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MG MGB Technical - Ignition problem?

Some time ago I had an intermittent problem while on the move, where my 1980 UK standard spec roadster would occasionally falter and misfire with a backfire in the exhaust.It would clear after a few seconds. (see thread "What Now" 8 April 2008 )
The problem has returned with a vengeance but now is apparent from starting.When cold the engine will start, but barely runs, seemingly only erratically firing on perhaps two cylinders.If it is left to run like this it slowly improves, with the misfire reducing as the engine gets warmer.However until the engine is completely warm, any attempt to open the throttle leads instantly to serious misfiring and backfire in the exhaust.The tacho only drops with the revs and does not fall to zero.Eventually the car revs freely.
I considered it must be HT or fuel, so I have replaced the carbs with a spare set and fuel delvery is OK. Once warm I checked the timing was spot on and got the carbs to a reasonable state of tune. So i'm pretty sure it's not a fuel problem.
Before I reset the timing I removed the distributor (a new item not long ago)and reset the points, replaced the condenser and the rotor arm and cap with new units.The plugs were also replaced.The leads are very recent.I've checked the voltage at the coil and substituted another coil. with both of these coils on the bench the resistance was the same.
A friend has held the coil and HT leads to the block and reported a spark from each to earth.I did put a "colourtune" plug in the engine and observed what appeared to be quite erratic flashing in the cylinder.It was also quite wet when removed.
The symptoms reminded me of those described by Paul Hunt in his piece on condenser failure, but that must surely be discounted now.
I have only had these problems since the head was exchanged for a lead free item, but there is no sign of a sticking valve.
So i'm at a loss now what to do. Why does it disappear when the engine warms up? What changes..any ideas gratefully accepted.
ph barton

PH - If the valve guides in the head were replaced with bronze guides, it is possible that the clearance of one or more of the guides is not sufficient. The bronze guides need more clearance than the standard guides do and with insufficient clearance, the valves may be sticking. Cheers - Dave
David DuBois

PH I don't know how the 1980 UK spec emissions differs from the 74 US emissions. I had a very similar experience to you with my 74 B GT that has an air pump, air rail, check valve and gulp valve and HIF carbs. I replaced everything on the ignition side like you have and then started with the smog stuff. My check valve between the air pump and air rail wasn't working which allowed compression gases back into the air pump system (I don't know how much this mattered) but I replaced the check valve. The gulp valve wasn't functioning which causes back firing on the over run, so I replaced that. I also noticed that previously I had installed new hose clamps on the fuel line between the 2 HIF carbs, and installed them with the clamp mechanism on the engine side. This prevented the choke mechanism from opening more than 1/3 open. After I turned the clamps around the choke could open fully. After all these changes my engine now starts again and runs well from cold.

Andy
Andy Preston

PH. You have ruled out the ignition electrics and fuel as the cause. Although his Californian car has a much modified inlet/breather system Andy's advice points at the inlet gasket. In short, you have had the head off. The inlet manifold may be loose when the engine is cold allowing the engine to suck in air. As it warms, the gap closes and the air is choked off allowing the engine to run normally.Try to tighten the securing nuts a little and see if it improves running. If the manifold is loose, the nuts will move easily. If this is the cause you will need to refit with a new gasket.
HTH
R Walker

i agree with R Walker, i had/have a similar problem with my vauxhall. When cold and idling it is incredibly lumpy and after removing cylinder 4 HT lead it had no difference in the running of the engine. It got worse over time and ony just over a week ago i placed a finger behind the inlet manifold and it tried to suck my finger in. The inlet manifold gasket was shot. Ive ordered a new one which i will soon fit, but when the car is running under load and warm its not as bad. Vacuum leaks can be a right pain in the a*se. Also do it quick as you may get cylinder washout causing premature wear in the bores.
A P New

PH, something else that I did which I did not mention in my previous post was that I removed the inlet and exhust manifolds and had them surfaced to ensure that the were flat and had the same flange thickness. The inlet manifold had a concave depression around one of the inlets and the exhaust manifold was warped. This ruled out any leaks around both manifolds. Every little bit helps.

