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MG MGB Technical - Running problem help please!

I have just had a complete engine rebuild on my 1967 MGB and am experiencing some running problems which I would appreciate some thoughts on if anyone has any!

As soon as the engine etc is properly warm (so at least 15 minutes of driving normally), the engine will suddenly drop down to running on three cylinders. Dipping the clutch and letting the engine go down to idle (1000rpm) for a couple of seconds then means that I can bring the revs back up with the engine running on all four again.

This doesn't always happen - can happen three or four times during an hours journey, or perhaps not at all. Usually it has happened when driving at around 50 or so mph. One suggestion is that is a valve sticking in the guide (brand new gasflowed cylinder head with bronze guides), but the head has Kent double springs which should be strong enough to stop anything from sticking.

To give some background, the engine rebuild involved taking the block out to +40thou, rebuilding with Kent 717 cam, fast road distributor and LCB manifold, whilst the head has been gasflowed to stage 2, (although stage three on exhaust ports), skimmed, standard valve sizes, three angle seat cut to hardened seats, bulletted bronze valve guides and Kent double valve springs (small inside big springs).

Any thoughts as to the problem possibly being electrical as it is so intermittent? No other running/starting problems at all and now into the 400th mile of initial running in.

Thanks in anticipation of any help!
Martin
Martin Port

I had the same problem with a leaky vacuum advance hose. When it got hot and started to expand/soften was the only time the problem emerged. Of course, the increased revs were the only time when I noticed the bad timing (could be mistaken for 3 cylinder running), letting the revs off equalised things up again and got the timing back into synch.

I thought it was a fuel starvation issue for ages.

Other obvious stuff - dodgy spark plug, leads etc I'm sure you will have looked at already.

Liam
Liam

What do the plugs look like? You can usually hear a stuck valve easily enough - by a chuffing in the intake if it is an inlet valve or in the exhaust if it is an exhaust. It sounds quite different from simply a non-firing cylinder. You can learn what that sounds like, when the engine is otherwise running normally remove an HT lead from the cap and listen at the exhaust. When the problem is happening remove each plug lead in turn, and the one that *doesn't* make any difference, or makes by far the least difference to the idling speed, is the faulty cylinder. However I don't like running the engine with an HT lead disconnected as it generates very high HT voltages that can break things down.
Paul Hunt 2

The plugs are all mid-brown, so none indicating a mixture problem. However, I am going to try and pull over the next time the problem happens and immediately check the plugs to see if that is the cause of the problem. I'm also going to change the condenser, rotor arm and points just in case they are to blame.
Thanks for the advice so far...
Martin Port

Hi Martin.

I would also try changing the distributor cap and HT leads, I believe it is good to have a set of spares in the car anyway, they are not so easy to buy locally these days.

Don
Don

A timing light (inductive pick-up type) is a good diagnosis tool, clip it onto the coil lead and each plug lead. Flashes on the coil lead but not plug leads indicates a faulty rotor or cap. If it is only one plug lead missing it is probably the cap, if all of them probably the rotor, but you might as well change both. Flashes from all of them but still misfiring could well be a faulty plug. If you are *really* tight you can swap the plugs round to see if the problem moves with the plug, and so change just that one. But again you are better off changing all four unless they are nearly new and you have a rogue one.
Paul Hunt 2

Problem is still happening, but I've managed to do a couple of quick roadside checks. As soon as it occurred, I killed the engine and checked all of the plugs - no wetness. I also looked at the float chambers - both about 3/4 full. I've put new points, rotor arm and condenser in - no change. The one thing that I have noticed is that if it happens at higher revs (4,000rpm for instance), you get a lot of carb 'popping' from under the bonnet, and some backfiring from the exhaust - does this give anyone any more clues?

I recently came across another thread from someone saying they had a similar problem at 4,500rpm, and they had just fitted an uprated distributor which could be causing it. I have also fitted a fast road distributor, so could this be an ignition advance problem related to that?
Martin Port

Martin,
The ignition parts you have changed effect all 4 cylinders but you sound as if you think you are loosing 1 cylinder. As others have said the items that may effect one cylinder only are distributor cap, plug leads and plugs. I would focus on these next.

Given the changes you have made to the head, cam distributor etc. a trip to a rolling road to have it properly set up would not go amiss. Needs to be someone who understands these cars.
David Witham

I had a similiar problem that turned out to be problems with my almost new 9mm hi performance resistor plug wires.

