i had an 82, it was controlled by the screw new2z was talking about, its hard to find, but look for a lil black cap on the side of the intake, take the cap off and if you see a flat tip screw inside, thats the idle adjuster.
It took a mechanic at Z Excellence here in Jax to show me that one. i saw all the pics in the books showing that it should be on the throttle body right on top, but I never saw that screw anywhere on my 82na, after I got my 78 I was like oooooh ok. i think the 81 might be the first year they changed it.