I wrote this article after encountering vapor lock issues after reinstalling the mechanical fuel pump when I did the HEI Ignition Upgrade of my 1965 Barracuda. With this car having a slant six engine, the fuel pump is located on the opposite side of the engine as the exhaust system so there is no heat gain from a hot exhaust manifold or exhaust pipe.
I had had enough of the electric fuel pump that I installed in 1988. It was noisy at idle and put an unnecessary 5 amp load on my [30-35 amp?] alternator, which was especially noticeable at night while stopped at a traffic light. While the P4070 was my only fuel pump, I never experienced any vapor lock issues.
Since the Carter P4070 electric fuel pump was already in place, I decided to keep it to prevent any hot-start issues by running it only during starting. This required keeping the start and run circuits separate and using a relay operated by the start signal. However, with the P4070 being a rotary vane pump, there was too much of a restriction for the mechanical fuel pump, which caused fuel starvation in the carburetor.
Because of the fuel starvation problem, I removed the electric fuel pump as I didn't recall the car ever really having any hot start issues a few decades ago. After then filling up the tank for the first time in 2018, I was dismayed that the car stalled shortly after pulling out of the gas station. It would not restart, even after being towed home. My first thought was that my newly rebuilt fuel pump had failed.
I did get it to restart the next day and had an uneventful 30 minute drive around town but experienced some fuel starvation while accelerating onto a highway. However, after pulling back into the garage and idling in the garage, the car once again stalled and would not restart even after a few hours. It looks like the mechanical fuel pump gets vapor locked during a short hot-soak. I was planning to re-install my fuel pump with a check valve bypass so that the fuel pump would run at starting as originally planned. However, I can foresee hot-weather idling issues with the use of the mechanical fuel pump.
I'm considering to remove the mechanical fuel pump and reinstall the electric pump for the time-being. I would use the fuel pump relay to power the pump during ignition run, which would reduce the electrical load on the battery during starting.