Your symptoms sound just like the ones my '79 450SL was exhibiting, and it wasn't the fuel distributor. It was the Warm up Regulator, or WUR as some call it. It controls the pressure to the fuel distributor, and it was making a very rich idle and had poor mileage due to also running too rich at speed. I took my WUR apart and decided the spring inside had gotten weak, so gave the adjusting screw on it a turn or two, and all the problems disappeared. I notice that Unwired Tools, or whatever their name is sells an adjustable WUR that sounds like the best fix for the problem. I think you should check the pressure on the line between the WUR and the fuel distributor - it will tell you whether to suspect the WUR or the distributor.

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Richard Wooldridge
'01 ML320
'82 300D 4.3L V6/T700R4 conversion
'82 380SL, '86 560SL engine/trans. installed
'79 450SL, digital servo update
'75 280C