Dirty Slots,
The screw allows more or less air into the pump/dashpot. They work very much the same as the screen door closures on a home. You will need to loosen the nut first as that locks the screw into place. Turning the screw clockwise will extend the spin duration. If you turn it counter clockwise it will shorten the spin time. Once you get the nut loose you can experiment with the timing. Now you have to be careful as the pump is diecast and can break easily if not careful.
Inside the pump/dashpot is a leather cup washer. Most of the time these dry out and the reels will shop quickly. If adjusting the screw makes no difference in speed, then that leather washer inside the pump maybe hanging up and you will need to take it apart and clean it.