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MG MGB Technical - '73B Running Bad at High Ambient Temps

Problem: At high ambient temperatures (above abour 85 deg) car runs really bad, no power like it is running on one cylinder. If I pull over to the side of the road I can shut it down for a minute, fire it up and it runs fine for a minute..any load on the engine makes it worse. Car runs/starts/idles fantastic when it is cooler out. The radiator is not boiling over. When it happened yesterday I tried to have a close look when I was idling on the side of the road..I took off the gas cap(in case the tank wasn't venting) and had a look at the carb pistons to see if they were sticking and did not appear to be. I left the air cleaner off and kept driving and as I drove it sounded like it was backfiring through the carbs (dual HIF's)

I have tried the following fixes:
new fuel pump, swapped coils, a new fuel filter, new points, distributer cap, rotor and condensor. Last year I replaced the radiator and fitted a bypass sleeve for the thermostat (This has been going on since last summer). I also fine adjusted the timing to make sure it was in spec.

I don't know what else to do?..I can only think that it could possibly have to do with the carb setting (rich/lean). I bought some "Water Wetter" and am going to add that to see what happens.

my wife doesn't want to go in the car anymore for drives and frankly I don't trust the car anymore until I can get this fixed.

Eric Willis

hi,do you have a heat sheald under the carb,or is the fuel supply pipe to the carb ok, don't know of any thing that will make the car play up like that ,
keep at it,
good luck.
da wright

Yes, there is a heat shield under the carb though it is a little older and the insulation might not be as good as it good be. I tested the flow rates mentioned in other archive posts and is fine. I do not have the tools to check the fuel pressure but I just replaced the old pump with a new electronic SU so I figure I can rule that one out.

I brought this up in a post several weeks ago and at the time I thought it could be due to the float level in the HIF's being a little low. When it gets hot out I may be experiencing just enough vaporization to cause some fuel starvation..? Not sure if this would explain what I gathered was backfiring through the carbs though..

The only other thing I can think of to mention is that when my car acts up it acts up like a light switch..cruising down the road all fine and good then in the flick of a switch..sputters coughs..etc etc, no apparant transition at all...

Eric Willis

Eric: There should be a strip of foam rubber across and on top of your radiator header panel. This helps seal the bonnet to the header and increases the airflow through the radiator. It's a small but helpful hint.
conrad sanders

My 73 with HIFs doesn't like to idle, but otherwise runs fine in high temps - my guess is that the bimetallic mixture compensator in my carbs does not work properly. If your car is backfiring out the carbs, the mixture is probably weak, this in combination with further leaning by the mixture compensator may be your answer. However I would expect the onset of problems to be gradual, not sudden.
Hopefully the electronic fuel pump isn't protesting the hotter weather - although it seems to me you had the same problems before changing the pump.
Is there possibly a vacuum hose the is cracked such that expansion opens a hole creating a weak mixture? This could explain the backfiring out of the carbs.
Best of luck sorting this one out - hopefully others will provide more potential solutions.

Erick Vesterback

Conrad, Yes I have the strip of rubber on the radiator header. Erick, you are correct I had the same problems before and after the new fuel pump. I haven't checked any hoses for vacuum leaks as the car runs great at lower ambient temps..The bi-metallic temperature/mixture compensator is something I hadn't thought of...maybe when I replaced the needles and seats a year ago (spring 2007) I could have put them together incorrectly?
Eric Willis

85 is nothing, and HIFs shouldn't suffer from vapourisation that the jet pipe of HSs can when the heat shield is faulty. You don't mention the temp gauge, if that is giving normal indications then it is nothing to do with the coolant itself getting too hot. Whilst the HIFs do have temperature compensating jets and the HSs don't, the compensating makes the mixture *weaker* in higher temps, but the difefrence is marginal. I can't see the problem being that the mixture is being weakened way too much if it is correct to start with, nor the problem coming on and off like a switch, which sounds more like ignition.

Is the tach flicking around when this starts to happen or does it remain steady? If flicking around it is an ignition LT problem. If not it could be HT, when it's running badly clip a timing light onto the coil lead and each plug lead and make sure you are getting regular and consistent flashing from all of them (assuming the bad running still happens at a standstill).

If that's OK check the fuel by disconnecting the fuel pipe from the front carb (careful, it will spurt a little fuel) and directing it into a container. Turn on the ignition and it should deliver at least one imperial pint per minute, and in practice closer to two, in steady and regular pulses with negligible bubbles. Compare this with good running when cooler.
Paul Hunt

Paul, Thanks for you input..
No, the tach does not flicker, even though I did not see this I replaced the points/ condensor/cap/rotor and swapped out the coil to be sure. The flickering would normally indicate a faulty condensor.

Though the temp gauge reads near H I'm not concerned, there are numerous threads discussing the topic and I am fairly sure the car is not overheating, the radiator is not boiling over.

It is definitely not the fuel delivery system, I checked the fuel flow with my old fuel pump and to be sure I replaced it with a new SU unit(wanted to do this anyway)and made no difference. I also checked the fuel flow when I was on the side of the road when I was experiencing this problem (as you noted) and was fine.

Now the HT side to the electrical system might be something I'll have a look at. When it is missing it seems to be more sporattic and not related to a single cylinder. This might be the HT lead from the coil to the distributor. Why it would only occuur at high temps is odd but electrical resistance does increase with temperature. The last time I replaced the HT wires was in '95.


Eric Willis


I would be inclined to replace the HT leads. At 13 years old they really don't owe you anything.

Might not help, but can't hurt.

C R Huff

Flickering would be caused by *any* intermittent bad connection or short in the ignition LT. A condenser going intermittent open-circuit wouldn't cause a flicker, but one going intermittent short-circuit would.

'Near H' is much higher than I would expect, especially for 85F. A significant increase in resistance with temperature in the LT circuit would cause flickering of the tach. Ordinarily any increase in resistance due to heat in the HT circuit it likely to have no effect, the leads are very high resistance anyway, and the ht voltage simply rises higher in compensation. However if a lead is faulty and its resistance goes beyond a certain point the HT voltage will rise to a level that it would rather jump through the insulation to a convenient ground than force its way through to the plug.
Paul Hunt

This thread was discussed between 17/08/2008 and 19/08/2008

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