Thanks for those tips. I ran the car again, then peeked at the throttle plates. Both appeared to be wet. I pulled the fuel line out of the bottom of the rear carb jet (not the float bowl connect), and fuel flowed freely, so I surmised that unless there was a clog in the fuel line to jet connection, that wasn't the issue.

Then I noticed something peculiar. With the air filters off, peeking inside each carb throat, I noticed that even though both jets were tuned 12 flats down, the rear carb's jet (from inside the throat), appeared to be much higher than the front, almost totally leaned out.

To test it, instead of tuning each jet height by nut turns, I tuned them visually from the jet height in the throat. I left the front carb at 12 turns down, and to get the rear carb's jet to the same visual height as the front, I had to turn it down 29 turns!

As soon as I started the car with that configuration, it ran super smooth. It's clear I was running way to lean on the rear. And before all this stuff, when I tuned just by the nuts alone, I had to tune them both down about 24 turns, basically running the rear carb about right, and running the front carb super rich. That would explain my fuel economy.

So the million dollar question. Why, when I lower both jets 12 flats, do they look exactly the same height on the outside, but on the inside, its clear the rear one is much higher, and in fact needs more than twice the number of flat turns to get to the same honest height as the front one. Do I have different jets? Is something sticking?