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MG MGB Technical - 79 MGB sputters and die when warm

Hi, maybe I'm too close to the problem, but this one is really odd to me. When the engine is cold it starts and runs well. As the engine is warming is still runs fine. When the engine is at operating temp. it still runs fine. After about 20 to 30 minutes, the idle starts to drop and the engine eventually sputters and dies. Even though this is an emission controlled application, the ignition system has been replaced by the standard points and condenser setup (no ignition box). I have performed a complete tune up, plugs, points, condenser, wires, coil. I have rebuilt the carb, replaced the fuel filter and the fuel pump (all with factory equivalent parts). When the engine dies, I immediately check for spark - it is quite good and hot. The fuel pressure is correct, the timing is correct, no restrictions in the air flow in or the exhaust out (when the engine dies and I turn it over, I can hear the exhaust popping out the tailpipe). So there are no apparent issues that meet the eye. I was wondering about a possible EGR issue? Any ideas? I am completely flat on this one. I would greatly appreciate any help on this matter. Thanks a lot!
S Schreiber

Does this happen after a period of idle or through stop start traffic? Is your manifold heat shield in tact?
Allan Reeling

Does the tach jump about when the spluttering starts? If so it is an ignition LT problem i.e. ignition supply, coil, points, condenser short-circuit, and distributor earth wire. If not then it could be condenser open-circuit, HT or fuel.

Clip a timing light onto the coil lead and each plug lead in turn and watch for the flashes when cranking and it doesn't start. If you have a 12v light you may need to power it from a separate 12v supply as it may not work with the reduced cranking voltage. While on No.1 and No.4 check that the timing is about 8 to 10 degrees BTDC.

What carbs? If HSs it's easy to turn off as soon as the spluttering starts and remove the float chamber lids to see how much fuel they contain. Even if HIFs leave it until you are pretty sure that it will start again, switch on and see if the pump just clicks a couple of times or chatters for a few seconds. In the latter case it indicates the pump cut out or was under-delivering. Fuel pressure is only an issue if you have too much, what is more important is delivery rate. Remove a pipe from a carb (be aware that it would normally spurt if the ignition has been on recently), direct it into a container, and switch on the ignition. It should deliver at the very least one Imperial pint per minute and in practice at least double that. If the single Stromberg then check the choke is coming off.
Paul Hunt

Hi, thanks for the quick responses. It doesn't seem to matter if I just let the engine idle or if I drive the car. I always seems to fail in about the same period of time. The heat shield does have some light damage but it is about 90 to 95 percent intact. The tach performs well and doesn't jump about. I replaced the condenser with a new one - no change. The carb is a Stromberg, and when I rebuilt it I noticed that the float level had been set too low (but not by much, about 2mm). When corrected there was no change. The old fuel pump did vary a bit while the engine was running so I installed a new one. The engine still dies. When the engine stalls, I did check the spark at each plug. Each plug is receiving good hot spark that flashes in time with the cranking engine. The fuel pump does pressurize properly with the initial turn of the key, and then just maintains pressure, as it should. It even has correct fuel pressure after the engine dies. The choke is indeed working properly now after the rebuild - it hadn't before. I'm wondering about the possibility of a faulty EGR valve or maybe a partially clogged cat?
Thanks again for your help!
S Schreiber

The EGR valve would cause your problems all of the time, not just when the engine has warmed up. The same thing goes for the catalytic converter. If you remove the gas cap does the problem go away? If so the fault lies in your emissions canister. RAY
rjm RAY

The fuel cap had been replaced recently, but alas, when removed the same condition still exists. I am able to stave off the sputter then die condition if I keep the engine at 1300 to 1800 RPM. But this only lasts for about 5 minutes before it dies anyway,
S Schreiber

My Sprite did this, exactly this, I could use it for very local runs and it would be perfect, but anything over 20 minutes and it would sputter then die. Had to wait for it to cool before it would start again. I changed the cylinder head gasket and swapped the cylinder head out for a rebuilt unleaded one (as I had been planning to do it anyway) and the problem completely disappeared. So that is not a very focussed answer, sorry, but I thought I'd throw it into the mix for what it is worth.

Good luck with it,

Piers Colver

"It doesn't seem to matter if I just let the engine idle or if I drive the car. I always seems to fail in about the same period of time."

This makes it more likely to be electrical from a marginal component warming up and failing, as in most cases that is not dependent on usage, just simply being on and running. I'd be monitoring the spark leading up to the problem, not only after it.

"It even has correct fuel pressure after the engine dies."

