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MG Midget and Sprite Technical - Good and Bad Day!!!!!!!

Its been a good and bad day, the good bit was the midget passed its MOT with no advisories, gave it a good run before the test, with no problems, got to the test station,turned it off and went to tell them i was there, the tester came out to the car and it would not start!!wasnt even trying,we pushed it inside,for a closer look, whipped out the plugs and they were dry, although the engine was quite hot,(it reached 29 here today)pulled off a fuel hose at carb and with ignition on fuel barely trickled out, there was about half a tank in the car,they diagnosed a faulty pump, so i popped over to watford classic cars and bought a QH electronic pump (50 Quid)they fitted it and it fired straight up and passed the test, i went and collected it several hours later and drove home fine,i turned off the engine at home and it would not restart!!!!i pulled the plugs and they looked quite dry, although i could smell petrol as i had been trying to start it, i gave them a wipe and it started fine, turned it off again and it would not start,anybody got any ideas as its driving me mad, when you release the fuel cap there is quite a hiss off air, so tryed leaving it off but no difference. Could i drill a small hole in the cap, its the original one that came with the car from new. Martin.
m a frame

Sounds like you might have a blocked fuel feed --- to the pump. Tank full of crud?
Lawrence Slater

you need to check the fuel flow again at the carb end if it's still a trickle then as Lawrence suggests could be a pipe full of crud.
RIchard Lawrence

could it be somethinbg in the tank , floating around and periodically blocking the fuel outlet?
RIchard Lawrence

I had a similar problem with my B, and it was a loose ground wire to the pump, which was grounded at the number plate holder.

C R Huff

All the fuel lines are clear, as is the tank as its all brand new, we blew air through the the fuel lines at the mot garage, and all seemed well, im thinking fuel vapourisation, is that feasible , the underbonnet temp was very high,i have the correct insulator blocks fitted and also the heat shield. really dont know what to do next. Any more ideas. Martin.
m a frame

Reading down your thread, I was thinking vapour lock, and it looks like you have come to same conclusion.
Doubt it is your fuel cap as that would probably not explain the fault being intermittant, but easy enough to check that.
Strangely I have been suffering the same problems in my MGA and had posted similar symptoms on that forum. I did read somewhere that different fuel can have differing volatility, meaning some is more prone to vapour lock than others.
My car, like yours, runs badly only after I stop for a short time. While you are driving along at 30mph, you will have plenty of cool air flowing through, but when you stop, even with engine turned off, the heat can really build up, and cause the fuel to vapourise.
Do you have a heat shield? You need to make sure that your cooling system is working properly etc. The MGA is particularly prone to it I think, so much so, one very clever and ingenious member of that forum designed a set-up to include a pump blowing cold air at the carbs, when they get too hot
Graham M V

I was thinking heat but wasn't quite sure - have you got the support straps on your heat sheild?

I can't remember what year your car is

what does your temp gauge show when you're going along, stationery and after switching off

if temp is high could fan belt be loose or rad pressure cap wrong pressure

is your tickover slow

route of petrol pipe

I've got one of those Q&H pumps they're fine
Nigel Atkins

Doesn't sound like a temperature problem per se to me. It's not that hot in the UK, and if this was a common problem it would happen far more often to others. If you didn't have the heat shield, ok, maybe, but that tends to make it run badly, rather than evaporate all the fuel in the bowls and prevent it running at all.

You've got a fault (intermittent) somewhere, even if it is hard to find it.

I would say go back and check again. Electrical (feeds and earth), and blockages.

And, as Nigel often says --- I'm surprised he hasn't --- even though you've swapped the pump, are you sure the replacement doesn't have a fault too? Unlikely but you can't rule it out until you double check.
Lawrence Slater

I should add, MGA excepted then, as I've just read that it is a common occurence on them.

But I used to run my sprite without the heatshield, (gave better physical access), and although in the summer it made it run rough, it never prevented it starting, and in the winter it was fine. Also, on the HIF44, I wonder if the heatshield is worth the bother it's so small.
Lawrence Slater

I am on the border of Herts, where Martin is and it has been pretty hot - 26 degrees yesterday. And I am experiencing similar problems. Go for a drive, stop for 5 minutes, and it struggles, but stop for half an hour and it then runs great. If Martin's symtons are the same, and that is easy enough to find out, then I would bet it is vapour lock.
Be interested to hear how often Martin uses his car. I found a piece on the web by BP that explains their fuel volativity varies from season to season, so if Martin is still carrying winter fuel, strange at it seems, this could be part of the problem. But as you rightly say, it is not happening to all the cars, so perhaps something in his cooling system is not 100%.
Graham M V

Hi all, Thanks for the tips, the whole car is fully rebuilt,it has a heatshield,new tank, fuel lines etc,should it have a vented fuel cap,(could i drill a small hole thru. my existing one or should i buy a new one? as i get a loud and long hiss when i remove it, put a gouple of gallons of summer fuel!!! in last night, its now starting(albeit struggling) hot after being off for a few minutes, it starts instantly when cold and runs very well, the pump is working well producing a strong flow of fuel with pipe off carbs.Cheers Martin.
m a frame


The interesting symptom was that there was a big suck of air when you undid the fuel cap. The USA spec fuel cap was unvented and sometimes exhibits this problem - to such an extent that the bottom of the tank on the Midget of ours with such a cap gets sucked up a couple of inches and there's a big metallic clang when the cap is taken off and the tank fills with air! It does occasionally play silly beggars with the fuel flow but not often. From memory the reason it interrupts fuel supply is when fuel is low and the bottom of the tank is sucked up the pick-up is left high & dry on an island of tank bottom with the fuel around the edges. And the gauge is fooled - reads nearly 1/2 too! We have learned that when the fuel gauge starts to increase it's a warning of this about to happen!

