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MG MGB Technical - What now?

I rebuilt my 1980 UK spec roadster a few years ago. The engine was sweet, with good compressions, so the only change I made at that stage was to fit an unleaded head.
To be honest the car hasn't run well since. All seems good when I have set it up in the garage, revving well and accelerating strongly on the road. Yet after six or seven miles it suddenly begins to falter when making only slight pressures on the throttle.It feels as though it progressively moves towards firing on two cyl. and there is a light backfire in the exhaust. If I back off the throttle it can clear instantly at times or indeed if I depress the clutch and rev the engine it can also clear. It really is an intermittent fault....ok it happens every time I drive the car, but it can go for miles without a problem. Then I accelerate hard and it will buck and falter..so it can happen with both large and small demands on the throttle.
It was worse when I first put the car back on the road, dying and then not wanting to start at all until it had cooled down.
Since then I have replaced all LT electrics (even though the Tacho needle doesn't drop when misfiring)all HT electrics including new distributor, exchange carbs, new fuel pump. Checked for manifold and servo hose leaks all without result.
So my plea is where do I go from here? Paul Hunt suggested the possiblity of a sticking exhaust valve and I have thought of the chance of a carb piston sticking. Do I try a cheap pair of spare carbs first..or should I try a spare head? What's the chance of a sticking valve being repaired if it can be identified? Is it possible to identify it visually?
Any input gratefully received.
Phil

Did you have your existing head modified or did you change heads? If changed there may be defects in the new head such as a crack. In some cases if the machine shop overbores the hole for the new valve seats they may be loose in the head causing leakage around the insert. You may want to perform a compression test looking for variations on the same cylinder at different time periods. If you are having a valve stick you should also be able to hear a loud tappet at the same time as if the valve hangs up you will have a lot of rocker arm lash at the same time.
John H

I chased a similar problem for weeks, with the car recovering before I'd have time to pull over to the side of the road. In my case it was a lump of dirt in the fuel line just before it entered the carbs that was randomly starving the engine.

I can't say you have the same problem but I guarantee it will be something as basic. I can recommend a bit of very methodical troubleshooting and some decent luck.
Steve Postins

Have you tried making sure the carbs are in tune ?
I also have a problem like this with my TR7 when I had the original carbs on. Yes I own an MGB too, and for a long time.... Now I just recently purchased webers carbs out of neccesity for my TR7 and I am in the middle of setting it up so I cant tell you what fault it was 100% as of yet as the old carbs are no longer on the car. I actually am tracing down the possibility of a vacuum leak and eliminating it. Which may explain your symptoms. Also have you checked the dashpot oil? The symptoms would clear up on the old strombergs when the oil would fully drain out of one dash pots that had a bad seal. Then it would run great. Now with those old set of carbs I wasnt sure if they were needing just a rebuild or if the auto choke was faulty. Seeing that there are no replacements for the tr7 dual stromberg auto chokes I opted for a nice set of dual downdrafts. Now with that said I hope some of my rambling does some good... Those are the paths I went down and Ideas ive had of what the fault could have been.
James

The fuel lines have been checked and cleaned,new filter, carbs off an checked for debris then reset. Dashpots topped up.Set timing again yesterday. I note the engine pulled best at 13 deg. btdc rather than the published 10 deg.
The head was an exchange item from MGBHive.
Had a drive today...still missing!
Phil

Phil,

I have had problems with both my MGB and MGC dying when they were hot and then restarting when the engine cooled. The problem was caused by faulty rotor arms.
Some repro rotor arms are manufactured with too much carbon black causing breakdown down of the insulation when the rotor arms get hot.

Regards,
David
D M B LEGGEAT

Another common component that will fail when warmed up is the ignition coil. I have also had a faulty condensor that failed whenever it felt like but only for a few seconds.
John H

It sounds like something intemittently covering the fuel outlet in the tank.
Tom

Part of the symptoms could well be dirt being sucked to block a fuel line under heavy demand. While it is doing that switch off and bring the car to a halt. If you had HSs you could now simply lift the float chamber lids and see what was inside, if it was fuel not reaching a carb one will be lower than the others, however an additional possiblity with HSs is debris in the jet pipe that screws into the bottom of the float chamber, this is unlikely with HIFs. With HIFs removing the float dcchamber 'lids' isn't practicable so you will have to stop in a quiet area then turn on the ignition again. If the fuel pump clicks no more than once it isn't fuel starvation - assuming the engine starts and runs normally as well. If the fuel pump clicks rapidly then one or both float chambers has indeed been emptied and is refilling. If the engine won't restart it could be HT (assuming the tach was reading normally until you switched off) or fuel. Clip a timing light onto the coil lead and each plug lead in turn and watch the flashes while cranking. The coil lead will obviously flash four-times as often as any plug lead, but other than that all flashing should be regular and consistent. If it erratic on one or more plug leads it indicates the rotor, cap, leads or plugs are breaking down. If erratic on the coil lead as well then it looks like coil problems. If the HT looks OK, and it still won't start, remove the air cleaners and overflow plumbing and with a tube on each carb overflow port in turn blow gently into the tube with the ignition on. If the float chambers are full i.e. float valve shut you should blow fuel up the jet and into the body of the carb.
Paul Hunt 2

