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MG MGB Technical - Popping sound from exhaust - engine cuts out


Whilst returning home after about 1 hour driving, I changed down a gear whilst decelerating to experience loud popping from the exhaust. I stopped and the revs dropped down towards zero and the engine cut out. The car would initially not restart but after about 30 seconds would start and then just die. I managed to restart the car and continue the journey by keeping the revs in the 2K – 3K region. As soon as I let them lower, they would go down past idle rpm and the engine would cut out.

The car would restart with the same symptoms when it was warm ( playing with the accelerator to keep it going, rough running and occasionally backfiring through the carbs when the revs dropped) but when allowed to cool overnight would not restart at all.

Initially, I suspected fuel or an ignition problem. The fuel supply is fine. I replaced the Magnatronic Ignition with an Optronic System and checked sparks at the plugs but still the car will not start.

The distributor cap and rotor arm have been replaced about a week ago. At the same time, the OE filters were replaced with K&Ns (the needles have not yet been changed as the car is due at a rolling road soon).

Any help troubleshooting this would be greatly appreciated.

James
James Stewart

The dizzy hasn't shifted has it? Have you checked the static timing.
Best bet here will be to start from the basics.
T Crossley

James,

Allow me to spread some doom and gloom!! I had very similar symptons to you when I had a burnt (broken) valve in the head. This meant very liitle compression, so at low speed the fuel wasn't burning and would go down the exhaust, and burn with a pop. At higher revs I think I was getting just enough compression to allow the fuel to burn. The only difference was that when cold I could get the car going, although it was a struggle.

As you have check fuel and spark a compression check sould be next. This would quickly identify a broken valve or other major engine problem. Halfords sell a compression gauge for about £17. I live near the Trafford Centre in Manchester, so depending where in Lancs you are, you are welcome to come and borrow mine.

See http://www.pumaracing.co.uk/comp.htm for info on the compression test.

My website at http://homepages.tesco.net/~rachel.cameron/Head_Broken_Valve.htm has a few pics of what I found.

A compression test is so quick and easy I now use it even before doing much in the way of other diagnositic checks.

Good luck and let us know the solution when found.

Cheers
Iain
I D Cameron

Ah, but the popping form the exhaust is on the overrun this time. Check compression by all means but I think this is a mixture problem. Is there a leak at the manifold or carb gaskets. Have the bolts slackened off. By the way what carbs do you have on your car?
Iain MacKintosh

Iain M,

I bow to your greater engineering knowledge! It may well be mixture or leaking gasket.

However I still like the compression test as a quick check. It takes about 10 minutes and provides a lot of info. I spent several hours trying to sort my problem (avoiding a compression test because I didn't have a gauge and thought it would be hard!). Once I got a gauge it only took 15 minutes the first time, and then I knew I could stop trying to mess around with the carbs and ignition and get on with taking the head off to finally identify the problem.

When my valve went I was travelling on the motorway, and just as I eased off going up the slip road did the problem begin. I had done over 50 trouble free miles that day and the car had been running well up till then. It is the quick onset that is in common with what James noticed that is familiar to me. One minute a happy car. The next a very unhappy one. Of course something else may just have gone then on James' car.

FWIW
Iain C

PS James, please do note that I am very much an inexperience DIY mechanic (had the car a year). Others, Iain M included I believe, who post here will have a lot more experience.
I D Cameron

Iain MacKintosh,

I confirm Lain's point of view. If dizzy did not move, have a quick compression check. It takes no time
and will give you plentiful informations about your valves/pistons sanity. Backfiring through the carbs is often due to leaking valves/ broken valves seats unfortunatly .
Good luck.
Renou

Thanks for all the suggestions!

I’ll see if I can spot any leaks in the manifold or gaskets and do a compression test. I’ve statically timed the ignition but will retest it with a timing light.

The carbs are the standard HS4.

I’ll let you know how it goes…

James
James Stewart

If nothing else turns up (or for a quick, cheap potential fix), replace the condenser in the distributor.
Good luck - Dave
David DuBois

...and try the OLD rotor arm back...or try another new one....they can crap out new!
P J KELLY

I had symptoms like these about 15 years ago when the inlet manifold was falling off. Some shoddy work by.... me when I was in a hurry, I did not tighten things up properly. Perhaps a week or two later this led to popping & banging in the exhaust when slowing down & eventually the car cut out.

Hey, was I embarrased when I found out the cause, but at least it was easily fixed.
John Prewer

The problem became obvious as soon as I removed the air filters. A spacer had come loose and was holding the carb open. I removed the spacer and now the car's back to normal.

I've also performed a compression test the results are: 1-175, 2-160, 3-165, 4-160. Are these reasonable results?
James Stewart

James, Your compression is good. Clifton
Clifton Gordon

Iain M,

I am impressed by your diagnosis!!

James,

Glad it turned out to be a cheap easy fix!

Cheers
Iain
I D Cameron

James,

Glad your engine is OK now. What you don't know is that I had a burnt valve last year an inlet one at that and I don't yet quite know why. Because of the way it was running about the first thing I did was go for the compression tester and it confirmed my worst fears. I've obviously long since sorted it out but it makes me fearful of going for the compression gauge !!

I must point out however that this type of fault would be more the exception than the rule.
Iain MacKintosh

This thread was discussed between 02/06/2004 and 07/06/2004

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