How does the idle mixture adjustment in a Stromberg NA-R9B float-type carburetor work?
The temp is nominally 68F, but the acceptable range is 40 to 80 ambient, by practice.

I can't say about that specific carb, but generally in the Stromberg and the SUs, there is a needle, which can be raised and lowered, which changed the metering.  By changing the taper of the needle, you can also change the mixture profile through the venturi flow rates.

We used to use a manometer to get the vac reading, and then look at EGT to make a decision on mixture.

The needle height will impact the overall mixture, and the shape of the needle will impact the mixture through the flow range.

What aircarft is this on?  I know some of the J-3s had Stromberg carbs, but this specific carb model is not ringing any bells on aircraft.