Do not remove the air filter permanently. It is just a test to see if there is a restriction in it.
EGR is an exhaust gas recirculation valve that allows some exhaust to re-enter the intake manifold. I'm almost certain you do not have one on a 76.

One clarification, if you're putting gas in the vehicle then we've found the source of your problem right there.

Sudden onset of black smoke is a certain indicator of a radical change in the air to fuel ratio. Either fuel or air delivery to your engine has changed. Did you leave a rag in the engine comparment that got sucked into the air cleaner? Did a squirrel build a nest in your air inlet.

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1977 300d 70k--sold 08
1985 300TD 185k+
1984 307d 126k--sold 8/03
1985 409d 65k--sold 06
1984 300SD 315k--daughter's car
1979 300SD 122k--sold 2/11
1999 Fuso FG Expedition Camper
1993 GMC Sierra 6.5 TD 4x4
1982 Bluebird Wanderlodge CAT 3208--Sold 2/13