I solved my own problem this time, but here's what I had in case it helps someone.
On cold mornings:
- I start the engine as normal (pump the gas pedal once all the way to the floor, release pedal, turn key).
- The engine starts immediately.
- After about one second the engine stalls.
- It restarts on the first turn of the key, without another pedal pump.
- It runs smoothly.
- After 10-15 seconds I put it in gear.
- After being in gear for a second or two, it stalls again.
- I shift into neutral or park and restart (again without pumping the pedal).
- After a few more seconds I shift into gear again and drive off with no more problems.
Turned out the choke pull-off was pulling the choke open way more than it should. (My spec is 3/32" and it was closer to 3/16".) I doubted the choke pull-off would be the problem because I had set it myself last year, but it was definitely out of spec. I guess that's why you do regular tuneups on old cars.
I'm still not sure why the bad adjustment caused the symptoms it did, but I'm no longer embarrassed to take passengers in the car.
