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MG MGB Technical - Bad MG Friday


Help needed from you MG pros.

Friday afternoon, hit the road with the 78B and daughter, heading for a weekend at the campground.
First 100km went fine but about 5km from the goal... not so fine. Suddenly almost no power from engine. (Looked like I'd try to ride 10km/h in fourth gear.) Hardly made it up a "not much steep" hill, finally made it to the top with a cloud of blue smoke from exhaust. Stop the car to let it cool (not very hot though but we never know). Started again but not better, still no power but a lot of smoke. Drove it a little more to park it in a safe place and have it towed back home.
Tonight I went to investigate. Started the car, it ran for a few seconds but stopped and wouldn't start again. I just floded the cylinders with a mix of gas and oil.
I made a compression test expecting to have, you guessed! destroyed rings. Here are the results:
1 140/dry 150/oil
2 150/dry 150/oil
3 135/dry 150/oil
4 150/dry 160/oil

not bad for a 28 year old smoker.

So that's where I am and here is a few questions:

Could the compression rings be good and the oil rings really that bad?

How can I check for the valve guides (2nd suspect) and can it be checked with the head (engine's head) on the block or I need to remove it?

or any other advice welcomed

p.s. I noticed, long time before, blue smoke from exhaust when accelarating from idle and shifting and planned to pull out the engine as a (quickly comming) winter project so if it's the only solution I'm comfortable with that.

thanks in advance.
M Gaudreault

Are you sure it was oil smoke, not gas? Sounds like the carb was flooding. Broken float?

Michel. An interesting problem. One that is very hard to define without some more information on which to base our diagnosis.

First, I am glad that you and your daughter are safe. My older daughter, learned to drive on an MGB and I transferred the title of the car--her daily driver, to her when she got married recently, have some great "father-daughter" times to remember when the "MGB broke down".

There are a number of tests which need to be performed to see what might be wrong. Please be so kind as to visit my website at and click on the MG section. From there, click on the "articles" section.

David DuBois has a tech article on how to test the fuel pump, giving the pressure and volume information to determine if your fuel pump is working properly. Dave used both his experience as a long time SU fuel pump rebuilder (I, too, have used his services) and contacted the current producer of the SU fuel pumps and carbs, Burlen, to verify that the data he listed was accurate. His e-mail is hot linked on the first article, perhaps others, and I can truly state that I have never e-mailed Dave on a fuel system problem when I did not a prompt, courteous, and informative reply. Please contact him if you have a problem in this area.

Also on the website are some articles that I have written about compression checks, checks of the ignition system, etc. Should, after your tests, please contact me through the website, or here, and I will do all that I am able to assist you.

Another fine resource for troubleshooting of such problems is Paul Hunt's "Pages of Bee and Vee". Paul is very highly qualified to provide advice as he is a "mechanical-electrical" engineer. (Did I get that right, Paul?) He has, in the past, answered questions from people whose best language was French, then, provided those of us who only speak English with a translation of the basic problem and his response in English so that we might learn from him.

I have learned much from Dave and Paul and all of us, the "three of us", and every other member of this BBS will do everything possible to assist you in solving your problem.

You can, to assist us, note what carburetor you have on the car (I believe the Canadian cars, being of "North American Specification" had the Zenith-Stromberg carb, which has its problems) and what distributor you are running. The basic "North American Specification" cars had the Lucas 45DE4 distributor in 1978 and these tended to fail rather rapidly. (My original 79, purchased new, had the 45DE4 distributor fail in the first three months of use. It was replaced, under warrenty, in Germany and worked well after that.)

Additional information will allow us to assist your more fully. And, to help other MG drives is what all of us are here to do.

Les Bengtson

This may be crancase ventilation problem.
Stan Best

I know it sounds odd, but check the ignition timing - if this has slipped seriously it can cause the rings to flutter and burn lots of oil.
Chris at Octarine Services

Les is correct. A person needs more information. However, think about this- it began suddenly. Since all 4 cylinders have good compression, and the valve guides would wear slowly, something radical happened quickly. If the plugs are wet and the engine won't start, it is either electrical or fuel. It could be the crankcase ventilation but if you dry the plugs, the engine should start and run for a short period of time anyway. Regarding a puff of smoke from the exhaust when shifting, I have owned two MGB's (1977 & 1980). When the club goes on drives I have noticed that the cars show a slight puff of smoke when shifting. I would suggest going back to square 1 and start sorting through the problem as suggested. Do the simplest and least expensive things first.

The discription given would ask for a check of the Stromberg carb, I think.
The symptoms explaned are most often found when the diaphragm becomes defective, allowing damper oil to be succed into the engine and a lost of power due to incorrect metering.

You should take off the dashpot of the carb to check it out.

BTW, the same problem is often found on TR 6 and old Volvos with the same carbs, burning the damper oil and loosing power.


Good morning

Great reply so far.
Tom. I saw blue smoke or maybe I tought it was! I won't discard the idea of a broken float and and put it on the "to do" list.
Les. I, too have a couple of great "father/daughter" time on the side of the road since I bought the B. Some where less funny but I think she begins to be "Brit Cars" minded. Back to the car: it has the original ZS carb, the distributor is a mecanical one, (45D4 I think) but I installed a Pertronics unit two years ago. I will take a look at the Websites you suggest.

Anyway I will print this thread and use it as a check list for further investigation tonight.

Thanks to all of you. I'll keep you informed.
M Gaudreault

Ralph is correct, but it is NOT the damper oil that is burning. Rather, the excessively rich mixture caused by the bad diaphragm is washing oil off the cylinder walls and out of the valve guides, causing the blue smoke. Additionally, it is likely that the diaphragm has had a small hole in it for some time and therefore the oil is severely contaminated with fuel, hence very thin and easy to suck out. Also, since the bad diaphragm causes the carb piston to stay down and choke the engine, it will develop very high manifold vacuum which is applied directly to valve guides and rings, exacerbating the oil "suckage". Change the diaphragm and the oil and filter; continued operation with contaminated oil WILL eat rings - and bearings etc.
Anybody with a ZS equipped car should have a spare diaphragm and the tools and knowledge to change it on the road; however, they rarely go without showing precursors of wrinkling and a distinct "gummy" feel, so if you get stuck it's either your fault or your mechanic's.
FR Millmore

Hello everybody

Problem solved !!

And the winner is: ......Ralph with his defective diaphragm !!

To be honnest, you're all winners with your quick reaction to help me solve this problem.

It's impressive how such a small crack in the diaphragm can bring so much trouble on an engine.

FRM: I had a spare diaphragm (in the garage, not in the car), tools and knowledge to change it. The only thing that was missing: I didn't know how a broken diapragm would react....NOW I KNOW!

New diaphragm is in place, I ran the engine a few seconds to check if everything is fine (it is) oil/filter change to come tomorrow and the B will B on the road again.

Thanks again to all of you
M Gaudreault

michel: thanks for sharing the outcome of your dilemma It is important for all of us to learn from each others experiences. This is how we keep the cars on the road.

Congratulations on your successful repair and to all who posted with advice

Gary :>{D
70 mgb

Michel. I would also add my thanks for telling us what you found. The more we can learn from each other, the less we have to learn on our own and the better our chance of solving the problem correctly the first time we try it.

Glad the car is working now. Do remember to change the oil as per Fletcher's advise.

Les Bengtson

This thread was discussed between 15/08/2006 and 16/08/2006

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