OK. Once and for all, I want to know what, exactly, the idle airscrews (or mix screws?) do on my 1990 GS500E.

When I tighten them all the way up, am I shutting off an air passage, or a fuel passage? I want to know what they do to the mix... I assume whatever it is, it's at very small throttle openings (like 0 - 1/8th open). Do they affect the slides operation? I'd like to understand them better so I can figure out what else they impact.

I'm playing with them now to get the bike tuned it. They were at 2 turns out, now set at 3.5 turns out.

I've noted that, in terms of tuning, turning them out seems to make the bike fast idle better / more predictably when cold. I've had a hellacious issue with hanging idle after 30 minutes riding, and this seems to be the final adjustment to address the problem. (everything else is stock on bike with Srinath upjet and washer on slides, valves gapped.)

The change (turning the screws further out) has created a new issue, a flat spot at ~4000 RPM.

Thanks!

Actually it depends where they are on the carbs- towards the air filter side or the engine side. On the air filter side, turning them out leans out the air/fuel mixture. And while they are called the idle mixture screws,  air/fuel mixture screws, whatever, they affect the MOST at idle. Their affect at WOT throttle is minimal,  but still there.

If you have them set at 3.5 out, I would say you need a size up on your idle jet. And a hanging idle at temp......could be an air leak developing after the carb mounts warm up.

Nevermind. I looked over the factory manual's circuit guide and figured it out.

Bottom line is that they are a mix volume screw. Opening them up enrichens the flow at idle, sending more of an air and fuel mixture out of the carb. This, in addition to the fixed orifice outlet that is slightly upstream. What's a bit counterintuitive about this is that since it's sending air and fuel into the airflow, opening up this screw allows you to run with a smaller throttle plate opening

So:

More turns out (Open) = More air and fuel, and need less throttle opening at idle
More turns in (Close) = Less air and fuel, and need more throttle opening at idle

I think that by opening up this screw, it allows the engine to run with the throttle plates more closed than they would be without this jet. Closing up the throttle plates more, at idle, prevents the floating idle issue.