Took a drop cord light and went over the engine. Sure enough, there was a vacuum connection off on the right side and next to the fuel distributor. Put it back on and the rough/missing idle smoothed out. However, rev the engine up and let off the accelerator and it nearly dies. Most of the time it catches before it actually dies, but occasionally dies. So, I'm going to put the first idle speed control unit back on and see if that helps. If not, I'll put the original Ignition Control Unit back on as well. Seem to be in a lot better shape at present than I was originally.

Appreciate the tip!

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Don
'85 500SL (Euro) - 186,000 w/a complete restoration and engine rebuild at 154,000
'95 C280 - 174,000