bcware is partly right. The screw isn't really either, air or fuel. Air is drawn in through a vent in the carb bell. This air gets mixed with fuel at the pilot jet.
The pilot jet feeds this air fuel mix to three holes by the throttle plate. Two feed directly into the airstream, the third hole is controlled by the mix screw. This allows you to adjust one third of the idle mix that feeds to engine.
There is a vent that feeds air to the main jet too.
This is why some of the cheapy pod filters are a poor choice. They have a lip that can block off the vents either partially or fully.
Leo