Vacuum leaks are a priority. Are you sure its rich. I would go over all of the obvious items first. The basics like air filter,timing, spark plugs/wires, cap and rotor. Then start looking at your warm up regulator.
I think in the tech help section of this forum you will have a lot of responses from others who are knowledgable. Also the Sl forum as well may be of help since it is a European edition vehicle. If you search Warm up regulator in the SL or Tech Help area you will find a lot.
Where are you. Do you have smog inspections in your state?

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Chris
84 280sl
82 300d euro