It was hard to locate as the screw holding the electronic ignition sensor inside the distributor came loose permitting it to move. Once I got the distributor all squared away, I then focused on the plug wires. The break was near the distributor cap. I shove an inch long piece of wire up the distributor end of each spark lead and the misfiring problem disappeared and I was back to a normal system.

I'm running an aftermarket coil and .050" plug gap. The wide gap results in a higher voltage being required to light the plug. This higher voltage will quickly find the weak link in your system.

I've found that cold starting and running is dramatically better with the wide gapped plugs, and throttle stumble is almost eliminated in initial cold running with no choke needed except for the first 15 seconds. It won't idle for about 5 minutes.(Cold winter mornings here drop down slightly below 32f or 0C)

Barry
Barry Parkinson

Popping in the exhaust can be unburnt fuel exploding in the exhaust ignited from the next cylinder which fires properly. Implies an ignition problem. Take a timing light with an inductive pickup with you next time, they are an invaluable diagnostic.

But popping in the intake implies an intake valve sticking open, and with that I wouldn't expect *explosions* in the exhaust, but you would get an uneven beat as the faulty cylinder hasn't fired and so isn't expelling hot and still expanding gases into the exhaust. This is a distinct possibility with a rebuilt engine if one of the valves is a bit tight in its guide.

I wouldn't expect any plug wetness if you switched off as soon as it started happening, the plug will be red-hot even if it hasn't been firing for the last second or two.

Depending on your centrifugal and vacuum curves you will almost certainly have more advance at part throttle high revs than a wider throttle at the samwe revs. Problems with timing are also likely to cause all cylinders to misfire intermittently, rather than one regularly. It is an easy thing to eliminate - retard the timing by 2 or 3 degrees. Paradoxically it *may* make the problem worse, which would also indicate a heat related sticking valve problem, as running retarded ignition results in hotter running.

A slightly weak mixture also causes hotter running, richen both carbs by one flat/sixth of a turn and see if that helps. Only as a diagnostic, if it reduces it and the mixture was set correctly before I'd be taking it back to where I got it rebuilt.
Paul Hunt 2

Martin
If you get intouch with Chris Betson www.octarine-servicing.co.uk he Builds engines for a living and will ans your problems
GRAHAM

Martin, how old is your distributor cap? You could have carbon tracking, which shorts your ignition to ground. The unburned fuel builds up in the exhaust and burns later, which causes a bit of noise. You can check your cap and plug wires at the same time by testing the Ohms from the terminal inside the cap to the ends of the plug wires. All 5 wires shouls be within about 10% of each other, probably in the 5k Ohm range, depending on the make.

If you are still running points, I would recommend trying another set. I have had problems recently with poorly made "Lucas" points bouncing at anything over 3000rpms. I'd recommend trying another brand! You may also want to make sure there is a good ground from your points plate to the distributor body, as well as checking that the advance weights are well lubricated. This may be a good time to go electronic?

Jeff Schlemmer

Well, the car is back in the garage again - this time they are going to try and get the Crypton analyzer to work. Having said that, last night and after doing the first 500 mile oil change since the rebuild, I brought the revs up several times to touch the 4k mark. The first time, and the misfire happened as usual when I brought the revs back down again. Second time onwards, I had a slight splutter and then it ran faultlessly. I am thinking that perhaps it was a valve sticking in the guide and that it just needed running at higher revs to wear down the tolerances a bit more? Car will be back again tonight so we'll see if they pick anything up on the ignition etc.

The plugs, dizzy cap etc are all new and I have been rapidly replacing bits since the problem began, but with no success. Funnily enough, Jeff, I was running Lumenition before fitting the fast road distributor, but have now fitted points. If the problem persists, then I might refit the Lumenition to see if that has any effect.

Thanks for all of the help guys!
Martin Port

Martin,

Your rebuild head has been bronze valve guides equipped. Refering to P.Burgess book, it's highly preferrable not to refit the valves stem seals due to narrower guides/valves stems tolerances when hot ?
Renou

have them hook up a vacuum trasducer to monitor individual valves opening/closing. It only takes a second to see sticking valves. Of course if you have large backfires in intake they may not want to hook up vacuum probe as it can be damaged.Checking for sticking valve by looking at ignition waveform takes fine resolution and a keen eye.
Joaquin

I think I've solved it - at least I did with the help and advice of Chris Betson at Octarine Services!

