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MG MGB Technical - Fuel Pump Location - 72B

YES! This is another one of my long inquiries!!!

My 72B came with an aftermarket fuel pump (the small brass box style) mounted on the shelf between the heater box & the carbon canister.

I've installed an inline fuel filter before the inlet side of the pump.

The original pump is not on the vehicle, at least I've never found it.

Having had previous cars with electic fuel pumps on carburetor engines, I was always use to the pump eventually shutting off, prior to startup. Like when it primed or built up adequate fuel pressure. Or at least it would really slow down.

My fuel pump does not shut off or really slow down.

If I turn on ignition & not start engine, the pump just keeps a pumping - it may slow down slightly but it won't reach a point of adequate pressure to shut pump off (with engine not running).

I have also observed on my previous clear fuel filter, that at idle the fuel filter would occassionally almost drain of gas, but the pump was slowly adding fuel to filter, and engine never hesitated.

Some say my present set up is allegedly making the pump suck fuel too far from tank. The pump would theoretically work better if it pumped the long distance instead of sucked from tank to engine compartment.

I've seen recommendations to change two things, based on the theory that fuel pumps are just that "PUMPS"

1. Move brass fuel pump from engine compartment back where original pump was by the battery box.

2. Leave inline fuel filter underhood, but have it after fuel pump and before carbs, not before fuel pump.

Your thoughts please & many thanks as always.

BobA


R.W Anderson

Some after-market pumps chatter or whirr continuously, maybe getting a bit slower or quieter once the float chamber are full. SU pumps chatter till the float chambers are full, then stop, only occasionally clicking as fuel is used or pressure seeps away.

The original pump would have been mounted in front of the right-hand battery box, visible from underneath or past the right rear wheel.

This style of SU isn't suited to being in the front of the car as they are pushers and not pullers. Other models *are* desiogned to be pullers and were mounted in the engine compartment of square-riggers, for example. Without knowing what your after-market pump is, it is impossible to say whether it is designed to be at the front or the back, maybe someone else will recognise the description.

Whether a fuel filter is full, empty, or somewhere in between seems to be random and variable, even on the same car. As long as the car is running as it should ignore it.

Fuel filters on MGBs are always between pump and carbs from the factory, but that is beacuse of the relative locations of each. SU pumps include one or two filters, whether yours does or not is again an unknown quantity.

Ultimately, if the car is running well then leave things alone - 'IIABDFI'. If you are having running problems, and it proves to be fuel delivery, than I'd take the opportunity to go back to the proper pump in the original location, leaving the filter in the engine compartment.
Paul Hunt 2

Les Bengston's web site has a lot of useful information, in this case, read the articles by Dave DuBois. At least you'll know more about how SU pumps are supposed to work, and then you can compare the performance of your little brass pump to them.

http://www.custompistols.com/cars/articles/index.htm

As a rule, SU fuel pumps hate restrictions prior to the pump, which cause them to pump until they croak, and don't mind restrictions after the pump (hence the fuel filters are after the pump and before the carbs.) I'm not sure what your little brass pump needs.

If you end up with fuel delivery problems, I would talk to Lew Palmer who is not far from you, or write Dave DuBois, and go back to the original style pump.

good luck,
dave
Dave Braun

The in-line Facet/Walbro/ million other names pumps have an internal bypass that makes them click continually; SU/Autopulse/million etc do not. The difference is largely that the continuous run ones are electronic, while the shut-off ones have points; but, some newer shut-off types are electronic also. There are also rotary motor driven pumps that run continuously, mostly high output items.

The electronic in-lines usually specify filters before the pump, but don't ever put a filter before the pump on the clickety shut-off points types.

No pump will ever pull fuel as well as it will push it - that's physics, not pump design. True vapor lock is a result of the fuel on the inlet side vaporizing as the pump drops the pressure on the suction line - it's a PITA, and easily avoided by putting the pump as low and as close to the fuel source as possible. And, for all pumps, it is desirable-to-essential to have the outlet above the inlet, to avoid trapping vapor in the pump chamber.

FRM
FR Millmore

"pusher" vs "puller" fuel pumps is a misnomer as Fletcher states above. Any SU fuel pump, regardles of where is it intended to be mounted will generate 4 to 7 inHg of vacuum, sufficient to lift fuel a considerable distance. The low pressure SU fuel pumps, which are intended to be mounted in the engine compartment (and thus have been called "puller" pumps are actually less able to lift fuel as high as the later, high pressure ("pusher") pumps. Fletcher states "True vapor lock is a result of the fuel on the inlet side vaporizing as the pump drops the pressure on the suction line (inlet side of the pump)", which is the reason that MG went to a pump mounted in the back of the car by the fuel tank instead of in the engine compartment as on the T series and earlier cars.

SU pumps are able to pass most debris right through them without causing any problems. The filters in them are designed to stop rocks and small birds. Placing a high efficency filter on the inlet side of a SU pump is somethng to be avoided because if the filter clogs to the point where it will not allow fuel to pass through it, the pump will stall in a current on condition. This situation, if the power is left on for any period of time (such as while troubleshooting) will cause the coil to overheat and will burn out the internal swamping resistor. Unfortunately, since the resistor is internal to the coil housing, the damage goes unnoticed but the result is that the points will start arcing excessively and burn out prematurely (because this internal swamping resistor is part of the arc suppression circuit). worse still, if the burned points are replaced, they too will burn out very rapidly, leaving another MG driver cursing the SU fuel pumps as worthless pieces of _____ (you fill in the blank).

Both Paul and Fletcher are correct about most after market pumps running continuously due to their having a bypass valve in them (this is how they regulate the outlet pressure). These pumps don't care if the fuel flow is interupted on either the inlet or outlet side, they just continue to chatter away happily regardless. All SU fuel pumps, points style or all electronic will shut off, once the float bowls are full and then only tick periodically as the fuel in the bowls is used up. Cheers - Dave
David DuBois

This thread was discussed on 02/07/2007

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