"Something has changed"
Yes! Gasoline. Since the advent of electronic fuel injection, the refiners don't need to worry about vapor lock. It's an index or specification called Reid Vapor Pressure. Modern fuel tanks are completely sealed, lines, everything. Carburetors were generally open to the atmosphere.
Fuel injection cured vapor lock. Since the fuel is under such high pressure 15 to 50 lbs., from the pump inside the tank to the injector it cannot turn to a vapor. Pressure raises the boiling point, that is why we use pressurized cooling systems. Also, with the pump mounted inside a tank of fuel it helps keep the pump cool.
With a mechanical fuel pump, the inlet side is pulling a vacuum on the fuel which lowers its boiling point inside the fuel lines which can cause the fuel to turn to a vapor (vapor lock) Especially on a hot day using low octane fuel, fuel lines routed close to the exhaust and even high altitudes.
On the outlet side of the pump between the pump and the carb the fuel is under pressure which raises its boiling point which helps prevent vapor lock with the high under hood temperatures. With a flathead fuel pump having such a low output pressure vapor lock might have been a more common problem than it was in later years when carbureted vehicles fuel pumps had an output pressure around 5 to 6 lbs. It might have also been common in the 1920-1940s when fuel was engineered for low compression engines.
I'll give you an example of what just one less pound of fuel pressure did. Where I worked at, the phone company, our fleet had about a dozen 1984, carbureted, Chevy Suburbans that were quitting up in the mountains (5000' to 8000' elevation). It always happened on very hot days. We would bring them down off the mountain to work on them and we could never duplicate the problem at 1000' elevation. The problem turned out to be the mechanical fuel pumps, as they got older their pressure output reduced from 5 to 6 pounds down to 4 pounds. They worked fine down in the valley with 4 lbs of pressure. For every 1000' of elevation you go up the atmospheric pressure drops one-pound so the fuel will vaporize at a lower temperature up in the mountains. With only four pounds of pressure on the fuel between the pump and the carb, and on hot days, with hot under hood temperatures the fuel must have been turning to vapor between the pump and the carb. The 4-pound pressure reading was taken inside the shop at the 1000' elevation, I imagine if I had checked it at a 5000' elevation, I would have gotten a lower pressure reading. The drivers figured out that when quit running to pour water on the fuel pump and fuel line and then they could on their way again. Pulling the fuel line and testing to see if the pump was squirting fuel would have lead you to believe the pump was fine when it wasn't.
Hardly anybody ever checks the vacuum and pressure on a mechanical fuel pump. They pull the hose and make sure its pumping, and that's usually a good enough test but when your having an intermittent problem kicking your butt its time to get out the testers and make sure your parts are within specifications.
