sorry but being new at this im not sure if you mean the screw on the butterfly valve or in the side of the carb?
It's in the second picture, right in the middle. There is a screw with a spring on it that takes a flat head screw driver (vertical slot in this photo). You count rotations by the slot. So try turning it clockwise until the slot is horizontal and that will be 1/4 turn. You can mark the top with a black marker to help count the turns.
If you take the screw out you would see the end is tapered. It fits into the gas stream in the carburator at idle. The more you screw it in the more it restricts the gas flow. When the butterfly valve opens up and their is more vacuum the main jet takes over. But if you don't have any restriction in the idle set screw it can add too much fuel to the mixture and cause a bog or stumble.
http://www.thecarburetorshop.com/Troubleshooting.htm
I had a computer controlled carburator on a Mazda pickup that was giving me fits so I replaced it with a Holley 5200 out of a 2.3L Ford Pinto. It took a little adjustment to get it working and I had the same problem. But I had idle and secondary set screws I could play with, as well as the jetting. So I got it fixed but left the larger jets in it so it ran slightly rich. I could accelerate faster by not pushing the throttle all the way in.
With this carb it looks like all you can do is adjust the one screw and the position of your butterfly valve by adjusting the small screw above the idle set screw. If you open your butterfly valve up a little at idle you won't get as strong of a vacuum on the backside, which could be adding too much fuel at lower engine speeds. But I would start with the set screw because it's easier.
Mark everything and write it down because it's easly to loose track when you're tweeking carbs.
