I have a 1987 rx7 sport. I can't figure out what this adjustment screw controls, so I have not move it. Not wanting to mess anything up.

 

if your engine runs fine, leave it be

its essentially a fuel/air mixture screw, you turn it one way to lean it out, the other to richen it

 

It's the variable resistor and is used to fine tune the fuel mixture while idling. Usually, the slot in the screw is closer to the R (rich setting) than the L (lean setting). The screw only turns a bit past the R and L designations so don't force it if you plan on any future adjustments. Adjustment is made w/the initial set coupler jumpered and the engine fully warmed.