1973 318 - daily driver for years - now won’t start

TrailBeast said

Believe it or not timing chains usually jump when the engine is shut off.
I don't think I have ever seen one jump while running.

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Imo I think they jump during shutdown. Then pistons and crank stop receiving power and start to decline quickly and cam is still turning the two are connected via the timing chain inertia takes over and bam.