That chassis uses Vac Retard @ Idle. If you take the vac hose off and no change , then the suspect can be a stuck dist advance plate [ check the mounting screws from the Pert] or No vac at the hose, or a bad diaphragm at dist. The timing should advance as soon as you take the hose off the dist
diaphragm as long as you know you have vac at the hose. There are also Emmissions SOV valves for vac control, but check that the dist can actually move when is has/has not vac applied to it first. With vac applied , the timing should be ATDC [ Retarded].... w/o vac , it should instantly jump to BTDC [ advanced]

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A Dalton

I remember when I was first starting out as a mechanic's apprentice in about '77. We worked on lots of these cars, and vacuum diaphragms of that vintage (really any Bosch vacuum diaphragm of the time) were a common failure. Easy way to check it... stick a vacuum hose on it, and suck on the end. You should be able to move the arm with your own suction. If it doesn't move, the breaker plate may be seized (not too common, as I recall), or the diaphragm is bad. Frequently with a bad diaphragm you can still suck it hard enough to move it. If you plug the end of the hose with your finger or your tongue the capsule should hold it's drawn-in position essentially indefinitely. If the diaphragm is bad it will slowly release over a couple of seconds.

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1987 W201 190D

While you have the cap off, give the rotor a little twist. It should rotate in one direction a bit, and spring back to it's original position. It should not rotate in the opposite direction. If it doesn't want to turn at all, the centrifugal advance is stuck and the car will be a dog.

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1987 W201 190D