"Phil's point about the check valve is interesting."

Yes. I was going to remark about this, but at the time felt I'd cluttered this thread up enough.

As a veteran of several fuel pump check valve failures, I can relate some experience. These failures I experienced all happened in the neighborhood of 20 years ago, and after a couple necropsies, were due to the spring rusting and breaking in two. This was during the time when our gasoline supply was not augmented (or contaminated) by ethanol, but MTBE was used as an oxygenator to our fuel here in Maryland.

The symptom of a failed check valve, if I remember, was slightly longer crank times, as the main pump needed to fill empty lines and not just the space behind the regulator's diaphragm.

Since ethanol replaced MTBE, any condensation moisture seems to be spread throughout the fuel mix instead of dropping out in the low areas of the fuel plumbing, or perhaps it is just our habit of keeping a full tank as much as possible compared with the previous owners of our cars. My spare check valves go unused.

But the reason I wanted to remark, was Phil's suggestion to test one by blowing. I can assure you that spring is more powerful than my cheeks. I am not able to blow through the new unused fuel check valve in either direction. Nothing like testing a check valve used in the vacuum lines.

"The odd thing about this is that the engine coughs once in the morning and then goes back to sleep, but runs fine after lunch. As a morning person, I find that quite disturbing."

Funny. I'm also a morning person, and not everyone in my house appreciates that. I've learned to make the most of my nose troubleshooting fuel issues, and this symptom, combined with an unburned fuel odor reminds me of leaky injectors or regulator diaphragm flooding the engine. I'd hold the pedal down to keep the throttle wide open to see if it would burn off, but I think OP has done this. One of the symptoms of covid (earlier variants I think) is loss of smell. I'd hate to lose that sense driving old cars as I do.

EDIT: By the way, Peter, I meant to thank you for your anti-seize recommendation in another thread. I have it on my list to find the Permatex product but so far the only place I ever visit for automotive supplies, Walmart, does not carry it. I will soon brave the NAPA nearby now that I know where they moved to. So many times I've gone in a local auto parts store and walked out empty handed, I just never bother anymore.

--
Art Benstein near Baltimore

"If you can't fix it with a hammer, you have an electrical problem." -vwbusman66