Andy
Andy Preston

I will check the inlet manifold, although when it was happening a year or so ago on the move I did look into that possibility and sprayed carb cleaner over the manifold to try and detect a leak. There was nothing obvious to see or hear.
I am going to check the resistances from Dist. cap to HT leads later to see if there's a clue there.
If you look at the previous post you will see that it was then a "sudden onset" problem that soon disappeared. With that in mind and the present problem that slowly disappears with engine temperature, is there something in the electrical system that could produce these symptoms?
ph barton

Erratic firing of a colortune definitely indicates HT problems, even better then clipping a timing light onto the coil and each plug lead, however the benefit of the timing light is that it is non-invasive i.e. you are unlikely to disturb an intermittent connection.

It depends on how far away the leads were held from the block, if they were closer than a plug gap then you could be getting breakdown of the cap or rotor with larger gaps, you should be able to draw it out to about 1/4". If it only sparked with a small gap then it could be condenser failure.

Whilst backfiring in the exhaust - intermittently and just as the engine picks up again - can be a sign of ignition failure, it can also be a sign of a sticking exhaust valve if it happens regularly, although it normally causes a regular chuffing in the exhaust or intake, different to a non-sparking plug, remove the plug lead from each cylinder in turn and compare.

You may be able to diagnose sticking valves with an adjustable timing light, funnily enough. A pal had a spate of only firing on three on a midget 1500, and by clipping the light onto a particular HT lead you can use the adjustable control on the light to 'freeze' each valve anywhere between fully up and fully down. All his seemed OK, and the problem went as suddenly as it arrived.

Another trick is to remove the plug leads and gradually increase the gaps, the bigger gaps causing the HT voltage to rise higher and fire what might be a dodgy plug (new set noted) or combustion characteristics.
P Hunt

I got the engine running yesterday, but again it was severely lumpy,barely turning over, gradually becoming smoother until it was normal as the temperature reached normal.
I took it for a run around the block and it pulled well, although I had a mild shock as I pulled into my garage. I had left the engine running and as I raised the bonnet/hood fuel was spurting everywhere! I turned the ignition off and left in a hurry, fire extinguisher in hand!
Once the fuel had evaporated away I found that the original fuel line between my old spare carbs i'd fitted had a 2 inch split that must have happened on the drive.I restarted the engine with fuel pump disconnected until the carbs ran dry, and with strobe on each HT lead and the coil lead in turn I got a regular flash as normal.
This morning I replaced the fuel hose. With the carbs dry I again checked each lead with a strobe and got a regular flash from all. After reconnecting the F. pump the rear carb flooded but after turning the ignition off/pause/on it stopped.
Trying to start the engine from cold produced exactly the same symptoms as before except it initially burst into life and after 2 seconds fell straight into severe misfire mode. I sprayed carb cleaner on the inlet manifold but there was no increase in rpm or evidence of a leak.
The engine stopped several times and the strobe showed normal flashes when the starter was operated.The engine was shaking a lot and turning over at a slower speed than when on the starter. It was not showing any reading on the tacho although this often sticks for a while on starting.
So where do I go from here?
I have a spare head that I could put on as a last resort to check for valve problems, but my gut feeling is that there is an electrical fault. Could there be an LT problem? Why is engine temperature significant?.....getting desperate now!
ph barton

As an add on to the last post, I did have the "colourtune" plug in no.1 cyl when started from cold this morning. The strobe was also on that plug lead.I had previously thought that the plug was firing erratically but now i'm not so sure. The engine was turning so slowly and unevenly it may have given the impression it was erratic. Certainly when the strobe lit the colourtune always flashed at the same time. The engine was turning over more slowly than when spun on the starter, it was firing so bad.The strobe flashed evenly on all leads when spun on the starter.
ph barton

Phil, I too had a similar problem in my 79 last year. The coil lead was working its way out of the coil and forcing a spark to jump from the coil terminal lead. It started up fine but after 2500 RPM the engine started voilent backfiring especially under load.

The other item may be the condensor. I have had brand new out of the box condensors fail especially after it warms up After warm up the condensor would fail and cause the tach to jump and engine to backfire.

