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MG Midget and Sprite Technical - Fuel pump location

Many moons ago I fitted a Facet square pump in lieu of the Lucas lump. Simultaneoulsy, I conducted all manner of minor engine mods, timing, changed out the carb for HIF44 etc. Since then I have also swapped the head over for a Metro head, tweaked the mix, new plugs and leads etc. For sure all of this yielded a noticeable improvement, but not quite the step change I was expecting.

Since then, I have had a problem with curtailed range. I only managed 130 miles or so before running out of fuel completely. Last week I decided to do something about it, so I dropped the tank to see if the pick up pipe had corroded. Peeking through the sender orifice, I could just about see the filter on the end of the same, so concluding all was in fact in order here, I noticed the fuel pump was sitting fractionally higher than the original - mainly due to the brackett I fabbed up to hold it in place. Purely on the basis of my sketchy fluid dymanics, and beacuse it diodn't look quite right - although not a million miles off - I decided to re-do the brackett holding the pump to allow the suction side to be lowered by about an inch or so.

What a difference! I have at least another 10/15 mph at full chat and the engine is far more rev happy....
I will gladly take all snorts of derision for my abject failure to take into account that this Facet pump may have a different NPSH than the Lucas, but I am somewhat surprised that in lowering it only an inch or so from the original Lucas suction side makes such an appreciable difference. It seems clear that previously I was starving the engine....Doh!

Couple of Qs then....

Will this also explain my lack of range?
Will I need to adjust the mix? It seemed spot on beforehand, and I am assuming the mix will remain the same although the volume of fuel/air entering the engine may well be increased.
Mark O

er, logically, no.
If you were starving the engine you would have had an unbelievably long (good) range wouldn't you?
David Smith

Mark,

What's your fitting into the tank connection, is it standard or something else.
David Billington

David S - logically yes. If the net positive suction head (NPSH) is insufficient, the pump simply does not work as it should, and it starves the engine of fuel. Carb demand outstrips supply from pump... By lowering the pump, static head and NPSH is increased, presumably closer to what the NPSH should be for that pump. Pump works as it should and thus supply meets/exceeds the demand from the carb. I presume it also increases the range - as the pumps ability to keep pumping fuel beyond demand is consequently increased.

David B - fitting into the tank is a male threaded union - fuel pipe on other end affixed with jubilee pipe clip. This is inserted into the threaded boss of the tank, with a conical brass olive inbetween the male threaded union and the pipe pick up inside the boss.
Mark O

Yeap... that will do it

You have to make sure the top of the pump is below the bottom of the tank .... if its higher, it will stop pumping when the fuel in the tank drops to the top of the pump

And yes... 1 inch is a big deal, easily 1/2 to 3/4 gallon of fuel

Prop
Prop and the Blackhole Midget

Prop

Not true. You have to ensure the NPSH is adequate which is not necessarily the same thing. Even as it stands, pump is well above the bottom of the tank, as was the original Lucas.

Regards

Mark
Mark O

Depends on the pump...

The lucas did work above the bottom of the tank... but some faucet pumps stop working when they are at a certian level of fuel in the tank

My pump pushes it dosnt pull... if the pump is above the fuel line I get nothing

If the pump is midway it will pump to the midway level on the fuel tank.... where as the SU kept pumping regardless of the fuel level

Prop
Prop and the Blackhole Midget

This thread was discussed on 30/11/2015

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