my bad, i thought you meant a cable adjustment that an aftermarket tors removal cap has.
you are talking about the stock idle screw on top of the tors box.
have you tried turning it in the other way ?
i haven't had a tors box/idle screw in so long i forget which way turns the idle up ? i think it's "turned in" that pushes a lever and lifts the slide for more rpm/idle ?
it's not uncommon for that idle screw to malfunction, a tors removal kit w/ standard carb cap and idle screw tapped into the side of the carb is the best option.
btw...
you're airscrew should be at 1.5 turns out normally, so it's also possible you have a clogged piloit jet ? turning the airscrew in limits the air thru that circut to correct the air/fuel ratio for less fuel delivered by the clogged pilot.