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MG MGB Technical - New fuel pump woes

Hello all,

Ive just had the BGT resprayed and it's been sat still for 4 months with the engine started a few times a week to keep it going. 1 month in and the fuel pump packed up. I bought a red-top Pacet performance pump and a malpassi filter king fuel pressure regulator.

A mate at work kindly fitted the new items but ran into a few problems. The original fuel pump seemed to be built in to the fuel tank so he couldn't remove it. It opened and closed it's points (i think) but didn't pump anything. He then fitted the regulator in the engine bay and then the fuel pump inbetween that and the carbs.

When i went to see it a while back it fired straight away and the run-on problem was cured as the two pumps stopped the flow of fuel dead when turned off. The pressure was set to 3psi and it seemed to run excellent.

When i picked the car up today it idled like a dream and revved well when stood still. When accelerating on the go it coughed and spluttered and almost felt like fuel wasn't quite getting to the carbs quick enough.

Is it because it's stood still for so long or is there a problem with the fuelling?

Will post picks of the new paint later :)

Thanks in advance and sorry for the essay!!!
K G Briscoe

Try 4 psi
Dan Robinson

With the pump mounted up front you have a long suction line to the tank, a small air leak at any joint or in the original pump could cause fuel starvation. The original pump is not mounted in the tank. On CB cars it is mounted next to the right hand battery box, on RB cars it is in the trunck.
John H

Hi Dan,

Will 4psi be ok for my SU's? i thought it had to be 2.5 - 3psi or it may damage the jets?

John, the car is a 1975 UK rubber bumper GT but the mate who looked at it said he's never seen a pump mounted in the tank before so maybe a PO had modded it? The lines were fine before hand so i guess it maybe either a problem with the original pump in situ or the new pump setup?

A few other people said it may just need a good run as it's been sat for a while??
K G Briscoe

KG - The item that is monted inthe tank is the fuel level sender and the pickup tube. The original pump will be mounted inthe forward bulkhead of the boot, pretty much centered. the end of the pump protrudes into the boot and is protected by a rectangular cover. 4 psi of pressure won't hurt anything in the carbs, but I don't think that the pressure is the problem, the carbs are designed to run with 2.7 psi of fuel purssure. Have you checked that the dash pots in the carbs have oil in them? Remove the screw caps fromthe top of the vacum chamber of the carbs and check that there is oil in the center tube. Check also that the proper damper/cap is being used in the carbs. some of the caps have a small vent hole in them and are used on the carbs that do not have self vented vacuum chambers. the self vented vacuum chamber will have, what looks like, a thick web angled down from near the top of the vacuum chamber (just under the cap) to the bell of the vacuum chamber. I believe that the carbs for the 75 should be the self venting type of vacuum chamber, which wouldn't use the vented cap, but ther is a possibility that the carbs may have been swapped for the type that needs the vented cap. Good luck - Dave
David DuBois

It may have been set up for fuel injection at some stage. You mention fuel lines plural which makes it possible, but very unlikly. An in tank pump produces much to high a pressure.
If you do infact have a pump in the tank you can recognise it by it having an access hatch in the top of the tank acessable from floor of the boot, leading down into the tank. Then there will be both a positive and a negative wire going into the tank at that point. It will be a circular hatch about saucer size with a ring of screws/bolts around the edge. There will also be the 2 fuel lines. This is in addition to the single wire on the fuel gauge sender that is on the right hand side of the tank.

If you don't have these things, you don't have an intank pump. This is not to be confused with, as David says, your standard 1975 MGB that has the SU mounted ABOVE and forward of the tank right hand side. It doesn't go down into the tank at all but it does stick back into the boot in an inconvenient way. You can get at it by taking off the rear right hand wheel and it will be right in front of you, above and forward of the tank. Since you have a new pump I suggest that you disconnect the old one. The two pumps fighting each other might be the cause of your woes. Get a piece of fuel line from an auto shop (about a meter should be heaps) and bypass the old pump. Go straight from fuel tank to fuel line. Disconnect the wire to the old pump as well. I'd suggest plugging up the fuel line ends on the old SU pump and leaving it there, you or someone may want to go for concourse with your car someday.
Use a few cable ties to secure the new bit of fuel line.
I have a '75 and prior to fitting the V8 did just that and subsituted a pump from a carby (not fuel injected) subaru wagon, which I put up in the engine compartment. Worked very well. Obviously when you pull the SU fuel line off the tank inlet you need to have the new bypass ready to go on, or some sort of plug.
Peter

