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MG MGB Technical - Flushing the fuel system...

Just awakening my '68 GT from its winter slumbers and it's managed to find another fiendish way to make me pull (what's left of) my hair out. It runs like a dream for 20mins or so and then starts to cough, seemingly running out of fuel, and eventually stop. If I wait 10-15mins it will fire up and all is well again (for another 20 mins.)

I've fitted a fuel filter/pressure regulator and cleaned out a little sediment from the float chambers, all to no effect.

I'm thinking of flushing the fuel system to eliminate the fuel lines etc.

My question is, what's the best way to do this?

The fuel pump chatters away quite merrily on start up so I think it's OK.........
Andy Hinson

Andy,

You say that the fuel pump chatters away on start up but was is it doing when the engine dies. You may have already considered this but it sounds as if your fuel pump is sticking in the traditional SU way of thing.

On start up an SU pump works away untill it has built up pressure and then stops. It pumps again when the presure in the fuel pipe between it and the float chamber drops either because the float chamber has accepted more fuel or because there has been a slight drain of pressure back to the fuel tank.

So if the ignition is on but the engine is off the pump will operate every now and again to restore the pressure in the pipe.

Next time your engine dies sit there for a few minutes with the ignition on. If the pump does not operate in that time at least once or twice then it is stuck.

Switching off and on will sometimes jolt it into action.
David Witham

David, Thanks. Yes it clicked a couple of times whilst I was waiting.

I'm now wondering if it's a blocked vent on the filler cap. It's happened before.........but not for quite a while. I'll try it next time it happens and hopefully get a whoosh when I undo the cap....
Andy Hinson

Andy,

Did the problem exist before you fitted the filter/regulator?

You could by pass your pressure regulator and filter to see what effect that has. These cars did not have them when new. Provided your tank is in good condition the filter has not got a great deal to do.

David
David Witham

David,
Yes. I thought fitting it may help. Still, it's a useful thing to have anyway...
Andy Hinson

Andy. A clear fuel filter will tell you if you are trapping sediment in the filter.

David DuBois has a tech article on the SU fuel pumps, including pressure and flow volume figures on my website. Look at the MG section of www.custompistols.com/ to see what you should be measuring.

If you have an SU fuel pump, a regulator is not needed, but does give you something else which can go wrong.

If you are seeing sediment in the fuel filter, or the flow/pressure readings are wrong, you will have to determine where the problem is. Sediment indicates getting bad gas/petrol or deposits in the fuel tank itself. Rust is generally the fuel tank itself.

Flow and pressure problems may be either the tank, pickup inside the tank, pump or lines. You need to check the system out, one piece at a time to see where the problem is and whether there may be more than one problem associated with your symptoms. Les
Les Bengtson

I've just fitted a new pointless SU pump which ticks away but only a lot of air and a few spits of fuel get as far as the carbs. I've been told that the pump may need priming - does anyone have any views / experience of this.

There is fuel in the tank!

Max
M Nunn

Have you you checked the float chambers for fuel after the engine dies? The fuel pump should not need priming.

Clifton
Clifton Gordon

Andy,

Sounds like a plugged vent to me. Try replacing the cap with a new one and see if that helps. Otherwise wait until it happens again and pull the cap off.


Max,

Sounds like you have an air leak on the suction side of the pump. Could be the fitting isn't tight, the hose is split, or there is a hole in the feed pipe somewhere.

Tom
Tom Sotomayor

Thanks - the rubber pipe on the tank side looks suspect but I didn't want to change it as its difficult to get at.
M Nunn

This thread was discussed between 10/05/2005 and 13/05/2005

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