There are two screws, a large “bypass” screw and the smaller “volume” screw. Here’s what I gather after dredging up a hundred or so posts about the subject on different sites and in different service manual PDFs:
On old (pre-PICT-34) carbs, where the larger (bypass) screw is on the bottom and the small (volume) screw is on the top, the bypass screw controls total air to the idle system, and the volume screw controls fuel. These carbs coincidentally oftentimes don’t have an idle shutoff solenoid, or the shutoff solenoid was located on the right side of the carbs, integrated with the pilot jet, alongside the choke. On these, screwing in the volume screw will lean it out.
On newer (34-PICT and onwards, including 30/31 PICT modern replacements, like the 30/31 PICT-3 I bought a few days ago), the large bypass screw is on top, and the small volume screw is on the bottom. Allegedly, according to aircooled.net, rob and dave’s VW pages, and a simple majority of TheSamba posts, on these the volume screw now controls idle air, and screwing in the volume screw makes it richer, as you’re controlling air. On these carbs, they have an idle shutoff solenoid located on the left side of the carb, right next to the idle adjustment screws.
From what I can gather, in the second scenario, the pilot jet controls total fuel available and therefore must be chosen carefully, as you cannot push less fuel than it provides, but you can adjust more air into the idle circuit via the volume screw. In the first scenario, (allegedly) you could oversize the idle pilot jet and adjust it down with the (confusingly-having-the-exact-same-name) volume screw, which was helpful as the idle solenoid being integrated with the pilot jet meant rejetting cost quite a bit more.
Added to all of this terrible confusion, we have the fact that there is a current split in the VW-ownership-matrix re: the gigantic number of original, pre-71 cars, and the absolutely mindboggling amount of data about those cars that is stored in difficult-to-access “offline-only” sources, like certain books, and greybeard word of mouth. Converse to that, you have the ACVW “resurgence” of the past 10 years or so, and the huge aftermarket that has grown in size around those enthusiasts. The new enthusiasts tend to be younger and more apt to share info “online” about their cars. Since that online information coincides with the new aftermarket that does weird things like create the mentioned “30/31 PICT” carburetors (named because they replace both 30 and 31 PICT carbs in a sort of hybrid arrangement where the manufacturers picked the best of both worlds) and their weird idiosyncracies (like the reversed bypass/volume arrangement) means doing a google search for information produces opposite and conflicting results.
These carbs are, I think, a perfect example.
Here’s the original, German 30-PICT-3. Notice there is no solenoid on this side of the carb, and in these, I’m 99% sure the small screw adjusts fuel:
Here’s the 34-PICT-3 I was running. Notice it has the solenoid, and the screws are backwards, and therefore theoretically the small screw adjusts air, not fuel:
And here’s the Chinese 30/31 PICT-3, that has the solenoid located the same as the 34 (different than the OG 30), and has the 34-style screw arrangement. Strangely, though, all of the parts from this fit on the OG 30-PICT-3, and is what I used to rebuild the one posted earlier.
The Chinese 30/31 PICT-3 is a hybrid that takes from both generations. Unfortunately, decades and decades of knowledge were built up upon the original 24/28/30-style carburetors, which is the absolute reverse of nearly all aftermarket parts, like the 30/31 replacement carbs. I have found enough posts stating that the later carbs are “assbackwards from every other carb on the planet” in that respect, though, so I’m pretty confident I’m on the right track. That being said:
Those who know ACVWs can chime in here, because I could be deluded in my search and be wrong about all of this on all fronts. No sarcasm: it has happened before.