But in summary our experience is that the non-vented cap doesn't seem to inhibit fuel delivery when the tank is at negative pressure. And presumbaly the very large number of cars sold in the USA would attest to that too.

You have noted that when you tried starting the car with the cap off it made no difference, so I doubt whether drilling a hole would make much difference. However, I don't see it would do any harm either so long as no remnant shards of steel are left in/on the cap to escape into the tank later.

If I understand where you have got to, you have good fuel delivery and good spark, but it won't start easily when hot. Although we haven't struck it with any of the Midgets, I have had this problem with my MGB. In that case, the fuel level was too high in the float chamber and heat soak made it expand and rise enough to dribble out the jet and flood the engine a bit. (Watched it happen). It was so bad that between 30 seconds and 20 minutes after switch-off it would not start without the starter regardless of throttle postion and had to be pushed. Every time! (So parking was always on a handy hill wherever possible!) Outside of this time it always started instantly.

In this case it was exacerbated by a combination of big bore engine, very high compression ratio and mild street cam, together putting a lot of load on the starter - and so dropping the battery voltage significantly.

Solution was multipronged:
- Dropped the float level a little lower than spec to reduce the amount of flooding (didn't want to take it too far lest it leaned out under power)
- Fitted a ballast resistor & ballasted coil so the coil now sees its full operating voltage when the starter is cranking
- fitted a geared starter to reduce the current drain and therefore system voltage drop.
This completely solved the problem. The engine still floods a little, but with the starter spinning the engine better and the system voltage staying higher and so allowing a fatter spark at the plugs the engine now fires after half a dozen revolutions at worst - usually it's instant.

I would suggest that next time it happens you whip off the dashpot & piston from one of the carbs & see if any fuel is dribbling from the jet or even very much higher than usual. This should confirm or otherwise whether your car is affected in the same way.
If it is, try the usual response to flooding of full throttle while cranking on the starter - if that overcomes it it's a cheaper fix than what I had do do!
And check the float level in the carbs.

Paul Walbran

Paul, Thanks so much for all the tips!! it all makes so much sense,by the way, Im coming to NZ in Jan. My brother and sister both live in Auckland, might have fixed the midget by then!!!Cheers Martin.
m a frame

Cheers Martin - we're in Auckland, look us up if you have the time (though I know that when I'm abroad I never have enough time to see all the people I want to!)
Paul Walbran

As for US Midgets, yes, the fuel cap is unvented... but the tank "breathes" through the vapor recovery pipes and the charcoal canisters, so unless that system is blocked, there should never be a vacuum drawn in the tank.

Gryf Ketcherside

Aha, that bit of kit must have vanished at the hands of a PO in our case.
Paul Walbran

I ran into this exact same issue last year at this time... I thought it was fuel and carb related then decided it was temp

Then just for grins when it got hot agian and wouldnt restart i checked te spark

Bingo... No spark when hot, but coil was good whent back to points ran fine

What i found was the pertronix chip was defective and was ... Phazing...

The next time you start it and run it for 20 minutes and turn off ... Check to see if you still have a spark at the plugs...crazy i know, but easy enough to test and clear... Esp considering i had the exact same issues as yours...ran perfect when cold, then rough as it earmed up, then wouldnt restart after 20 min unless i cooled the dissy, but would restart when the car cooled down
Prop and the Blackhole Midget

Have been very interested in this thread as experiencing similar problems. Did you have the problem when the engine got really hot, regardless of whether you stopped?
Mine only happens once I have switched off and left the car for a few minutes without cold air blowing through.
Graham M V

Mine was dead as soon as i turned it off... But the engine had to be hot..usally after about 20 minutea of normal driving

Just a thought and someting to consider that ive seen here several times... A screw into the wiring harness of a broken insulation and contact with car sheet metal....basically the car starta.fine and continues to run worse and rougher as you dive into time... About a 1/2 hour it will die

Check the coil if its to hot to touch... Then the wiring harness is about to catch fire so disconnect battry
Prop and the Blackhole Midget

Car runs fine until i stop the engine, then it will not restart, until its cooled a bit, going to check float levels tomorrow, also had a mini years ago that was a pain to start from hot, that was a faulty condenser, that broke down when hot, but cooled off and was fine, took me forever to find that!!also have a new spare coil, so i will try each in turn, thanks so much for all your interest. Martin.
m a frame

This thread was discussed between 25/05/2012 and 28/05/2012

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