An excess rich mixture is not good.......the worst scenario would be excess gas washing the piston walls. Over time this will shorten the life of the piston rings.

The best (and only) proper way of setting the mixture screw is with the use of a CO/HC gas analyzer. This is done with the O2 sensor disconnected.......to get the proper baseline setting.

If yours is a KE-Jetronic CIS system, then the idle is adjusted using the air-bypass screw........a large knurled screw with an Allen head on it. It is found on the throttle body.

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1997 SL 320
1988 Ferrari Testarossa
1983 Ferrari BBi Boxer