%O2 sensor adjustment screw

[vancouver86/911]

I have an '86 Carrera ROW that I recently dropped the engine to change the clutch and I am now just putting a few miles on it which is allowing me to become familiar with the car. The car has an 02 sensor, no cat and a 2 in 2 out aftermarket muffler installed by the previous owner. When started cold the engine would rev to 1200 rpm and then shorty after drop to 1000 rpm but I could not get it to go lower than 1000 rpm. When the engine warmed the idle rose to 1200 rpm and stayed there. In stop and go traffic it was a neck breaker. Letting off the throttle would nearly put my head into the steering wheel. The car idled rough all the time, warm or cold. I pulled the ICV and bench tested it and the mechanism inside opened and closed no problem. After reading the forum I decided to jumper the B&C plugs and I was able to get the idle down to 800 rpm. Unfortunately the idle then rose up to 1000 rpm again after revving the engine. I suspected a vacuum leak so I got a length of rubber hose and stuck one end into my ear and begin hunting for the leak. I checked the vacuum lines and then when I got to the oil filler cap I found the snake...hissssssss. I removed the cap and put my hand on the oil filler spout and the engine smoothed out quite a bit. I replaced the oil filler cap and now the engine stays right at 800 rpm although the engine still idles rough. The car now coasts when letting off the throttle and is much smoother to drive. While I was checking the air flow sensor I found that the cap was gone but to my suprise so was the adjustment screw. I was able to insert any size allen key, long end first, at least an inch and a half into the hole which leads me to believe there is no screw in there. Suprisingly the car passed the air pollution test easily in this condition. Are these adjustment screws available? Can driving the car damage the engine in this condition as I'm getting a little bit of backfiring? I realize that the air/fuel mixture is going to need to be set now that I have found my vacuum leak and obviously I need to find and adjustment screw. Thoughts and comments are most welcome.
ps, title should read %CO sensor adjustment screw

[rusnak]

the mixture screw can fall out.

What I'd do is look for a fairly new, used AFM (air flow meter) on ebay or in the classifieds.

Be careful, because these things can go bad or wear out. Then you have two supposedly good AFMs.

Then I'd have a second O2 sensor bung welded into the cat bypass, so you can use a CO meter or Air Fuel Meter on the car while the DME O2 sensor is working. Then you can adjust the CO mixture after the car is hot, and you can see what is going on with the DME while the car is under load. Otherwise you will have some difficulty knowing if you have the baseline CO mixture correct. Since you already have a bypass, for just a little bit more you will have a good way to check your air fuel ratio, and health of the O2 sensor in the future.

When setting the baseline CO mixture, disconnect the O2 input connection first, and then disable the ICV, by jumpering the pins. Manually set the idle to 880 RPM (I think).

[Wavey]

You could use a mirror to see if the screw is really gone, or you could unbolt/unclamp and rotate the AFM to get a better view. The screw is brass-colored. Might it be sitting on the engine tin somewhere?

[Talewinds]

The AFM adjustment screw sits quite deep up in that hole. It has a conical/funnel shape which directs the allen wrench into the center.
The allen size is 3mm.
Use either of those methods Wavey suggests. To me, it is not fairly apparent even by looking into the hole that it even accepts an allen wrench.

[vancouver86/911]

Wavey and Talewinds,
You were right...thanks for the tip..I went and bought a tiny mirror and a flash light and then checked up inside the hole and there it was..a little brass screw with an allen head. I foolishly assumed just because the Bentley manual shows a picture of the guy inserting the allen key short end first that the screw must be near the opening of the hole but it isn't, it is way down deep in the hole.
Now that I know I have an adjustment screw which way should I turn it to get some sort of reasonable setting to stop the backfiring so I can get it to a shop to set the mixture correctly? I had to turn the idle screw one whole turn to get it to come down to 800 rpm. I figure since I found and plugged the vacuum leak there is less air going into the system now which means it must be running rich? I'm hoping this smooths the engine idle right out when adjusted correctly.

[Talewinds]

If you have some time, get familiar with this thread:
idle bounce

It should tell you more than you want to know.
Good luck.