To be honest, I happened.
It did not stop running is about all I'll say. Won't say in public what happened. Nobody would believe it. I don't even believe it. I only have negative suspicions about the actual cause.
What I did troubleshooting wise first was get over wanting to light it on fire and watch it burn. Sort of kidding about that of course.
In order of appearance here is the troubleshooting I've done so far:
Start draining the oil. Lots of bronze in it. You can guess what parts that came from.
Check timing: It was a degree or so retarded, so I added a degree or two.
I had added a set of .125" phenolic spacers in front of the intake manifolds during this rebuild that were close but not perfectly port matched. I started working on them to correct the openings in the troubleshooting phase, but decided to just put them away. The motor ran fine without them and was tuned previously without them. I thought maybe they were causing a low RPM issue, because fuel had to get over them to get into the combustion chamber with low vacuum off throttle. That was not the major issue, just a street tuning thing. Also drained the float bowls. Still using some oldish fuel, which might not be helpful. It's got Stabil in and octane boost in it. It usually holds up OK.
While checking the timing I also made sure the pistons still go up and down in the bores via the kick start. Also did the best I could with a flash light to check for scoring from piston wrist pin spiral clips coming loose and grinding ditches in the bores. All looked good of what I could see. Pistons also went up and down in the bores normally. Whatever normally means. Tops of the pistons still looked new, but 10 miles isn't much run time.
Removed used spin-on filter, filled a new filter with oil and installed it. Refilled the oil tank after I sucked all the oil goo out from below the oil screen line in the bottom sections of the oil tank.
Currently letting it wet sump a little. Might start it tomorrow.
If it still does what it did that made me say I broke it, I'll be taking it apart and digging into the bottom end.
There is one constant related to this rebuild and it is I. I might be suffering from early onset dementia.
Can one still type semi-coherent sentences with dementia? Then again I might be fooling myself about the sentences being coherent. lol