I have personally had this experience, back when I was a young man, in the early/mid 70s .
I was called in to help with a 318 that would only barely idle with the 2bbl WFO, and the ignition very advanced. But in fact, changing the timing while the engine was ticking over, had very little effect, except the back-firing stopped. It didn't take long to figure it out, and when the front cover came off, there were just stumps remaining on the cam-gear, as the nylon was mostly gone. As I recall, the timing chain had jumped several teeth.
The point is this; it only takes a jump of one or two teeth to create chaos. If I could remember how many teeth are on that gear, then I could tell you how many degrees each tooth is, but, I cannot.
