I run an electric fuel pump (on a 51 3100). I have an oil pressure switch that closes a set of contacts when oil pressure is present. These contacts feed a relay that runs the fuel pump so that when the ignition switch is on and the engine is not running, the fuel pump is not operating. I have a small 'momentary on' switch hidden under the dash to run the relay to prime the carburetor if the pickup sits more that a couple days.
The oil pressure switch I used is found on irrigation engines and is used to turn the alternator on when the engine is running and shut it off (so it won't drain the battery) when the engine is shut down for any reason. You can see the switch @
http://www.fwmurphy.com/products/pressure_vacuum/psb.htm I use a Bosch or Hella relay. Terminal 30 on the relay is fed from a 15 amp circuit breaker. Terminal 87 goes to the fuel pump. Terminal 86 on the relay is connected to the common terminal on the oil pressure switch. Terminal 85 on the relay is grounded. The normally OPEN terminal on the switch is fed from the same 12 volt source as the #30 terminal on the relay. When oil pressure is present current will go through the switch and energize the relay, which turns on the pump. The momentary on switch I mentioned earlier is connected to the nomally CLOSED terminal on the pressure switch. The momentary switch is fed from the ignition switch.
To prime the system the ignition switch must be on and the momentary switch must be pressed for the fuel pump to operate when the engine is not running. When the engine is running (and has oil pressure) the momentary switch no longer has any effect and the fuel pump will run until the engine is shut off or loses oil pressure.
