The screw on the bottom is not for the idle. It affects the main and the idle. It affects the vacuum on the bowel. The screw on the bottom is factory set to the carb it's attached to in order to give thar carb the correct volume of fuel mixture based on the cylinder's volume it's attached to. Basically you pick the main jet and needle jet and sync the carbs on a bench and adjust all the floats then set the screw underneath at 1 or 1 1/4 out. Then start the bike and use vacuummate and 4 colortunes at the same time to adjust all the carbs so they are running at the same vacuum at constant higher rpms by adjusting the bottom screws and screws on top of the carbs closest to the cylinders until you get a nice blue in the colortunes and the vacuumate shows them all equal. Then you can start to work on idle and the blips and transition from idle circuit to main circuit. It's an iterative process. There's a very good paper on it on gsresources website on the vm26. These carbs are not like most carbs. Yes lots were made in the 1970s but the smoothbore sliders were gone by the 80s and there are fewer and fewer people who can tune these. The pro tuners of the 70s are few and far between. I don't claim to be a pro. I just found lots of papers and read the factory manuals and talked to shops until I figured it out. My carbs are really well tuned and my bike pulls good and my pipes aren't blue and I don't smell gas in my exhaust.

You are correct. This is a very advanced carb. Most bike shops are used to only 1 air screw or 1 fuel screw. This carb has 1 air, 1 fuel, and 1 main screw and then it has a slide lifter on each carb top under each lid and then a pilot needle size and then needle drop heights and a main jet. It was the Superbike of 1976-77 fir suzuki and highly tunable. It's old tech now, but if you know how to tune it it can be dialed in. It takes some doing and for sure understanding how the vm26 works to get it running right.