What exactly did you "turn down"? Hopefully not the governor adjustments internally. These massive pumps should have an external ALDA of some sort which you can adjust if necessary. You are running an upgraded huge turbo (hopefully with an intercooler) to supply enough air for that pump right? Black smoke means you need more air.

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Current stable:
1995 E320 149K (Nancy)
1983 500SL 120K (SLoL)

Black Sheep:
1985 524TD 167K (TotalDumpster™)

Gone but not forgotten:
1986 300SDL (RIP)
1991 350SD
1991 560SEL
1990 560SEL
1986 500SEL Euro (Rusted to nothing at 47K!)