Idle diaphragm thing

[Hard Drivin Johnny]

So, when I first got my car, it idled at about 850 with the AC off, and stalled with the AC on. At first I just shut the AC off when I was stopped at a light.

Took the car to Trackdog, got a big pile of work done, they set the idle, but it still stalled with the AC on. I set the idle using the air screw with the AC on, and it would be about 850, but jump to 1500-1750 with the AC off. I just left it like that since I pretty much ran the AC all the time the last few months.

I put new NGK wires on it, and a fuel filter. Trackdog had put new plugs in it.

The weather cooled off, so I decided to close the airscrew and get the idle where it should be. I closed it until it stopped, and the idle is still around 1000-1100 rpm. It will idle around 700-800 with the AC on now.
Looking at the throttle butterfly arm, I see this diaphragm type thing that I'm guessing reacts to manifold pressure and kicks the idle up if it drops. None of my manuals show it.

Can someone help me out with adjustment of this to get the idle down, or anywhere else I should look? Trackdog set the timing for me also, so at this point I'm assuming that's right but I will check it if nothing else pans out.
Also, should I replace that part if the idle isn't coming back up when I turn on the AC compressor?

The clean engine bay is thanks to the gentlemen at Trackdog. The thing was inches deep in dead leaves when I got it.

[POS Racing]

You adjusted the idle using this procedure? -> http://www.miata.net/garage/isc.html

[Hard Drivin Johnny]

The dashpot! What does the dashpot do?

And no, I didn't check it that way, my idle is damn good (i.e. I don't wanna do all that stuff. )

Dude... over 500 bucks for the ISCV. Shite.

Thanks for the link. Great info.

[Hard Drivin Johnny]

I found it in the workshop manual. It's part of the deceleration control system, prevents the throttle from snapping shut.