Thanks for the inputs!

I think I found the source of the hard starts: the fuel return valve on the carburetor was leaking: it was a little "damp" under the valve, suggesting fuel weeping. I replaced the sealing washers with new copper; since then it starts much better.

I assume the hard starting was the result of one bowl being empty, the other full. Perhaps the car was able to start but ran roughly until both bowls were full.

(I have not had time to look at timing, yet. The points look very good and the distributor inside is clean, but I have not put a dwell meter to the motor.)