Thanks to the suggestion of tangofox007, who tested and adjusted my injectors, I pulled them all again and noticed that one injector has noticeably less soot than the other four. The cylinder corresponding to this injector is presumably not firing.

The pre-chamber looks in order, alike to the others.

I'm still waiting for my compression tester, but it seems obvious which cylinder is problematic. I'll triple check this valve and post back.

I also edited the link to the original thread in the first post.

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1984 300CD