That's not good enough, you have to check the flow rate. With a full carb and a stopped engine the pump may well maintain the correct pressure, but be incapable of delivering the required flow. I'd do this even if you have monitored the pressure before, during and after the problem, simply because it is flow that is important, not pressure.
Paul Hunt

Try a new rotor arm. Sounds like its getting hot & losing its resistance to me.
G Britnell

Another thing that can cause a bit of spluttering/misfiring is a spay of coolant. The dizzy is near the heater valve and pipe and a spray from the rad could do it. It could coincide with the system pressure build up.
Allan Reeling

Hi, sorry bout the delay. I tested the fuel flow before startup, while it was running, and after the engine died. Initial pressure was 3.6 psi and the flow was 1 litre in about 80 seconds. While the engine was running and cold, the pressure would flex between 3.2 (lowest) and 4.85 (highest) psi. As the engine warmed and at operating temp, pressure was flexing between 3.23 and 4.3 psi. Before the engine died and was running rough, pressure was flexing between 2.8 and 3.6 psi. During this time the exhaust was popping. When the engine died the pressure was 3.1 psi and flow was the same at 1 litre in about 80 seconds. I had performed a complete tune up before, however I will try new tune up parts to see if that changes anything. No coolant leaks at this time. The spark was good and strong at startup, during the time it ran, and after the engine died. So, I can only surmise that I have fuel, spark, air in, exhaust out, timing is correct, the sky is blue and the birds are still singing.
S Schreiber

Two things you did not mention in regard to your tuning.

Have you checked/properly set the valve clearance settings? These are important as a base to other tuning settings you make.

Have you checked for manifild/carb mounting gasket leaks? these can develop with engine temperature rises and might be a contributing factor to the problem you are having.

R Taylor

I have checked the valve clearance two times, when this first developed, and again today. Cold they are .015, a little loose but this would prevent them from expanding and holding the valves open. When I rebuilt the carb I replaced the carb to manifold gaskets. I also checked the manifold to head gasket and it is holding as well. I honestly can't think of anything else it could be.
S Schreiber

4.85 sounds too high. 4 is generally said to be the maximum and the norm is usually given as 2.7 to 3. Could well be overwhelming the float valves especially with the vibration of running. It might be worth putting a switch in the fuel pump supply and turning it off immediately the splutter starts.

Is it possible to see the fuel level in the jet with the piston removed? Compare normal running with stalled.

Also you must monitor the spark constantly through normal running, the splutter, to stalling, on the coil lead and each plug lead. Mentally compare the flash rate with the engine speed, missing pulses are easy to spot, even on the coil lead at idle.

"when the engine dies and I turn it over, I can hear the exhaust popping out the tailpipe"

I wouldn't expect popping with an almost closed throttle and not firing, there should be very little air flow. What happens regarding this noise when you crank it with the coil disconnected?

Paul Hunt

Well, I did a little more testing. I also installed a fuel pressure regulator and set it at 2.8 psi. The engine ran for about ten minutes longer this time. I did a engine compression test before the engine was run and then right after it died. Cold compression was cyl 1 144psi, cyl 2 138psi, cyl 3 135 psi, and cyl 4 120 psi. After the engine was run and then dies it was as follows; cyl 1 135 psi, cyl 2 130 psi, cyl 3 136 psi, and cyl 4 131 psi. I watched the spark curve on an engine analyzer and cylinder 3 was very low. Also cylinder three had sign of oil burning on the spark plug. After the engine ran for about 15 minutes, I reved it and a lot of oil smoke came out of the exhaust. I had spark the entire time, start, run, die, and cranking. Also, it does not pop out the exhaust with coil disconnected

S Schreiber

"it does not pop out the exhaust with coil disconnected"

Then if it still does with the coil connected it is firing to some extent. With the hot cranking are you opening the throttle at all?

Cylinder 3 could be either chicken or egg, i.e. oil could be fouling the plug and stopping firing, or lack of proper firing could be causing it to foul (or should that be 'fowl'?). However losing one cylinder after running for a while should definitely not be enough to make the engine stop altogether.

Idling for several minutes, then smoke out of the exhaust when revved, indicates worn valve guides, also if you do a downhill on a trailing throttle then open it up.

I assume you have the single Zenith carb with auto choke? If so I'd remove the water jacket and manually weaken and enrich the mixture and see if either of those make any difference.
Paul Hunt

I had an MGB that would run perfectly for about 30-35 minutes and then stop. Would not start for about 20 minutes then would start and run again as if nothing was wrong fo about 30 minutes then it would repeat by stopping and no statr.
It turned out to be a small crack in the cylinder head in front of #3 spark plug. Coolant would spray on dist cap and short out the ignition.

It seems that I have multiple problems here. I've resigned myself to the fact that the head has to come off and be rebuilt and that I need a new heat shield. I was wondering, since this car has a cat and it directly under the carb, could the carb body have warped due to exposure to heat? I disconnected the coolant lines to the choke and fiddled with the mixture. I was able to get another 11 minutes of run time out of her. If this is possible, I was also wondering if the Weber conversion kit is a good alternative? I wish I did have a coolant leak, as that would be easy to mend, however such is not my luck.
S Schreiber

Taking a step or two back there has to be something basic wrong here. Given fuel, air, compression and spark in approximately the right quantities at approximately the right times a simple engine like the MGB should start and run, even badly. People with two plug leads swapped over can get theirs to start, so there has got to be something really basic wrong to cause yours to stop. It's very easy to convince oneself that one or more aspects must be OK so aren't checked, so there really is no substitute for looking at every single thing from first principles, and assume nothing.

Whilst the compression figures are a little odd I don't see anything that warrants taking the head off at this stage, that would introduce even more variables into the equation, likewise fitting a Weber, which has its own problems.

