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MG MGB Technical - Fuel starvation when starting from warm

My newly aquired 1976 Roadster starts and runs without problem, but after standing for about half an hour, it starts ok but runs for 150 yards or so and then loses power drastically. When I continue to drive, it "staggers" along for about a mile or so then picks up and is fine again. I've changed the obvious ignition parts, but I think it is a temperature related fuel problem. It happens every time when starting from "warm", but never when starting from cold or hot.
I've noticed an increase in temperature of the carb bodies after standing, so I assume the bi-metal mounting of the main jet will be adjusting the jet position, could it be related to that?
M Goode

what's your heat shield like
Nigel Atkins

Hello Nigel, The heat shield is no longer "bright and shiny" of course, but otherwise seems to be intact. I think the temp rise is from conduction after switching off.
Do you have any experience of temperature causing this kind of symptom?
Rds
M Goode

What's the tach doing during the staggering? It is jumping about? Or simply registering the actual rpm?

Could be a pump problem. Do a delivery check when the problem occurs by removing a pipe from a carb and directing it in to a container. Note that if the ignition has just been on you may get a spurt of fuel when removing the pipe. If you do, that almost certainly means the pump is OK. But in any case the pump should deliver a minimum of one Imperial pint per minute in a continuous stream of pulses, with minimal bubbling.

Carbs normally run colder than their surroundings as they are vapourising liquid fuel, which takes energy, which comes from the carb body, so lowering the temperature. Exactly the same as a fridge. When a hot engine is switched off there is no more vapourisation, plus there is heat soak and the outer parts of the engine get hotter than when running as the coolant is no longer circulating. Heat soak is shown as a rise on the temp gauge after switch off, which immediately drops on restarting.

HIF carbs as used from 74 do have temperature controlled jets, but unless there is something wrong with the parts or how they have been assembled they won't cause this problem - just consider how many marques and models had these carbs over they years. Something being wrong with the carb components or their assembly can of course apply to any of the carb components, or many other engine components, not just the jets. While it is technically summer it's hardly heat-wave territory, and these cars run perfectly well in desert states elsewhere in the world.
Paul Hunt

yes I have, as Paul's put when you switch the engine off the water pump stops so no circulation of 'water' coolant, engine not running so no circulation of secondary coolant the engine oil and no cooling air from the car's movement so under bonnet temperature rises

but I found the car would start and I'd give it a few revs to move the cooler petrol petrol along but as Paul has put this would only be in very warm weather and when things weren't fully running and set up properly

Paul will talk you through this problem but to find and help prevent more problems I have the same advice to all new owners, learnt by my from expensive mistakes and experience over 20 years of using various classics as daily use cars

buy and read and later often refer to the relevant Driver's Handbook - (Ref: 0053) - http://www.mgocshop.co.uk/catalog/Online_Catalogue_Handbooks_5.html

give the car a 36k-miles service/check-up staggered between driving it regularly for reasonable length journeys, the service will find and prevent many problems, then regularly drive the car all year round on reasonable length journeys and carry out full and proper regular servicing, maintenance and repairs

this will get the car to be reliable, handle and go well and you'll learnt what the car is cable of and how to drive it to take advantage

I'm a big believer in a thorough clean (and if required overall) of the cooling/heating system to help prevent problems help the engine run well and provide a margin if problems arise - it's not a direct thing with your problem but I include it as part of the 36k-mile service/check-up, my simple method is all over this site

good luck with your present problem
Nigel Atkins

Thanks Nigel and Paul for your helpfull advice.
I checked the Tacho early on and it is stable, so I decided if it was an ignition problem it was on the High tension side and changed the low coast items such as points; capacitor; and plugs. But since the problem only occurs under the one specific condition I am fairly sure it is fuel. (There is no "popping and banging" just loss of power).

I was hoping I might not have to remove the carbs, but I will do that now, in the next few day, and have a good look.

I will post any interesting findings.

M Goode

never assume just because a part or component is new that it is fully or properly working or fits properly
Nigel Atkins

Paul and Nigel, looks like it's fixed.
Stopped the car during the symptons and removed the carbs. fuel pump was good, and the front carb float chamber was full, but float chamber in the rear carb was almost empty.
The float in the rear carb was so badly adjusted so that the needle valve was barely open. The other carb was also badly adjusted, but in the oposite direction, so was lucky not to have fuel flowing from the overflow.Re-adjustment to recomended float levels cured the problem.
It would seem that normal running with cool fuel flowing, the needle valve was just open enough to keep going, but the slight increase in temperature when stopped closed off the needle valve tight. Then after re-starting, when the chamber emptied it was only running on 2 pots, untill cooler fuel came through again. Can't think of any other explanation.

I noticed that the overun valves on the choke disc have been soldered up, so no longer working. I understand this feature is to eliminate backfiring on overun, so could this be some kind of "Boy Racer" enhancement?
Anyway we're happy for now, many thanks for your help.
Judging from these early experiences it seems I could have an interesting project on my hands, and you might well hear from me again in the near future.
Rgds
Mike
M Goode

The usual reason people solder up the overrun valves is that they were leaking to the extent that the idle speed was too high even with the throttle stop backed right off.
Paul Walbran

The poppet valves were an emissions reduction measure, giving a more gradual reduction in throttle closure than would otherwise be the case if the throttle were simply snapped off. I remember seeing a throttle cable damper on 80s BL cars that did the same thing.

Backfiring on the overrun is caused by too much oxygen in the exhaust, which can come from a leaking exhaust. North American cars with exhaust air injection needed a gulp valve to prevent the backfiring that was caused by the air injection.

I had to solder my V8 valves shut as they would stick open shortly after a below-freezing start, possibly due to icing, and the engine raced madly.
Paul Hunt

boy racers would want the back firing

you've now found out why I suggest what I put in a previous post about having the Driver's Handbook and carrying out a staggered 36k-mile service/check-up between regularly driving the car to help find and prevent problems you've no real idea what was done to or with your car before you got it by previous owners

keep at it you'll get there, good luck

Nigel Atkins

This thread was discussed between 08/06/2014 and 13/06/2014

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