A UK 75 shouldn't have the pickup pipe in the sender, that came later in the UK at least. Whilst the electrics end of the pump on a rubber bumper car sticks into the boot the petrol end is on the other side of that panel (pretty-well on the RHS of the car on mine). It's also not a good idea to have two pumps in series especially if the original pump isn't pumping properly, it could well restrict the efforts of the other pump, it should be removed from the circuit. There are always a lot of problems reported with after-market pumps, much better to stick to the original SU type, for both pressure and connection reasons. A standard pump has more than enough capacity to supply the V8, let alone the 4-cylinder.
Paul Hunt 2

Thanks everyone - from all the comments it seems to point toward the fact the old pump maybe causing problems. When i pick it up from the paint shop i'll have a good long look at it and this mystery old pump!
K G Briscoe

Hello,

You mention that;-

"He then fitted the regulator in the engine bay and then the fuel pump inbetween that and the carbs."

The best position for the Fuel Pump is at the rear, close to the tank, that way the fuel in the fuel line to the engine bay is under higher pressure and less likely to boil causing 'vapour lock' than if the fuel is 'sucked' at a lower pressure, which would further lower the boiling point of the fuel.

Anyway, with the pump at the rear, the regulator should then be fitted close to the carb (anti-vibration mounted in the engine bay) and set to 2.5psi to 3.5 psi depending in the tune of your engine. I would suggest that if you have fitted the pump "between the regulator and the carb" this will not work!

Regards, Mike.
MG Mike

Hi Mike.

Just picked her up and i think i got myself confused!! The pump is mounted in the engine bay as my mate said there was nowhere else to put it at the back (will investigate later). The malpassi is then inbetween the new pump and the carbs.

As i was driving her back i noticed she was getting better the faster i drove her. A good bout at 70 for 20 miles and she was behaving a lot better.

This leads me to believe it's something to do with the dashpot oil - maybe one of the carbs is a little low. Now she's safely tucked up for the winter i'll strip them down and geive them an overhaul.

Thanks everyone - and i'll definitely have to sort the old pump out before it hits the track again!

K G Briscoe

Dashpot oil only affects acceleration, not ultimate speed. Low oil causes stumbling with significant increases in throttle, you should still be able to reach normal high speeds (eventually) with gradual acceleration. It also won't get 'better' as you go along, that does sound more like fuel or ignition. To check the dashpot oil level unscrew the plastic damper cap, lift it up 1/2" or more, then press it down again, and if you feel the resistance of the oil before the plastci cap reaches the piston cover you have enough. How much before tells you how much of a 'reserve' you have.
Paul Hunt 2

The MGB engine is a notoriously dirty running motor and the fact that you have been starting it up and running the motor without driving the car for several weeks has resulted in you fouling the spark plugs. Taking the car out on the road and running it at a steady pace to clean the plugs will resolve the problem as you have almost discovered
Iain MacKintosh

Hello K G Briscoe are you related to Ivan Briscoe of Stoke on Trent?

Sorry everyone.

John Dale
John C. Dale

Paul - The stalling on acceleration was getting better and better the more i drove it but it's still not running at it's best.

Iain - Thanks for that - hope that solves the problem though a good run out this weekend if the weather is good should help.

John - not too many of us Briscoe's around but you see a few around as it originates from France i'm told - the Briscoes cam over with Norman the Conqueror but i don't have any family in Stoke.

The whole fuel pump thing has me scratching my head as it was a complete rebuild a year ago, entire new ignition and good tune up from Pete Burgess so it's practically brand new! The old fuel pump was the only weak link but now it's replaced it runs worse. Thanks for everyones help - i'm sure a bit of TLC when i get some time and she'll be purring like a kitten again!!!
K G Briscoe

This thread was discussed between 17/11/2006 and 22/11/2006

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