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MG Midget and Sprite Technical - Fuel pressure dropping?

I have an SU fuel pump mounted at the back, actually one of Burlen's electronic ones. It's only two years old. Carb is HIF44. I find that when the level of fuel in the tank gets down to about a third, turning the ignition key causes the pump to tick rapidly for several seconds and then stop. The engine will not start properly unless I wait for this process to finish, then it starts instantly. With the tank half full the pump gives a couple of ticks and the engine starts ok.

It appears that the level of fuel in the carb is dropping overnight. But there is no visible leaking. I have watched the carb like a hawk for several minutes with the ignition turned on, and there is just a tick about every 10 seconds. I have replaced all the seals more times than I care to remember, and just about any other part that's subject to wear.

I did notice recently a slight hesitancy from the engine when opening the throttle at high speed, but I can't be sure that wasn't an ignition timing problem.

I've also found that the fuel consumption is considerably higher. It rarely gets near 35 mpg, although I do drive it like I stole it. Again, I can't be sure if this is related to the main problem because it coincided with having the distributor recurved.

My thinking is that because the carb is higher than the pump and the tank, overnight fuel is gradually draining back through the pump and into the tank. When the fuel level is up around the position of the pump this doesn't seem to happen. This suggests that the valves in the pump are not up to the job. What do you think?
Les Rose

sounds like leaky pump valves--
maybe get one of those little inline check valves and put it in the line between the tank and the pump

https://www.google.com.au/search?q=8mm+check+valve&sca_esv=1b23332890ff0c63&sxsr









William Revit

Thanks Willy, they have them in a local store. The tank is full at the moment so I have to wait a bit to find out if it has worked.
Les Rose

If fuel is draining back from the carb' fuel bowl, surely it's the float valve at fault. But given the fuel level in the fuel bowl sits below the level of the fuel inlet port (1), how can fuel be flowing back to the tank via the pump? -- Even if it does need new non return valves in the pump.

1 - FUEL INLET PORT
2 - FLOAT CHAMBER OVERFLOW/BREATER
3 - CRANKCASE BREATHER PIPE CONNECTION
4 - FUEL MIXTURE ADJUSTMENT

As it happens, I have the same arrangement, albeit I still have the points tyoe pump (which is yonks old), and I too find after leaving overnight, the pump ticks as you describe.

My own guess is that the fuel evaporating via the overflow/breather(2).

Modern fuel is far more volatile, and I had exactly the same issue with the weber on my Capri. Overnight the fuel level in the bowl was dropping. And there was no leak. I had also converted the Capri to an SU points pump, and it too ticked after switch on overnight.

As for high consumption, other than your lead foot, if the float is set too high the pump will effectively pump directly at the jet. Maybe it needs resetting.

Setting the float height. John Twist.

https://youtu.be/vPCfwkIdWpc?feature=shared






anamnesis

As Anam noted, fuel draining back is easy to check. There is a rubber line between most of the hard lines and the carbies. (At least, I have never seen a hard line between the fuel line and he carbies on all of the Brit cars I have owned. But, my experience is limited to MGs and Austin-Healeys.) Thus, it would seem that a very simply experiment would be to use a set of "Vice Grips/Mole grips" to clamp off the line between the hard line and the carbie. If, when turning the key, the fuel pump operates the same as before, the problem is before the carbs. Fuel draining back into the tank. If the fuel pump only clicks once or twice, it would indicate that the problem is with the float bowl in the carbs. A couple of 1/8" pieces of steel, and "mole grigs" might supply some useful information.

Les
Les Bengtson

Thanks for the advice chaps. I have fitted a non return valve between tank and pump so we'll have to wait for the fuel level to drop. After fitting the valve I did find it necessary to get the hose clamps extremely tight, or the pump was sucking a bit of air. I'll report back in a few days.
Les Rose

I have pretty much the same set up and it behaves exactly as you describe on start up after sitting, I hadn't considered it to be a problem... until now.
Paul MkIMkIV

Bit of a left field shot but could it be the orientation of the fuel pump?
My SU has been refitted at a bit of a jaunty angle and I'm sure it ticks more often than it did before - it's one of those jobs which is at the bottom of the list to do.
Jeremy MkIII

Well the tank is down to 1/4 full and the problem is exactly the same. Pump ticks madly for several seconds after switching on. Further testing needed.
Les Rose

Les,

Is the connection into the tank tight, if loose it might be letting air in and allowing the petrol to drain back, also is the nut pressing the cone fitting into the tank pick-up tube to form a seal against the back of the tank boss, I've had that issue and it took a while to find the source of air in the fuel line.
David Billington

Yes Dave, the tank connection is tight. I replaced the pickup pipe not very long ago. Maybe this is just a "feature" that I will have to tolerate. There are others!
Les Rose

This thread was discussed between 18/02/2024 and 05/03/2024

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