Do you have a vacuum gauge? I'd hook that up to the inlet manifold and watch that through running and stalling. But I'm clutching at straws now.
Paul Hunt

I think I had the same problem as you. I brought my 1976 MGB about two weeks ago. I couldn't take the car out on the road but everything seemed fine. After I purchased the car I would be driving along and the car would go dead. Sometimes after a couple minutes it would start right up and run fine for a while and then die again. I spent four or five days trying to figure this out.

1. Check for spark....ok
2. Changed fuel filter. same problem
3. Previous owner installed new fuel pump. My car has a
Carter Electric Fuel Pump. Disconnected
the fuel lines, and tested pump by hooking up to
spare battery. same problem
4. Tested all fuel lines for blockage. same problem
5. Previous owner installed a new gas tank. Checked
tank blockage. same problem
6. Reattached fuel line to gas tank thinking vacuum
leak. same problem
7. Ran a new fuel line from gas tank to fuel filter.
same problem
8. Removed SU HIF4 single carb and checked float needle
and seat. Adjust float. same problem
9. Ready to sell car....going nuts! I began to think
that the previous owner had the same problem and
sold the car. New Fuel Pump, New Gas Tank, and the
Carb was spotless on the inside.


It turns out that the later MGB's have a running-on control valve. All I did was disconnect the vacuum line that runs from the valve to the intake manifold
and plug the intake.

The running-on control valve if activated dumps air into the engine effectively killing the gas air ratio.
Result.....dead engine. BINGO!!!!!

I don't know what is causing this problem. I believe it is in the ignition switch......but right now I'm so glad the car is running well.

Let me know if this helps. MG's are fun!

gbp Pratt

The factory anti-runon valve actually works by sucking the fuel out of the carb jets, not by dumping air into the inlet manifold as after-market valves do. The line from the inlet manifold supplies vacuum to the valve to do that, it is way too small to dump enough air to kill the engine.

Problems with the plumbing can have a similar effect, to determine whether it is electrical or plumbing reconnect the vacuum line and check the problem still happens, then remove the wiring from the valve and see if that stops it. If it does, it is electrical, if not, it is something else.
Paul Hunt

Paul Hunt is correct about how the anti-runon valve works, but by disconnecting it I solved my problem and maybe that will solve your problem.
gbp Pratt

Similar problem with a V8 - if you did not see the thread - the HT lead from the coil was arcing onto a coolant pipe. Very simple but difficult to spot.

Like Paul, I would persevere with all other avenues before pulling the head. If you are not losing coolant, odds are on an electric issue or an air leak. Last time, my similar symptoms (on a Volvo) were caused by an air leak by the inlet manifold gasket. A liberal spray of WD40 while the engine was running revealed the leak.

As a general point for MG owners - and more modern - electrical connectors older than seven years are always suspect - they all need to be looked at - opened and cleaned. Silicone spray is useful here.
Roger W

Not sure about silicone, it was considered death to relay contacts in my BT days for example. See - 'Silicone Contamination'. People talk about using dielectric grease (even though a dielectric is an electrical insulator) but that is pretty expensive compared to Vaseline.
Paul Hunt

Hi everyone, sorry that I've been away for awhile. I have discovered the problem. 1979 MGB have the catalytic converter directly under the carb, protected only by a heat shield. It would seem that through the years, the heat shield has deteriorated and lost it's protective qualities. This created a condition that heated the carb to very high temps. After a number of years of this heating and cooling effect, the carb warped internally, which created a vacuum leak which then caused the engine to stall and not restart. When everything cooled, it resealed and then the engine restarted. I resolved this by replacing the carb with a weber and putting an exhaust header on. It runs great now - better than before actually.

Thank you everyone for your help!
S Schreiber

I don't know about the catalytic converter requirements in Washington, but I would think by now it is not functioning the way it should anyhow. My 79 also has a catalytic converter, which is needed for New York State inspection (just don't look inside of it, wink wink). So if your catalytic converter is re-configured as mine was, you could eliminate the problem forever.


79 MGB
gary hansen

Coming to this thread a bit late, I appreciate, but it strikes me that for the engine to actually STOP requires more than just one cylinder to be iffy.

So, even a poorly performing cylinder would not seem to be the cause of the stopping, albeit a recipe for bad running.

Stating the obvious perhaps, but shouldn't this be a pursuit of a 'common' problem (fuel starvation, electrical failure, heat-induced component failure) as opposed to a unique one (such as spark plug, piston ring etc).?

Obviously, I'm trying to prevent an 'engine out' if the fault can be traced.

Reminds me of a friend who rebuilt his VW van engine after it started smoking. Took weeks. Couldn't find much wrong until he finally got to put the carb back on, to find that the friend he'd lent it to prior had put some diesel in mistakenly...

1. Ballast resistor wire (if you have one)
2. Dirt in fuel tank
3. Exhaust 'popping' sounds like ignition-related

Once it dies, how easily and/or how long until it restarts???
R Kelly

This thread was discussed between 17/09/2013 and 01/10/2014

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