Adjusting air flow meter (Opel Vectra A 1994)

You don't adjust the air flow meter, the meter's job is to give accurate air flow readings to the engine computer to it can adjust the fuel-air mixture. If the air flow meter isn't delivering accurate information to the computer then it needs to be cleaned/repaired, not adjusted. Most allow no type of adjustment anyway. It may be it just needs a clean, how to depends on the type, it's also possible it has broken beyond repair and needs replacement.

However, a rough idle could be many things: idle control valve, low fuel pressure, vacuum leak, faulty oxygen sensor, etc. Cleaning the air flow meter makes perfect sense, but you should check the computer for codes before spending loads of time tracing things.

Edit: the picture added to the post shows a pretty badly scraped sensor ... plate? Thingy? Not sure what you call it, but it's the part that the sensor arm makes contact with. It works on the principle that the voltage drops the more graphite (assuming it is graphite) it passes through. However, over time the friction rubs off the surface, leaving bare spots where contact is lost. When contact is lost the air data drops, causing the wring fuel-air mixture. I had a similar problem on my Porsche 944S, I resolved it by bending the contacts in a bit. Yours is a different construction though, so that might not work. I diagnosed there were issues by using a continuity test, and listening for dropouts as I moved the sensor arm.

You may have a similar problem, and that's how I'd test first. If you have a drop in continuity at low air flow it may be your issue, if you don't then it is still working. You can also run a voltage through it and probe for fluctuations as you move the arm.

There are some alternate fixes which may work if your sensor has bad patches:

1. Use a pencil to replace missing graphite. It sounds nuts but it apparently works. I don't image it will last forever, but it could be a useful temporary fix, and help diagnose your issue
2. Resurfacing compounds: these replace the original surface better than rubbing a pencil
3. Relocate the contacts to a different part of the arm, so they touch different parts of the plate. This may or may not be achievable depending on the sensor arm construction

If it was me I would, since it's open already, give it a thorough cleanup and test it using the methods above. If there's dropouts try the pencil trick and see if it works. See how long the fix lasts

One thing: make sure you seal the sensor up again, you don't want grit getting in there. If it was sealed before do it again with high-temperature silicone.

If your engine is non-catalized (20NE as you say), it should have a trim to ajust the %CO at idle.

A wrong trim can cause bad idling.

Anyway you should do the paper clip test and check if there are any error codes saved.