Those screws very often hold the rotating drum in the carb body. Their point often runs in a heilical groove in the drum and serve to move the drum a controlled amount sideways that matches up with the amount the venturi is opened.

No matter what brand of carb, it is wise to verify that the loss of that screw isn't screwing up other things or might not risk other more important problems. On your ST, try to pull the drum out from the side.

But keep in mind that some carbs shouldn't be affected at all by the loss of that screw. And it's possible your ST carb is one of those. I'm pretty sure their barrels on most of their carbs don't move laterally.

And with our R/C throttle control, an idle stop on a carb is really a waste of time and a setup for a problem. There really is no reason to have the carb limit it's own closure. Our radio should be adjusted to give perfect WideOpen and perfect idle (which requires the throttle not be stopped early) and engine shutoff. And what additional closure should be available? That sucker can cause a big problem if it keeps the engine from shutting down, and at any other throttle position it's worthless.