Before my last drive with these symptoms I blew through the fuel lines, fitted a spare SU fuel pump (this cured flooding problems in the rear carb caused I think by too much pressure from a Moprod pointless pump)and fited TWO new fuel filters before the carbs.So I'm pretty confident it's not contamination. I have a spare pair of carbs and I will fit these over the weekend to see how the running compares. I will change the HT components again if there's no improvement. As a last resort I could fit a spare head. Anyone like to comment on the possibility of problems with exhaust valves being the cause...the car was running well before I fitted a replacement unleaded head. Could the head be cracked? The car drives well for 6 or 7 miles before symptoms occur.
Phil

Phil,

It is not clear to me whether or not you replaced the ignition coil as John suggested. It may not be your problem, but it sure fits your symptoms.

Have you done the compression test, also suggested by John. A stuck valve should show up. Also, you should be able to remove the valve cover and possibly the rocker and push your valves open and let them close to see if they move freely. While that stuff is off you can chect to see if the valve stems are at the same height, (or at least all in alike and al ex alike) which they should be if the valve job was carefully done.

Charley
C R Huff

You might consider a sticking piston in one of the carbs. If you have a spare, you might want to change it long enought to check it out.


Dan Robinson

Yes the coil was changed a few weeks back, condensor, points, new dizzy. I will change the rotor again just in case.Compression is good...the problem is intermittent so any valve sticking is occasional not permanent.
My first task will be to explore Dan's suggestion and swap the pistons ans suction chambers from the spare carbs to see if one is sticking on the fitted pair.Fingers crossed!
Phil

Phil. Carb pistons are fitted to the individual carbs and may not interchange.

A running compression check can be used to diagnose a valve which is sticking. If you have one of the compression gauges having the rubber hose between the gauge and the screw in connection at the cylinder head. One spark plug is removed, the engine is started and the gauge read, pressure released, and read again for several repetitions. Running compression should be in the area of 80-90 psi. If you have a bad/sticking valve, you will get significantly lower compression readings. This check is performed on each cylinder, replacing the spark plug and connecting the lead on each cylinder that has been checked so the engine will operate properly for the test.

Also, if it is possible to do so safely, stopping, raising the bonnet, and spraying some carb cleaner around the joints on the intake system might be worth trying. If there is an air leak developing as the engine warms up, this test would find it.

Can the symptoms be reproduced when the car is sitting still in the driveway? That would make it easier to test out various things without having to make a test drive each time.

Les
Les Bengtson

We still don't know what carbs you have.

Moprod electronic pumps, if they are of a similar appearance to SU with the main difference being a plastic body compared to metal, work on the same principle as SU i.e. fuel pressure is only developed from the diaphragm return spring and so are very unlikely to deliver excess pressure.

Backfiring in the exhaust indicates ignition problems, missing ignition allowing unburnt fuel to accumulate in the exhaust to be ignited by a subsequent ignition. Relatively easy to detect if, once it starts happening on the road it will continue to happen when stationary, less so if it is only when under way but still possible.
Paul Hunt 2

The carbs are standard HIFs for the year. There is no sign of a problem when the car is in the driveway..only on the open road. There is no drop in the rpm guage when misfiring..but I have replaced all the LT components anyway as they are not that expensive..no change.HT parts have been replaced as well , but always possible a rogue part in there. Rear HIF was flooding badly even after fitting new float, pivot and grose needle. As soon as I replaced Moprod pump with SU all was ok.
I really would love this to be an ignition problem!! BUT having replaced everything I can think of , how likely is that now??
ph barton

Les thanks for the compression information, I will follow that up. If I replace the piston AND matching suction chamber from the spare carbs will this be a problem?
ph barton

With having done so much in the fuel and ignition department and still having the problem, I'd say it was an even bet that one of them still has a problem.

Do the test where when it starts happening leave your foot where it is but turn off the ignition and bring the car to a halt, then when all is quiet turn on the pump and listen for how many clicks it makes.
Paul Hunt 2

Thanks Paul, I will try that tomorrow if it stops snowing! What about the carb parts transplant...is it worth a try?
Phil

Phil - personally I'd try to prove it to either ignition or fuel first, changing things on spec could result in more confusing results, but if all else fails then it's a case of any port in a storm. Certainly change piston and cover together as they are matched, but if you can lift the piston with a finger (against damper resistance) all the way up and then it drops smartly when released it should be OK. You could check the inside of the cover and the edge of the piston to check they are clean and see if there are any rub-marks.
Paul Hunt 2

Thanks Paul, I swapped the pistons and suction chambers today after a good clean and managed an 8 mile uneventful drive before a huge snow shower made me run for home. I will give it a longer run in the morning. Fingers crossed for a cure...otherwise it's back to square one.
Phil

I managed a 50 mile drive today. All well but there was a hesitation briefly at 41 miles, shortly after I was stuck at roadworks for 10 minutes.Other than that it drove really well...so perhaps changing the pistons/suction chambers has helped.
ph barton

This thread was discussed between 01/04/2008 and 08/04/2008

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