How does the idle speed adjustment in a Stromberg NA-R9B float-type carburetor work?
The idle speed adjustment in any engine is to control the minimum RPM that an engine reaches, in other words how quickly it will run with the throttle all the way off. You want the idle speed to be high enough that the engine will not stop running when the throttle is pulled back, but not so high that it wastes fuel. On a carbureted engine the idle speed is typically adjusted using a set screw which allows a mechanic to quickly adjust the idle speed. This would be done when the airplane is serviced as part of its regular maintenance, or out of the service cycle if there's a reported problem. 

The idle speed adjustment is unrelated to the mixture control and is not adjustable in flight.