So check the connectivity of the coil lead and all leads to ensure they are seated properly and try a different condensor.

cheers

Gary
79 MGB
gary hansen

Thanks for the comments so far...will re-check lead connections.
Keep any ideas coming!
ph barton

Check and clean the plugs, replace and restart and keep running in miss-fire mode as long as you can. If it looks like it is starting to run better, or can't be kept running any longer, take the plugs out again look at the condition. If one or more of the plugs are sooty that indicates an over-rich mixture, which would normally affect a pair of plugs, and would normally be caused by a carb. The rear carb flooding points in the same direction. If one or more plugs look still clean then it may be that they aren't getting any fuel. Perhaps a better test for that is to clean the plugs, disconnect the coil, crank for 30 secs or so as if you are trying to start it, then take the plugs out again. Plugs getting fuel may be wet i.e. flooded, and certainly should smell strongly of fuel, dry/no smell plugs aren't. It does sound more like fuel starvation on one carb than flooding, if it seems to get better as the engine warms up.

Is the choke cam in each carb working the same? Unfortunately with HIFs (I'm assuming) you can't see the actual enrichment device, which is an internal valve with O-rings, which can wear out and leak.
P Hunt

I will try that this morning Paul. I went over to Summit MG in Maidenhead yesterday, and their view was (without seeing the car)it is a carb. problem OR a fault with the cyl. head.as the symptoms have only been apparent since it was fitted.
The symptoms were identical when I replaced the HIF carbs with a spare pair. Apart from giving them a good clean I did not change any mixture settings from when they were previously on another car.I would not expect them to be so far out of tune to cause identical symptoms.
Remember the car was starting perfectly normally and running pretty well( subject to an occasional faltering and backfire)...so if the mixture was undisturbed, would it overnight suddenly start displaying this very great change?
ph barton

If there is one thing I have learnt in 40 years of tinkering with cars like this is that you can't assume anything, especially on a car you can't see, which is why I always strongly recommend diagnosing from first principles, even when logic tells you such a thing can't have failed. And I'm still learning.

I can't remember the whole thread when I suggest something, but I do agree that the same symptoms with two sets of carbs do tend to rule out the carbs themselves, but perhaps not the way they have been installed or adjusted, and is also why I have suggested ignition and valve diagnostics.
P Hunt

I did as you suggested today Paul, and cleaned the plugs before attempting to start. The engine fired but again was extremely lumpy and died after about 30 secs. I inspected the plugs which were all sooty again. All smelled of fuel and the rear two plugs were a little more fouled than the front pair.The engine was fired up again with choke ( it would not run without )but I noticed that shortly after, pushing the choke fully in made it run smoother.
Eventually got smoother as the engine temp increased.
I adjusted the jets to two turns below the bridge and got the idle and balance reasonable and took the car around the block...accelerated well in 1st. & 2nd. gear but in third when I opened the throttle the misfire started in ernest although no backfire this time.
I will have to try and set the mixture with a friends CO meter tomorrow. I hope this may make a difference.
ph barton

A CO meter is no good as that will only measure the average mixture from both carbs, one could be way rich and the other way weak. That can only be used after both carbs are fully balanced, and then only to adjust both carbs by the same amount in the same direction to achieve an emissions figure, if required for an MOT.

If the correct mixture is not close to two full turns down then there is something wrong. But that is only a starting point, you then need to adjust each jet in turn to get the highest idle speed. Richer and weaker will make it idle slower, so you should be able to turn it from rich, through correct, to weak and back again to find the highest idle. If you can't i.e. fully up or fully down it is still improving, or running really badly everywhere, there is something wrong. If you have to turn it too far from the starting position there is something wrong, it should only be a quarter turn or so. With that set, you use the lifting pin and very small movements of the jet until lifting it 1/32" causes the idle to momentarily rise then settle back. If the revs stay up it is too rich. If it tries to die straight away it is too weak. And all that is after getting the air-flow balanced between carbs with a listening tube or balance meter.

