>I'm just thinking that if the engine stops, and for some reason I don't turn
>the key off right away, the electric pump is going to build up pressure and
>blow something out. Or am I just being a bit over protective?
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No, and it would be a really good idea to install the pump in this way,
which is how fuel injection systems are set up. It isn't because of a
pressure buildup though, as most pumps, especially ones for carburetted
systems, have a built in limit of around 2 to 5 psi anyway. In fuel
injected engines the pressure can be substantially higher than a
carburetted system. The reason it's done in injected engines is because in
the event of an accident, or if the fuel line for some reason ruptured,
fuel would be pumped out at a very high rate. With a mechanical system
when the engine stops, the fuel pump stops. If you were to wire an
electric pump in series with an oil pressure switch, not one used for
gauges but one specifically designed for such a purpose I might add, and
the motor were to stop running, the oil pressure then falls below a preset
level in the switch (typically around 15 psi), the switch opens and the
pump shuts off to prevent fuel from spewing out all over the place. In the
case of injected engines there is a timed routine in the computer that
allows the pump to run at startup, when there is no oil pressure, to make
sure that everything is primed with fuel, at the proper pressure, and ready
to go. The problem is that carb systems don't rely on computers to operate
and as such don't have a "circuit" to by pass the oil pressure switch. If
the car sits for any length of time, then fuel WILL evaporate out of the
float bowl, and with no pump to replenish it upon startup you won't be able
to get the engine started, let alone running. What you could do to avoid
this problem would be to connect a relay that supplies +12 volts to the
pump when you hit the START circuit (when the starter is cranking), not
just during "run", bypassing the oil pressure switch. That way at least,
when you start cranking, the pump will fill the float bowls allowing you to
start the car. Myself, with my car as it is now with the carb, I just have
the fuel pump connected when I turn on the ignition - I should take my own
advice :-), and will when I switch to fuel injection in the very near
future!
Barry Schwartz (San Diego) bschwart@pacbell.net
72 V6 Spitfire (daily driver)
70 GT6+ (when I don't drive the Spit)
70 Spitfire (long term project)
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