Turning them in makes them "Leaner"
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Since the fuel coming out of them is already mixed properly, they act like teeny tiny throttles.
It gets really complicated when you factor in the transfer ports. When the throttle is closed, the transfer ports lets more air into the fuel idle circuit making it leaner. When the throttle passes the transfer slot, it' lets out more fuel making the mixture richer.
But it's just easier to think of idle screws controlling the idle mixtures so lean-in rich-out
You can see the taper on the idle screws and turning them in will limit the amount of mixture coming into the engine.
here is a good picture of how the transfer ports work:
It's for a progressive Weber carb but the theory is the same.