I have a 1972 s. beetle that I can only get to run on two cylendars. It is getting fire to all four, but it is getting gas to only two cylendars. Any helpful suggestions.

I’ve come across a few problems (the latest being what I think is a clogged heat riser) and I’m hesitant to go back to the shop (especially since I’m low on cash).  I think I can fix them, with the help of Muir’s book, but I have little automotive experience.  I’m a little concerned that time/money in the upkeep of my little bug might be beyond what I can do or afford, and I’m hesitant to jump in and start tinkering with something that could potentially be life threatening…  Do you have any advice?

Thanks for the reply.

I guess it is getting time to re-write this post and incorporate everything from the comments into the main body.  Lots of good conversation has come in from readers.

Dual carbs have the potential to get very good gas mileage and much better performance though.  All progressive carbs are by necessity a design compromise – do away with the long, relatively cold intake manifold and you can get closer to optimum.  But you will need a different distributer than the one shown here.