Ill what i said it a little more simple. This is what I would do if I were in your shoes. I would go down to a smogcheck / repair station and ask them remove the O2 sensor and probe the Exhaust gas analyzer in that hole Pre Cat. The fuel mixture should be adjusted from this point. Depress the allen screw adjuster on the fuel dist. and turn only a little at a time. You will feel the adjuster screw catch when depressed. You want to get it to about .5% CO pre Cat. the HC should be between 120 and 200 ppm. at idle. Then replace the sensor and test the exhaust from the tail pipe. If the Cat is working properly you will have little measurable emissions after the engine is tuned.
If the reading pre cat shows readings that are extremely high, for example, 700ppm HC that points to engine malfunction somewhere. Either AF mix is way off, A fuel injector is bad, or many other possible issues. Start with AF since that is easiest and has to be correct. If other areas are at fault they must be fixed then AF adjusted again. Its all kind of connected.
Remember that these engines were to be tuned in Germany without a Cat. so tuning pre Cat is best. If you call around to repair places ask if they have a Exhaust gas analyzer that they can use to help you fine tune your European muscle car. If you could find a person that would let you poke your head in to help it would be nice. You could try performance shops. You want someone who will do as you ask and not try to wring you for the diagnostic flat rate.

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Chris
84 280sl
82 300d euro