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MG MGB Technical - 69 B bad sterter when hot

Hi everyone, a second plea for help for the same problem.My 69 B wont re-start after a few miles ie when engine is hot.I have changed the coil as that was one suggestion and removed recently fitted fuel filter.I have now realised there is a vapour lock forming (an audible hiss when removing fuel cap},but even with this released its still reluctant to start.Give it an hour and its fine. Is there a breather hole in the fuel cap ? as I cant see one.Any suggestions welcome especially after being stranded on a crossroads today!
p stafford 1

Phil - The hiss from the gas tank is more an indication of a vacuum forming than vapor lock. If there is no charcoal canister with your car, then the vent for the tank is in the filler cap. The vent is rather difficult to see and I would not bother trying to find it, just replace the cap with a new one. The hard starting after releasing the vacuum from the fuel tank may not be related to that problem at all, or it could be some kind of blockage in the fuel line. After releasing the vacuum and turning on the ignition, did you hear the fuel pump start up? If not, you may have a fuel pump problem. If you were driving for awhile with a vacuum building in the fuel tank, the fuel pump probably got quite hot from trying to pump against the vacuum and that may be the reason that you had a problem getting restarted. Now that it has cooled down, you should not have any further problems as long as the tank vent is not clogged, but it would not be a bad idea to carry a spare pump with you (such as the little square, run all the time, make a lot of noise, Facet pumps) to get you home in case the fuel pump was damaged. Good luck - Dave
David DuBois

Phil, David is spot on, a UK car of your year will need a vented cap. My car, just purchased, had the exact same problem and the po had also changed the coil! I found an aftermarket locking cap causing it. Although it said vented on it, it wasn't, and there was a great hiss removing it after a few miles. Change the cap then re-assess.


Rich - The only vented cap that I ever had that caused a problem was a locking cap. When I got it home (we were int he middle of a 2500 mile trip when the problem occured), I proceeded to disassemble the cap and found that it had such a convoluted method of venting that it was a wonder that it worked as long as it did. To get home with it, I cut a section out of the sealing ring. Cheers - Dave
David DuBois

Thanks guys ,looks like a new cap then
p stafford 1

Hi All,

After a very warm saturday (this is Scotland and it was T-shirt weather, so that was summer!) I had the same hot starting problem. At cold the car started first time, it always does, but after being stopped at a local supermarket for about 20 mins it refused to start until it had cooled for another 30 minutes. And then started first time and ran perfect.

Annoying. Is this the reason for coil replacement ?

Apologies for (potential) stoopid question.


Assuming you do not have a tank venting problem.

There will be a number of items in the under bonnet area that get some cooling benefit from airflow when the car is moving. If you leave a hot car parked there is no airflow and some of these items will initially get hotter due to heat soak from the engine.

If your coil is bolted to the inner wing it is less likely to suffer from heat soak than items such as the distributor and carbs.

The fuel in the carb float chambers may boil due to heat soak leading to a temporary vapour lock. There is a heat shield and insulator blocks between the carbs and the manifolds to reduce the risk of this.

The MG Montego Turbo had an air duct and fan specifically to cool the carb float chamber.

If you have electronic ignition with the traditional "grey box" the fitting instructions will probably have said to put it somewhere cool. Mine is on the inner wing in front of the radiator diaphram.

David Witham

Thanks David,

It has the old weights and bobbins ignition system, the coil is on the inner wing. I was told it sometimes doesn't have enough "puff" to start it when hot so I was trying some deft crank/off attempts to set it spinning and let it go. After 20 mins first crank and brum brum.

As a coil's not too expensive I'll give that and some new carb spacers a go and see what happens.

Mind you that will be as hot as it gets here for the next 5 years...


Chris, My po had been this same route with hard starting when hot and running-on. When I got the car it came with 2 petrol caps (standard which was fine and a locking one with blocked vent which I don't think he knew about as he said he just used the standard one for showing. If he was swapping them over that way it is understandable he was missing a problem).

He'd also fitted a new coil and his mechanic, who was a modern car mech really, had fitted insulating wrapping to the exhaust manifold and reflective alu wrapping to all the fuel lines citing heat as the problem. It came fitted with an MGOC anti-run-on valve (which turned out to make bugger-all difference). It had been running leaded fuel and, later, unleaded with these same problems btw.

Now, I've had these cars before including during the 2 best British summers of later years, in 74 and 75, without any problems at all and loads of people run them abroad. So, I took off all that rubbish and started again.

The carb heatshield was new and complete with insulating lining which was a good start as were all the correct insulating spacers. The carb float levels were set way too low as he'd set them when the fuel valve springs were compressed rather than just seated. One float chamber was bolted solid instead of floating on rubber bushing so would have got hot/vibrated. The jets were worn, it had weak needles fitted and the piston springs were weak and short. 2 plug caps were way over ohms rating (reading Mohms!) so spark would have been weak. Timing was retarded and tappets all over the place.

Fixing all that, with another new Lucas coil and plugs, which is really just restoring original spec and a good service, has cured all starting problems. Setting tickover to 650 stopped running on.

If I had to pinpoint things they would be weak spark, weak mixture and retarded timing. Oh, a duff condenser can behave this way.

I'm not sure that you could suffer actual vapour lock problems with carbs, though the heat may mess up the mixture. I had an old trials bike which boiled the mixture in the carb on hot days but still ran fine.


I shall check all that out, thanks for the advice.



The vent lines coming off the carbs could be filling with fuel and causing the problem. Try removing the vent lines when the engine is hot to see if it will start. Its worth a shot, right?
Jeff Schlemmer

Jeff, Chris, The vent pipes are simple overflows on this model and should exit down by the sump.


I'd be interested to know if anybody with starting problems when hot, are running with a stainless steel exhaust manifold rather than the original cast iron one.
That's one modification I've made that I now wished I'd stayed with the original design. The stainless steel manifold seems to generate a lot more heat just below the carbs. On a hot day when parked for a short while, the carbs get extremely hot to touch, even with the proper heat shield and spacers in place.

Charles Goozee

Mine's stainless tubular and fine.


This thread was discussed between 18/06/2005 and 21/06/2005

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