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MG Midget and Sprite Technical - Any ideas on Intermitent Death...?

Hi everyone,

Well, after many months of trying, she's still stuck in my garage.
The car runs sweetly for anywhere between 10-60 mins then starts to miss on one cylinder and then dies. After 2/3 minutes she will restart then run fine for another 2 minutes before dying again.
Had an old coil on it before, and that used to get uncomfortably hot, so that was changed for a brand new one.
We have fitted new coil, Alden distributor, HT leads, refurbed carbs, fuel pump, filter/regulator, even converted the car to alternator and negative earth! Good constant 14.2 volts across the battery while running. We can see fuel in the line into the carbs.

Any ideas that might help? I really am stumped...
Many thanks!

Josh Spooner

Dodgy rotor arm?
James B

Check fuel pressure.It's possible you have a vapor lock. Try removing the fuel cap and see if that helps.
Remove fuel line at carburettor and check fuel flow as there could be a restriction in the fuel line.

Just a guess, but is the heat shield between the float chambers and exhaust manifold in good condition?
I have heard that if the shield is removed the heat from the exhaust can cause fuel evaporation, it's never been a problem on my 1500 though.
BH Harvey

Does the rev counter have a bout of spasm flickering as it dies, or just drop as the revs drop? If it does twitch, it identifies the fault as LT electrics. If not, then it is most likely to be fuel related. Simple first step diagnostics.

As Guy rightly states diagnosis is the only way to find problems
Random changing of parts and guessing problems is a total waste of time and will simply waste money and time

The rev counter is a very good starting point as Guy states. I also used to keep the timing lamp inside a car to determine if the problem was fuel or ignition before tracing it down further.

If it stopped flashing then the problem was not fuel
Robert (Bob) Midget Turbo

after the amount of time and money you've spent on this I'd be temped to strip the car, sell the parts and buy another one..... When someone comes back and complains the part you sold them was duff then at least you'll know it was that bit! ;0) LOL...

Joking aside.. when it stops is it because of a lack of spark or fuel? Anyone close to you who can help?

M McAndrew

As Darnoc31 said, does it happen if the fuel filler cap is off? It could be that the cap isn't vented, in which case the car runs properly until the pump can't suck petrol against the increasing vacuum in the tank.
Mike Howlett

My questions.

Why have you got a fuel regulator?
Is this a 1500 or 1275?
Have you still got the original points distributor laying around?

Are you on, or at least VERY near to, the kent surrey border? If so, would you like a rank amatuer to take a peek and see if he can make it worse?
Lawrence Slater

"converted car to alternator and negative earth" suggests it is a pre '67 car doesn't it?

Yup that's true. simplifies my thoughts only having to think of one type of fuel delivery system then :). I'e pump under arch. Tickking or not ticking at moment engine stops.

Fuel filter? As the engine draws fuel, any crud gets compressed in the filter. At some point it blocks off. Engine stops. Rest a while, crud settles more losely, and allows flow again, until the next repeat.

A new filter can fill very quickly if the tank is full of crap. I've had this happen, and is why irrespective, I never install a filter now.

But all is conjecture without clear answers from Josh? :)
Lawrence Slater

Agree, so far the symptoms of a car that stutters and stops after a variable length of time could be just about anything! The list of guesses could be very long!

That is why some logical process of diagnosis is needed. If my car hesitates I ALWAYS look first at the rev counter to see if it is flickering, then at the temperature/oil pressure, fuel gauge, and ignition light in that order! If it stops I then listen for the fuel pump.
One can get quite a way on diagnosis without even getting out of the car!

hi josh,
i had this once on one of my cars. was the dizzy cap. i changed it to one i had in garage and no change. after weeks of changing and checking parts i had someone take a look at it and said it was dizzy cap. i said no because i had changed that and he said change it again. so reluctantly i did and all was well for the next 2 yrs i owned the car. everything you say was happening to me.
good luck bob.

Hello Josh?
Come out come out, wherever you are. :)
Lawrence Slater

If you used a dynamo originally & changed to an alernator I think the new setup could fry the coil.

When you put the new coil in was it a ballasted type?
Gavin Rowles

This was happening to my 69 midget when I first got it. Then one day I was going to put fill the tank I heard this giant sucking sound as I took off the gas cap. The vent was plugged. Fixed it and no problem since.
S E Bryan

Hi everyone,

Thank you all so so much for your ideas; really very much appreciated. Apologies for the slow response - I'm both busy at school and inefficient!

Having worked through most of the ideas above here, had a look at the fuel pump and it looks like it only works in certain orientations - meaning that once we'd spun it around it seemed to pump only rather than fuel and air, as well as the fact that it wasn't cutting out...

Let's hope that does it, I'll report back to let you all know!

Thanks very much again for all of your ideas, I really do appreciate them - I'd have been stuck long ago without you all!!

Josh Spooner

If it stops again, feel the pump body. If it's hot, chances are the points are fried. Assuming it's a standard electric SU pump. Then hit it with the contact on, if it starts ticking again, then it's knackered.
Alex G Matla

Not knackered Alex, highly servicable, Just get the parts and fit them.:) If it is the fuel pump causing this, then the faults intermittent and likely to just need a points clean or replacement.
Lawrence Slater

Hi again,

After a long and very painful/expensive process, I think I've now found the problem...(but not counting my chickens quite yet!)

Problem was the orientation of the fuel pump (SU electronic), very sensitive.
Turns out it has to be pretty exact in the way it is mounted to continuously pump without stopping/producing air bubbles. (Impossible to tell when bench testing it because it just seems to work!)

So many many thanks to everyone for your ideas - I may still need them yet, and definitely will in the future!

All the best,
Josh Spooner

I've got a QH electronic pump and didn't take any special care on fixing orentation

I can't say about this for the (expensive) SU electronic pumps but others may be able to comment
Nigel Atkins

Josh, you may be on to something. The following was posted by Dave Dubois, probably., the, SU pump expert. Go to his website to learn all about SU pumps.

"The pump will probably run ok in that position, but not to full efficiently as it will in the correct position. With the inlet/outlet ports pointing up, the pump will have trouble expelling any air that gets trapped in it.
Dave DuBois
1953 MGTD
1966 MGB "

Lawrence Slater

another reason to me not to use those expensive and now fussy SU electronic pumps then :)
Nigel Atkins

This thread was discussed between 27/02/2012 and 05/03/2012

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