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MG Midget and Sprite Technical - Fuel flow issue

I have an SU electronic pump mounted on the rear bulkhead. When the level in the tank gets down to around 1/4, on starting from cold the pump ticks rapidly for at least 20 seconds until settling down. It has done this for a very long time, and a few years ago I fitted a non-return valve into the line between the tank and the pump. I was wondering if AC Dodd would have any ideas on this when he came a month or so ago, but it didn't arise because the tank was full at the time. I have just taken the non-return valve off and tested it, and it seems to work perfectly. Yes I have got it the right way round! I have recently renewed all the rubber hoses, and there is no suggestion of fuel leaking anywhere. Once the fuel system is fully primed the engine operates normally. I would really like to get rid of this irritating feature, but can't think of anything I have missed. Any ideas folks?

I should mention that while the pump is doing its frenetic ticking the engine will start, so the float chamber seems to be full. It seems like the pump somehow empties itself overnight. But there must be fuel in it or it would not work at all. If the system has been deliberately drained of fuel I have to disconnect it at the carb end and run the pump until it's primed. If the pump had emptied itself I don't think I would be able to start the car.

Forgot to mention that I replaced the pickup pipe in the tank many years ago because it was cracked. It has no filter on the end, but I have one at the carb end.
Les Rose

Les,

As I understand it, an SU fuel pump can push or pull fuel, but only one of them.

So on a Spridget the fuel pump is lower than the tank so fuel runs down the pump and then pumped up to the carbs.

On a Moggy Minor the pump is high, so pulls the fuel up the pump on the bulkhead, then it runs down to the carb.

So first check you have the fuel pump correctly installed and orientated, - I might be able to help with a photo as my rear brake cylinder has just spat a dummy.

Then I guess you need to check if you have the correct pump, I'd assume the correct pump would hold pressure above it, but the Minor one would hold vacuum below it.....would this let the fuel line empty past your one way valve ?
Malc Gilliver

Here is my pump. I don't know why the forum insists on rotating the image. The inlet is at the bottom. Yes I really must restore the bracket.





Les Rose

Rapid fuel pump clicking beyond the first half dozen or so clicks usually signifies a leakage. Either downstream of the pump leaking fuel - typically via the carbs with a stuck float chamber valve. Or an air leak into the pipework upstream of the pump. Leaks in the rubber sections of the fuel line don't necessarily show up as dripping petrol. Small splits may well self seal and not appear to leak. But when suction is applied by the pump the split opens up and admits air at which point the pump goes into an over excited state.

I can't see why that would relate to your 1/4 tank reference though. Other than the commonly reported complaint that appears of the gauges never dropping below 1/4 full, though there's another reason for that.
GuyW

I had a related issue with a new Hardi electronic fuel pump, which was used to replace an SU pump in the engine bay. I then found out (MGOC? memory not serving me well) that the pump was a pusher, not a puller. Moving it to the rear bulkhead by the tank resolves any issues.

Perhaps your pump works better as a puller, not a pusher ?
Tim C

All the hoses are new and clips fully tightened. The pump is an AUF214EN, correct for negative earth Sprites as mine is.
Les Rose

Interesting I have always used a Facet silver top pump (recommended as a pusher) and it seems to pump normally whatever the level of fuel in the tank.(Just as well as I nearly ran out of fuel once on a Spanish motorway when the gauge was firmly on the E. It took almost 6 gallons to fill it) Its mounted at the rear in approximately the same place as the Spridget one.
Bob Beaumont

My observations.

1. My pump, mk4 Sprite standard pump location, is above yours in the arch. Have you lowered yours?

2. With your pump, lower than the tank, I can't see why you need a non return. And anyway, as far as I know SU pumps have non return valves built in.

3. You say it happens around the 1/4 tank level.

That tells me it's more likely the feed pipe to the pump, that is empty; and then the pump has to suck that full again.

You say "Forgot to mention that I replaced the pickup pipe in the tank many years ago because it was cracked."

Was this 1/4 tank level ticking happening BEFORE you replaced the pickup, or has it started since?

4. If I leave mine for a few days, especially in the recent hot weather, my ticks a lot too. It didn't do this before 'modern' fuel. The bloody stuff is far more volatile than it used to be; you can tell by the stink in the garage. I suspect evaporation rather than a leak per se.

You said, " --- while the pump is doing its frenetic ticking the engine will start, so the float chamber seems to be full."

But you don't *know* it is.

Drive it, get it good and hot, then leave it for a few days before driving again. Then before switching on the ignition, lift off one of the fuel bowl lids (I assume twin su carbs) and see if it's full or empty.

If it is empty/part full, if it hasn't leaked, it's evaporated from residual engine bay heat; -- modern petrol.



anamnesis

Thinking it 'might' be that check valve
SU pumps don't like restrictions on their inlet side
If the check valve is a little on the strong side, then the weight of a full tank of fuel possibly openss it ok but when the fuel gets down maybe not.
Would it be worth a try fitting it to the outlet side instead, it'd still operate as a check valve , the pressure of the outlet of the pump would easily pop it open and no restriction to the inlet-------
William Revit

Yours mounting does seem low compared to my mk2 1965.
Mine only ticks on ignition for 5 seconds, so 20 seconds does seem a lot of pressure / fuel to be loosing.
Either there’s a leak or it’s flowing back into the tank?



James Paul

Same location as mine James.

I can't see how the petrol could possibly drain back to the tank.

