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MG Midget and Sprite Technical - 1500 Fuel Pump Spacers

How many spacer blocks should a 1979, 1500 midget have between fuel pump and engine block? Mine has two each about a quarter of an inch thick am wondering if this could be affecting the performance of the pump as getting fuel starvation when engine is hot. Have filled tank with new fuel.
K Hughes

The 1500s used two different pumps during their production run - the first one fit right up to the block, and the later one stood out via that spacer. My guess is that they were trying to minimize heat soak from the block, and the fuel vaporization that could result.

So - the pumps themselves are different, in that the later one has an extended lever arm to reach all the way to the lobe on the camshaft. Personally, I'd recommend the later pump with the insulating spacer. But make sure you have the correct combination of pump and spacer, as the earlier pump won't work with the insulator. It's odd that you have two spacers; sounds like their combined thickness equals that of the original, but I'm not sure.

By the way, carburetors were fitted with the same type of spacers for the same reason - to prevent heat soak.

Cheers,

-:G:-
Gryf Ketcherside

Yes it does seem strange that there are two so what I really need are a couple of kind people to nip out to thier garages with a ruler and check the total thickness of the spacers. Mine add up to half an inch and my thinking is that if this is too much then arm will not be bearing fully on the cam so will not be pumping fully. I do know the dangers of not having any spacers on these pumps - rips the arm off completely!
K Hughes

Ken,
check the condition of the diaphragm inside the pump. These can split - in which case it will still pump fuel, but not a the rate that it should.

Guy
Guy Weller

I'm pretty sure that 1/2" is the actual thickness of the original insulator, so 2 spacers @ 1/4" ea. would do the same deed.

As for the diaphragm, Guy's right. Remove the top of the pump and have a look. Also check the "filter" screen inside for crud.

If you didn't know, the pulling action of the lever arm doesn't do the actual pumping. It merely primes the pump by drawing the diaphragm downward, which pulls fuel into the pump. The pumping happens when the arm releases and the diaphragm spring pushes the diaphragm back up, sending fuel to the carburetor. The spring is calibrated to deliver a specific amount of force, and that's what regulates your fuel pressure; meaning that adding an external fuel pressure regulator really isn't necessary with the mechanical pump. It's designed to work with the carbs normally fitted to Midgets, be they SUs or ZSs.

Speaking of oddball pump glitches, the one that came with my car apparently had some tolerance problems with the upper section - or it was getting hotter than necessary. The inlet pipe insisted on working its way out of the pump, and I had to rap it back in a few times. Then the outlet pipe came out, while I was driving home from work in heavy traffic. I smelt fuel, and suddenly the car lost interest and I had to make a dash for the shoulder. I found that the pump had spat out the outlet pipe, and had pumped fuel all over the starter before the engine stalled. I whacked it back in smartly, resumed the trip, and ordered a new pump as soon as I got home.

On a related note - with that same pump - the car once stalled dead while I was pulling out of the garage, and I traced the fault to one of the little check valves in the fuel pump falling out of its seat, again due to loose tolerances in the pump's upper body. I found the valve lying on top of the filter screen.

So as simple as the mechanical pumps are, they can still have personalities. Having said that, once I'd replaced the pump the new one has functioned flawlessly for probably twelve years now.

Cheers, and good luck,

-:G:-
Gryf Ketcherside

Have just had a chance to whip the top off the fuel pump and all seems fine with the didiaphram. It is pretty new although as I know very well this does not always mean it is OK! One thing that I did notice though is that the clear in line fuel filter I have fitted seemed to be pretty much empty. The engine was only ticking over in the garage so maybe not enough revs to fill properly but could this indicate a problem elsewhere?
K Hughes

Fuel filter doesn't have to be full - it is common for them to have a permanent air bubble but still deliver fuel effectively. However, if it is more air than fuel it could be drawing in air somewhere "upstream" of the pump i.e. possibly one of the connections or rubber hoses between the pump and the fuel tank.

Another thought - if you think you are getting fuel starvation after it has been running for a bit check that the vent on the fuel tank filler cap isn't blocked!
Guy Weller

Ahh! - Fuel cap. I initially thought no because I filled up with fresh fuel during the last run which would have released any vaccuum - BUT I do remember thinking that it made it worse. If there was less free space in the tank then it would make any vaccuum easier to create. I shall have to get brave enough to go for another run and then take the cap off when it starts to play up. Too wet to contemplate another breakdown today but might risk it over the weekend!
K Hughes

This thread was discussed between 26/04/2010 and 30/04/2010

MG Midget and Sprite Technical index

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