Turns out the third spark plug was not fully installed and seated. I discovered this issue while viewing my video clips. It’s difficult to see with your eyes alone if the plugs are fully seated, even using a flashlight. And since the old plugs were cross-threaded, or perhaps the threading in the head was not done properly when the engine was rebuilt, it was not possible to tell based on increased resistance when the plugs were fully screwed in. Normally, you should be able to screw them in by hand or with a little coaxing using a spark plug wrench, then you torque them down to specs. This was not possible to do.

I expect to get criticized for using an open end wrench instead of a spark plug socket wrench. I have several, but they were not large enough for this spark plug. And I was trying to get them changed in time to go to the Model A cruise.