The symptoms all pointed to carburettor icing - same point in the same journey, and always at the fastest moment in what are still pretty cold conditions here in the UK. So, a couple of flexible duct tubes fixed onto the air intakes on the air filters meant that I could draw warmer air into the carbs and... bingo! No splutter or misfire!

Since then, I found a pic on a later rubber bumper BGT where the air filters already had longer intake tubes manufactured to curve down and draw air from the manifold - was this a standard item on some of the later cars?

Thanks for all of the advice though - finges crossed that it stays like this and the solution has been found!
Martin Port

Amazing, I've often heard about carb icing but (like vapourisation) never experienced it in 40 years. All V8's, and North American spec cars from 75 with the single Zenith (and many other BL models of the era), had a temperature-controlled flap in the intake which drew air from around the exhaust manifold when cold and cooler air from elsewhere when warm (based on intake air temperature). This was supposed to allow a quicker warm-up and hence reduced pollution from running with the choke out rather than prevent icing, and BL got an award for the system. UK twin SU cars never got it. If it was icing it has to be a precise set of circumstances relating to your particular car, weather and driving. Otherwise it would be more common ... but then there would have been a factory mod to prevent it! My V8 (along with many others) has the K&N filter conversion as the original manifolds needed for the temperature controlled intakes break and are very hard to find, and so is breathing cold air (to start with at least!), but has never had the problem despite being a daily driver in all weathers for a number of years. Ditto a Marina which I ran for 6 years and 60k miles with a cold-air duct pipe onto the air cleaner.
Paul Hunt 2

Paul - Chris said that he has problems on his V8 BGT with carb icing, so what you said rings true. I actually used to get carb icing on this same car before the engine rebuild but it had to be much colder than it is at the moment. Again, it used to happen at the same point in the journey (different journey to the one I do now) after hitting some very cold air through twisty B-roads.

I guess that the fact that it is happening again now is due mostly to the only real change in the car that could be affecting the heat - the stainless extractor manifold as a replacement for the old heavy cast one. Since rigging up the ducting it has only happened a couple of times, but only in the last couple of days when it has been extremely cold in the mornings.

The forecast is indicating that the temps are going to rise into double figures next week so it will be interesting to see what happens then.
Martin Port

Hate to say it Martin, but I don't think you've killed your dragon.
1. There have been an awful lot of Bs with the same set up as your own, and they don't ice.
2. You've ducted warm air so no way should it still be happenning.
I'd go though the basics again and maybe concentrate on electrics for a bit. Is it still on twisty bits that it dies? Carb icing occurs at light throttle openings, so think long straights at a cruise rather than the throttle blipping of bends. Problems on corners bring back memories of smoking wiring. Borrowing a tip from Paul Hunt, what does the tacho do when the gremlin's at work?
Steve Postins

Steve - no twisty bits nowadays - the problem ALWAYS occurs on dual carriageway and motorway. Practically all of the electrical components have been replaced or tested now with no change - the ducting is the only thing that has made any difference. When it used to happen pre-engine change, it was always at 6.30am in minus conditions on consistently fast B roads (twisty or otherwise).
Martin Port

I was driving over some hilly terrain with the temp in the low 40's f and my car began to misfire after winding it up to 6k to pass a couple cars in 3rd gear.

It began to misfire. A few minutes of cruising at partial throttle and the problem disappeared. I'm thinking it may have been icing. Because of the hi compression pistons and modified cam I cut some big holes in the side of the air cleaners cannisters to let in more air.

It doesn't seem that likely, but I can't find any other explanation for the transient problem.

Barry
Barry Parkinson

I think that part of the problem is not so much the ambient temperature but the relative humidity - my V8 only does it on damp days and does it even when the temperature is quite mild. The evaporation of the fuel can drop the temperature of the mixture around 15 degrees.

I also think that fuel from some sources can contain a certain amount of water!

Martin - are you running on Shell Optimax or BP Ultimate? Or is it supermarket brew?
Chris at Octarine Services

Chris, I am normally running supermarket standard unleaded, and usually with Millers VSP Plus which boosts the octane rating a touch. Do you think it is worth keeping an eye on which fuel I am using and if it has any effect on the misfire?
Martin Port

This thread was discussed between 11/03/2006 and 26/03/2006

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