I have an 88 300SE with the same engine and the black knob in the middle of my cleaner according to the mech is a fuel richment adjuster. Assuming were speaking of the same knob. The one I'm speaking of is actrually accessable with the cleaner in place. There's a "hole" in the cover that allows you to place a longish screwdriver with the cleaner in place. I don't believe this affects idle speed just fuel richness with no air adjustment. I'll keep an eye on this thread as I'd like to raise my idle a touch as well.

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Thanks,
Ron Brooks