If you have been pumping the hell out of the hand primer pump it may have an air leak.
all my cars i have the newer style primer pump. This may not be your problem i thought i would say something because i had a similer problem.

it defentley sems like you are only runing on a couple cilenders.

crack each injector line if the idel changes move on intill nothing changed there you should start looking for the issue.

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1976 240D
1976 240D parts car
1970 220D daily driver