How does an external fuel pump work

[killerpirate]

Hello, im looking to buy an merc xr2, and it has an external fuel pump. I was wondering, how does this work.
I assume you have a switch in your dashboard to turn it on, and then you start your engine, and then leave the fuelpump on?
Does it regulate the fuel? What if the engine is off but the fuel pump is on? Will it just build pressure? And what about different RPMs? The engine needs more fuel at high rpm then at low rpm.
How does the external fuel pump does this?

[David - WI]

Mine had an electric pump that came on with the key "on" and a regulator... both near the floor behind the fuel tank. The pump can supply more fuel than the motor can use, so it maintains the pressure set by the regulator regardless of rpm or load.

[killerpirate]

So does the fuel pump turn off when the max pressure is reached? What happens if i have te fuel pump on and the engine off?

[rock]

Feed the pump with the key on wire. Use a relay and trip it with the purple wire.

Rock

[David - WI]

Pump runs continuously. If the pump is on and the motor is off... it will run until it kills the battery and/or flood the carburated motor if the needles & seats leak at all.

[killerpirate]

Pump runs continuously. If the pump is on and the motor is off... it will run until it kills the battery and/or flood the carburated motor if the needles & seats leak at all.

So it just keeps pumping and doesnt stop when max fuel pressure is reached. Isnt that bad for the pump?

[David - WI]

They are not positive displacement pumps I think max pressure is like 7 or 10 psi, they just kind of cavatate and stop moving fuel (more like a leaf blower than an air compressor)... but why do you want to leave the pump running when he engine is off????

[roadkill636]

It all depends on if your mota is carburetor or fuel injection. Your is probly carburetor and will need a low pressure pump. Such as a Holley RED.. Red is made for carbs and put out about 4lbs. No return line needed.

[killerpirate]

Thanks! Because i wont switch it with ignition key but ecternal switch. So i might forget to turn it off

[racervboat]

Turn it off with the key,im sure you won't forget to turn the motor off. like you would turn off your car it has an electric fuel pump.

[David - WI]

Thanks! Because i wont switch it with ignition key but ecternal switch. So i might forget to turn it off

Only if you're completely deaf. Mine was about 3 times as loud as the bilge pump, and I can hear the bilge pump when the engine is not running.

[rock]

Thanks! Because i wont switch it with ignition key but ecternal switch.

That is a recipe for disaster.

Rock