When I arrived in Salt Lake from the low altitude East Coast, I needed to turn down my fuel.  The same screw is also used when you do a turbo installation to allow more fuel through the system.  When you turn up and down your fuel control is makes adjustments regarding black smoke and must be turned according to your EGTs.

Here is a pict of the screw.  It has a three prong lock washer that you must create a special tool to loosen.  My tool took about 10 minutes to make using three nails, a drill, some super glue, and a piece of 1/4" thick, 1" wide steel.  The steel was actually an old hinge.

Turning the screw in (clockwise) makes the engine leaner, turning it out (counter clockwise) makes it run richer.  Turn in only 1/4 turn increments (the screw is very sensitive) and always record your fuel adjustments in a ledger in case you need to return back to your stock setup.

HJ and BJ motors have a similar setup.