CIS A/F conundrum question.......

[CatSkynr]

I have an interesting question. I am a little confused and want to know if this makes sense.

When I first acquired my ’77 911S, my local mechanic after about an hour of fussing with it told me the air screw was threaded backwards (clockwise = lean). For the last five years, that’s how I have always adjusted the A/F. Well this weekend while adjusting for emissions, I couldn’t get the car to lean out far enough (gunston went out of calibration while working). In the meantime, all of a sudden while turning clockwise to go lean, my car passed a threshold and started going rich again. Now my air screw is correct (clockwise = rich). Since I took possession of the car, I have had trouble getting it to idle smooth and had symptoms of a stuck exhaust valve. I had to run extremely rich (approx 11/1 AF) and both symptoms went away. NOW, with the air screw apparently turning correctly, these symptoms are gone (except while tuned lean for emissions) once I get my CO @ about 2.0 .

Does this make sense with a CIS system? I wouldn’t think the air screw could be tuned beyond its threshold point. How would passing this threshold reverse the A/F effect? I am happy that symptoms have gone away, but why would these symptoms occur with the air screw where it was?

[CatSkynr]

ttt