Ok, now I understand.

I guess I just thought that when the car was idling and the throttle plate was closed, the only air to the engine would be coming through the small venturi hole. This made me think that possibly the additional air coming from the vacuum pump would be enough to change the mixture in a meaningful way. Guess not.

The line from the vacuum pump to the intake is quite clean but the line to the brake booster is pretty dirty. Regardless of the diaphragm's condition when I finally get it open, as you said, it's probably original and should be replaced anyway.

So now I'm kinda back to square one. For those keeping score, the car I'm working on is a recently acquired 1972 220D.

Within the last 2 weeks I have replaced both fuel filters, installed 4 fuel injectors rebuilt by Greazzer, installed new pencil style glow plugs, adjusted the valves, replaced the injection pump regulator diaphragm, and renewed the regulator poppet cam seals.

The car still smokes like crazy and I'm running out of things to try. Next up is trying to check the injection pump timing.

Pictured below is the car in question.