Setting idle queston

Turning the throttle stop screw in and out on the drive side carb seems to have no effect on the idle speed. Original carb not a Premier.

If it's not physically moving the throttle slide your cable needs slacking off?
(Unless you have an air leak further upstream)

then you may well have too much slack in the cable so the slide does not come up?
I would take the air filter off and look in the back of the carbs with motor off while you turn the throttle
and you can see not only if the slides move together but also you can see if the drive side slide lifts as you
turn that adjustment

Huh...Too much slack? The idle screw is the slide stop on an Amal.