"Vapor Lock" is indeed extremely rare. Another possibility is deterioriation in the short rubber section of fuel line on the inlet side of fuel pump, case distortion as described above, leaky or cracked diaphragm in the fuel pump, or a worn fuel pump pushrod. Putting an in-line "primer pump" only covers up your real problem. Check for vacuum leaks around the various fittings on the intake manifold, they can cause problems with fuel delivery too. (I found a leak in my windshield wiper motor vacuum hose that when fixed really made a difference at idle and low RPM, especially hot)
Rare in some cars perhaps, but not at all uncommon with any Ford, Mercury or Lincoln flathead V8/V12, due to the placement of the fuel pump on top of the engine, all the way to the back, just in front of the firewall, which is the hottest area of any engine compartment. This meant that gasoline had to be sucked upward almost 18" from the horizontal run of the fuel line, the fuel line necessarily feeling the heat output of the exhaust system as well. That, coupled with the ordinary hot-running characteristics of these engines, made vapor lock a regular operating feature.
When Ford Motor Company finally moved the fuel pump to a more conventional location on the lower front right side of the crankcase, the problem pretty much went away--however, vapor lock was always a possibility with mechanical diaphragm fuel pumps that were called upon to draw the gasoline several feet from the tank, particularly in very hot weather.
Art Anderson
June 12th, 2006
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