Surely there isn't enough 'head' to build the pressure needed, from the top of the fuel bowls or the inlet to the pump. It has rise up and over yhe axle. Just doesn't seem possible to me.

Unless lealing out of the jet pipe at the bottom of the bowl, petrol can't escape the bowl other than by evaporation.

That leaves what's in the outlet pipe from the pump, to flow back through the pump, the non return in the pump, and the add-on that's been installed, and climb over the axle. The feed pipe to the pump is over the axle, and is higher than the tank.

Must be an air leak or evap', imo. 😉


anamnesis

Agreed the only way would be a faulty valve in the pump or the wrong valve fitted on the inlet port.
If Les can’t smell or see the leak considering the amount of fuel that should be escaping the system, where else can it be going?

Les have a read of my blog and see if anything on yours pump looks different to mine.

https://www.mgmidgetrestoration.com/2022/09/fuel-pump-rebuild.html



James Paul

As the pick-up tube has been replaced I wonder if the flared end isn't sealing 100% in the tank boss so allowing air in when the fuel level gets below it, not so much to effect running but enough to allow the pick-up to drain when the pump isn't in use.
David Billington

Les you said.:-
“If the system has been deliberately drained of fuel I have to disconnect it at the carb end and run the pump until it's primed. If the pump had emptied itself I don't think I would be able to start the car.”

That is not normal - an SU will push fuel through from the tank and refill the carbs without intervention.

The pump does appear to be mounted low mine is right up high on the heal board.
No need for non return valve.
I do also wonder if your pump is delivering the correct volume and pressure. There is a simple volume check pumping into a container ISTR in the SU guides.
Also I’ve had three SU pumps with very solid diaphragms - hardened due to Ethanol that had difficulty pumping or stopped completely- one an electronic version only a few years old.
Replacement diaphragms are silly money £(40+) so I changed to a Hardi - so far so good and much cheaper than an SU.
richard b

Don't ever park it up with the choke out.
GuyW

Reading back through this Les, you have mentioned that you have to remove a fuel line at the carbs to get the fuel up after being empty----
Does your car have the little strainers behind the needle valves---maybe they're blocked if you have them
William Revit

Les,
"fitted a non-return valve into the line between the tank and the pump."

Have you tried putting the non return valve downstream of the pump? (delivery Pipe to carbs side). It seems to me that if the pump is ticking furiously it is trying to fill the delivery line up to the carbs, implying that the fuel has emptied out. Maybe when the tank level is low, fuel in the pipes is draining back down through the pump valves (?!) and back into the tank until it finds its level. The pipe along the car floor may remain full, but from where it goes up in the engine bay might drain back out. Perhaps?

Do you also use an in line fuel filter?

GuyW

Thanks guys, you have given me a lot to think about.

The carb is a single HIF44, recently tuned by AC Dodd. No sign of leaks.

The pump was new about 3 years ago. I have had this problem since before I fitted it. Of course, a Mk 1 has a mechanical pump, and I fitted the electric one as near as I could estimate to be the correct position. It didn't occur to me that it might be lower than standard for a later car. Or that being too low could cause a problem. I have used an electric pump in this position for over 20 years, and this problem is much more recent.

I fitted the check valve some while before fitting the new pump, in an attempt to fix the problem. I wondered if the valves in the pump were faulty, but it's a new pump now anyway so that doesn't seem likely.

I'm very doubtful that the diaphragm is at fault, because once it's primed the pump produces oodles of fuel. I'm now thinking about the pickup pipe. I've tried tightening the tube nut and it won't go any tighter, but as somebody said maybe the flare on the end isn't quite right.

To be clear, this problem appeared some years after fitting the new pickup pipe. Since then, because there is now no filter on the end, I have had an inline filter at the carb end.
Les Rose

The tank is now just under ¼ full, and here is a video of starting from cold this morning. Strangely the pump only ticked briefly, which is what it should do, but the engine only coughed and I had to try again.

https://youtu.be/c6uSMt3aTNA

It almost always does this from cold, worse in winter and when tank is low. Needless to say, when AC Dodd came it started perfectly!
Les Rose

Sounds ok to me.

" Strangely the pump only ticked briefly, which is what it should do, --"


Yes. And exactly as to be expected now the weather is cooler. It's the volatility of the modern fuel.

No leaks. The fuel bowl on the hif has no jet outlet under it to leak like the hs2 can sometimes do if the o ring is toast. And you say no leak from the rubber seal holding the lid.

And because the fuel inlet is above the fuel of course, draining back out of the fuel bowl is impossible.

If there is no fuel in the bowl it can only have evaporated due to residual engine heat.

**Or as Guy said, don't leave the choke pulled out.**. That'll partially empty the bowl and you'll hear ticking to refill it.


Not unusual to 'cough' on starting either.

That could be simply tuning. Mixture, or timing.

But again, because modern fuel is more volatile, the heat makes it harder to ignite, so you need a good spark and sometimes fresh fuel.

My Ford Consul Capri had a mechanical pump. And, just like everybody else with one today, I found I had to crank the engine for ages to get fresh fuel into the carb, in the summer, before it would fire properly. So I put a nice big SU pump on it; in exactly the same place as a Spridget. In the summer, it would tick like crazy if I hadn't driven it for a day or so. Not leaking. Not draining back. The Weber fuel bowl was ALWAYS full, and yet it would cough sometimes, and the pump would tick. Eveporated in the lines somewhere.

It may be irking you, but I don't think you really have a problem.






anamnesis

This thread was discussed between 01/09/2026 and 12/09/2026

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