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MG MGB Technical - Update on my Miss firing thread

Still no joy - starting to tear my hair out in frustration!

So far:

Removed the carburettors and cleaned them out ( with compressed air )
Hooked up an auxillary fuel supply to bypass the on car fuel lines etc.

No joy.

Changed condenser, coil, bypassed the ballast resister and fitted a ceramic one.

No joy.

Changed the distributor and distributor cap for a spare one and obviously checked the timing etc etc.

Still no joy.

Had the rocker cover off to check valve clearances and to make sure a nut hadn't fallen off - all ok.

I haven't changed the plugs and leads next but am starting to think there is something more serious wrong.

The car starts fine but just back fires through the exhaust and feels like it is trying to run on 2 cylinders very roughly!
I took out all the plugs and they were black and sooted up. Not surprised at this as I been on the choke to start it all the time.

I am wondering now if the head gasket has failed between a couple of cylinders. I will have to get a compression test done and see what that reveals.
AdrianS

Similar circumstances led me to remove the suction chambers from the HIFs. I noticed that one jet was very high - enough to close off the two cylinders connected. I pushed it carefully back into place. The car then ran normally for a few days but stopped somewhere extra inconvenient.
I then removed the offending carb with the jet once again protruding and found that it was in two pieces - the metal sleeve had detached from the plastic base which was anchored by the bi-metal connector as it should be - the sleeve had moved quite quickly upwards finally coming to rest on the needle.

The carb worked on the choke but as soon as the choke was pushed in - shut off fuel.

This is a one off - of course it could never happen again and certainly not to you!!!
Roger Walker

Adrian,
I have just discovered the cause of a similar issue to yours. Looks like it was a leaking servo vacuum pipe. The same issues I suppose could also be cause by a faulty servo diaphragm.
The problem started with just a miss on over-run but gradually developed into full scale back-firing and a reluctance to idle.
Worth a look!
Allan Reeling

Thanks Allan

Did check the pipe as had probs in the past with a vacuum unit failing. In fact I blanked it off at the manifold but it made no difference.
Awaiting a friend to lend me his compression gauges to check the cylinder figures before I start taking the engine apart!
AdrianS

You should be 110% sure that all aspects of the ignition system are in tip-top condition.

Today new LT & HT components are not necessarily good, particularly those in green boxes. I have fitted Distributor Doctor dist cap, rotor arm & HT leads to solve misfires that persisted after so called new components were installed.

Richard.
Richard Thompson

Updates should always be added to the original thread, for the convenience of others coming along later.

If the tach is still jumping around when the problem starts, then given everything that has been done I'd be suspecting the earth wire. This is a very flexible cloth-insulated wire running from the points plate to the distributor body. It's continually flexed as the points plate twists back and fore under changing vacuum, which is why it is not a standard plastic insulated wire, but does eventually fail. Originals maybe available from Dist Doc, Moss Europe have an alternative.
paulh4

I really feel for you, having gone through this trying to bring a 79 spitfire back to life. It gets frustrating when you have tried everything.

Compression test is a good step. I am still putting my money on either the ignition or the carbs.

Have you sprayed carb cleaner or starting fluid around the manifold checking for leaks? If the car still has the egr, also look for leaks there.

Remember that just because a part is new does not mean it is working correctly.
Bruce Cunha

All the fuel and ignition components have been changed. Very frustrating that nothing has made any difference. Away for a few days so when I get back will start again trying to sort it out. Will start with compression check and take it from there. A couple of mechanics have had a quick look and they are baffled too. Will report back when some progress is made'
AdrianS

Is the tach still jumping around when it happens? If so it has to be ignition LT. What does the ignition warning light do when the tach is jumping?
paulh4

This thread was discussed between 31/05/2017 and 10/06/2017

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