I have a 1989 RX7, GTUs. Idle has been a problem since it was new. While attempting to clean the BAC valve, I unscrewed a plastic plug in the end. I put some pipe thread sealer on it and screwed it back in. After reasembly, I found that car idled very fast. I reajusted the idle speed and turned the plastic plug out slowly (this is done with a metric allen wrench. I found that the speed decreased then it became unstable. Turning the screw further out stablized the idle and it appears to have fixed the problem.

I first removed the BAC valve.

With an allen wrench I removed the plug in the end. (Note how far the plug is in before removing it.) This was difficult because it appeared to be factory sealed with blue paint. I used WD40 and worked the plug back and forth until it could be unscrewed. There is a spring inside, so be careful to reinstall it properly.

When I put the plug back in I used some paste pipe thread sealer. I screwed it in past the point where it was initially set.

The plug evidently sets the spring tension which affects the idle speed servo stability. Use your own judgement on trying this.