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MG Midget and Sprite Technical - Air bubble rushing through the fuel filter

This is quite sudden. We were miles from home when she began spluttering, and a lot of air was going through the filter. I assumed I had a leak in the fuel line somewhere.
Found a friendly garage with a four-post ramp, and he had a good look from tank to pump - nothing obvious. But the rear float chamber overflow was running, which he fixed. (big chunk of crud).
After that, and after the tank had been filled, she ran okay, but now - a few days later, the same thing is happening, rather worse than before. We are 450 miles from home.

Can mechanical fuel pumps let in air internally?
Nick and Cherry Scoop

I suspect you're a victim of ethanol fuel. Depending on what type of mechanical pump your car uses.
If it's an AC Delco type Y pump (very common indeed even today) most variants have the two halves of the pump held together with five screws.
They are susceptible to decay of the diaphragm from ethanol fuel and the diaphragm is also the seal between the upper and lower halves of the pump. Air ingress is quite possible.
This might also be consistent with the lump of crud you found.
On a happier note there is a rebuild kit available for the pump and it's a super-easy task.
If it's crimped together (as some are) it's not rebuildable but complete pumps are inexpensive, although you have to be selective about things like the length of the operating lever.
HTH.
Greybeard

Thank you, Greybeard. I'll probably treat her to a new pump when we get home. Though the pump seems to be strong - enough to run the engine while pumping fuel out through the overflow at a good rate a few days ago.
Nick and Cherry Scoop

Might be worth smelling the oil as some pumps can let fuel into the sump when the diaphragm fails. Also what was the fuel level when it occurred, might be the pick-up in the tank if it starts when the fuel is around half a tank or below.
David Billington

Hi Nick. I know you said the garage had checked, but rubber fuel pipes between tank and pump can develop splits. Hard to see and may not let fuel drip out but when the pump is working it can draw in air.

I have also had a pump where the diaphragm developed a split. Pump still operated and ran ok at a low cruising speed but wouldnt pull as soon as we came to a hill. Forced a roadside improvisation repair.

Are you in Scotland. Or maybe France? Not that i think it makes any difference to a fuel problem!
GuyW

Sorry to be so long, Guy. We've been on our way home from Northumberland.
All well until about Hadrian's Wall, where we stopped for coffee. That rear float chamber overflow was at it again, so I fixed it while Laura ordered drinks. Seemingly okay, but after we reached our overnight stop, petrol started flowing from the footwell, or rather the narrow space in front of the footwell, via the rubber bung in the floor. I guess it had built up from the various overflows, and had finally softened the rubber.
So we called it a day and got recovered home.

I'm guessing a failing filter has been letting crud through - smaller stuff going through to the carbs, larger stuff holding the valve open. But I will replace the pump as well - no diaphragm being available for my original pump.

I remember your roadside repair: Sainsbury's bag, wasn't it?
Nick and Cherry Scoop

. . . . . and replace the pipes.
Nick and Cherry Scoop

I had a problem with the B sucking air. There was a pin-hole in the pipe from the tank, where it went over the axle. It was right at the very top.
Dave O'Neill 2

You have a good memory Nick. That was a long time ago! It was definitely a plastic carrier bag. Though I couldn't now say if it was Sainsbury's. It could have been Waitrose. Certainly a 'better class of carrier bag" as it lasted pretty well.

The overflow from the rear carb has rubber pipe that is often tucked into the cut out in that triangular reinforcing panel so excess petrol could well drip from the bottom of that, where it abuts the front panel of the footwell. More likely than actually sloshing around in that footwell void. But you never know.
Nice Sprite roads in Northumberland. And big sky landscapes!
GuyW

Nice to hear from you Nick, it's been a while... Sorry you had to be recovered. Ethanol, that Grey referred to, might also have attacked the rubber pipes feeding and linking the carbs - I've had that.
Bill B

I have never added a fuel filter on my Spridgets. I don't believe UK fuel makes it necessary, though a rusted fuel tank may do so! Anyway all of the glass or plastic ones I have seen always have air in them which doesn't seem to impact on the fuel delivery. It makes me wonder if, with the engine running and a large air bubble visible in the filter glass, could the vibration make it look like this air was rushing through the fuel line?
Maybe it isn't drawing air in and the spluttering and rough running was just down to a flooding carb and over rich mixture because of a stuck float bowl needle valve. Are you persuing the right fault?
GuyW

Thank you, everybody. A bit late, but I found the laptop charging lead had been left in Northumberland so haven't been able to reply.
Just got it back.

I will work out a diagnostic plan, and might have the courage to submit it here.
Nick and Cherry Scoop

This thread was discussed between 18/06/2026 and 25/06/2026

MG Midget and Sprite Technical index

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