P Hunt

Thanks Paul
I borrowed a volt/ohmeter and ran some checks on the power at the coil. An article from a very old "Enjoying MG" on coil ignition suggested some test for a late car like mine with a resistance wire ballast and 6v coil. Test 1 with contacts closed, ignition on and the voltmeter between earth and the +ve terminal suggested I should see a reading of 6v, however the reading I got was 0.5 v. When the starter was operated the voltage was going up to 10/12 volts.
Voltage at the -ve terminal and earth, contacts open ign on showed battery voltage.
On the last few starts from cold the engine has burst into life normally but almost instantly faltered massively barely running at all. As described earlier the car running gradually gets smoother as the engine warms until at normal temp it appears to run well except for occasional misfie/backfire on acceleration particularly in third gear.
Could it be that the problem is a very weak spark (once the starter cuts out) that is barely able to ignite the charge when cold. As the engine warms this becomes less critical and the engine appears to be running well? Is that why a spark to earth from the HT leads looks strong when the engine is cranked? How can I run further tests to isolate the problem if the ballast wire is hidden in the loom?
I measured the resistance between the coil terminals with wires removed at 3.7 ohms..is this correct?
I also noticed a fairly thick black/white wire coming out of the large loom cluster to the rear of the coil which had a rubber bullet terminal on the end but wasn't attached to another wire....can anyone identify it?
Your advice gratefully recieved
ph barton

Have you fired the car up at night to see if you have stray sparks around the engine bay?

I had a problem with a hairline crack in the head, a fine spray of coolant shorted out an HT lead. The amount of coolant was so small that it instantly evaporated. Only diagnosed at night when you could see the stray sparks.
R Fowler

As an update since my last post I have had the cyl head off and replaced it with another spare one I had, just to see if it was a sticking valve or cracked head. Sadly there is no change in the symptoms...after a fairly long crank the engine does burst into life momentarily and again almost immediately starts a very serious misfire.
I did manage to keep it going enough for a warm up and short drive and reset the carbs again, but it was hesitant.
Cleaned all connections and found power to coil 5.78v.
There was an article in "Enjoying MG" recently that claimed the resistive wire in late UK spec cars can give problems...although it didn't give any detail other than suggesting a 12v coil wired directly from the fusebox to eliminate it.Has anyone experienced a problem like this? Presumably a 5.78v reading at the coil without the engine running is OK? It's just the way it fires up OK but starts to misfire a few seconds after the starter cuts out that makes me wonder.
ph barton

I have read this article too. There were mainly two kinds of coil, 1.5 ohm for 6V power and 3 ohms for 12 volts, voltage mesured at coil conectors. The difference resides in the wiring, on RB cars, there was either a ballast short circuited for startup, and in serie with coil under normal conditions or a balast resitor + resistive wire included in the coil circuit. There was another variant as stated by EMG 9V coils, but I never saw one. The simplest rule: if you have a 12V feed coil you must read 3ohm with a multimetre, and a 6V feed coil will match a 1.5 ohm reading. Some electronic ignition kits need 12v to run properly too. You may a a look on advanced autowire site were electric drawings are available in pdf format files for every MGB.
Rgds.
Guy RENOU

I've seen the figure of 1.5 ohms expected resistance in a ballasted coil, but I have a resistance of 3.7 ohms across the terminals wires disconnected.Today I replaced that coil with a new one but no improvement.
ph barton

Philip,

I bet for a 80 B the coil resistance should be 1.5 ohm.
If yours don't match that figure, your coil primary voltage will be half the intended value and so on for the HT one. But current will be twice the designed one, and coil must excessivly warm on time.
Guy RENOU

Philip,


Got back EMG article.
Please try to apply the given solution for a 12V coil. Connect a wire from fuse 2 brown/white wire and feed the coil directly in place of the original white/light green wire. Just a simple test to carry on.
Rgds
Guy RENOU

Thanks for all your help. Despite having changed the dist. cap, rotor, points, condenser, leads, coil etc. I only just managed to keep the engine running enough for it to warm up slightly yesterday. I got a strobe on the coil lead and found the pulse to be very erratic . Also on leads 2 and four. Again as the engine got to temperature the symptoms disappeared.
In desperation I had bought a new distributor off EBAY (a bargain at 36 pounds albeit pattern)and once the engine was warm I fitted it.
Bingo! The engine now runs smoothly and started first time later from cold.
So the distributor was causing the problem even with the new components. A lesson learnt there....any ideas as to what the fault was?
ph barton

My guess is that the distributor shaft bushings and and the distributor cam itself are worn, causing the spark to jump around and fire at random. RAY
rjm RAY

Philip,

Might be interseting to fit your original distributor cap onto the good one, jsut to see. Root cause of issue could be only a wrong carbon brush/spring?
What do you mean by erratic pulse, is it disappearing and back or fluctuating on time?
Guy RENOU

This thread was discussed between 25/01/2010 and 03/03/2010

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