This should be your carb http://www.osengines.com/parts/ev15460aa.pdf
As you can see item #6 and it's spring are the idle stop screw. It's used to set the absolute low end of the throttle and NOTHING to do with mixture.
Item #2 is the idle mixture screw and can only be seen from the exhaust side of the engine and is VERY close to the prop so do not attempt to adjust while running.