Oh, that's right, the screw is a stop on the barrel rotation. These do not close all the way, so in effect they do bleed air. The low throttle needle keeps it from downing on idle. These also meter during the rotation to full throttle.
I think the barrel moves in two directions as it rotates? At least it looked like a typical newer OS style. Maybe that is just on my 4 strokes. It has been a while since I messed with them.
I had heard sand would work as long as it does not drain out of the bend area. I have always thought the increase in volume made the sand loose and it still collapsed on me.
DC