You guys are going to love this....Root cause analysis actually found something very interesting. I dodged a "bullet" by thousands of an inch on this one. I could so easily have been totally disabled on the side of the road, in the desert, during my 1500 miles journey here to Tucson....or during driving around for two weeks on these terribly rough bumpy roads.
The rotor is actually the root cause, from failure of one of the two cheesy little "plastic weld nipples" that hold the metal rotor contact arm onto the plastic rotor assembly. I found the broken little nipple inside the distributor. The symptoms had become progressively worse because the rotor arm's one remaining retention weld was working looser and looser every minute. The loose rotor arm was starting to gouge the cap and had damaged the #5 contact. If it had let go, instant motor off and no restart.
I find this alarming because, how the hell do you protect against that? Proactive replacement of cheap rotor before failure I suppose?
Crazy huh? Cheap plastic (OEM?) rotor failure cost me about a grand in parts and labor...(chasing plugs, wires, opti cap & rotor, I wont count water pump)
She's running great now though.

 

Interesting. If you had simply traded in that distributor for a new one without any inspection, some tech would have replaced the rotor, cleaned it up and put it back on the market ;-)

 

It's not all that uncommon. I've seen several instances of the rotor metal tab sticking out of the side of the opti cap. Their scenario was sudden shutdown at high RPM. Sudden opti rotor failure is real. Yours would have been there soon.

People seem to forget that the cap and rotor are wear and tear items, just as they were on any old SBC. They do last longer and are difficult to get to for replacement, but need maintenance just the same.

 

Hi Gary - yeah, except I always keep all my old parts removed, for salvage later
Hi Rob - True, it's easy to forget the that the hi tension path is totally old school and rotor & cap wear can still degrade or kill your spark quality

thanks again for all the help. I do like getting to exact root cause, rather than replacing parts and fixing the problem, but not having full understanding of the failure mode.