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MG MGB Technical - Erratic firing #3 cylinder

Hi,

Having now done a few hundred miles in my MK1 B, with an 18GB engine, I decided to give it a tune up, as it was idling a bit lumpy, but runs well.
Set the timing, and started on the carbys. For some reason I used cylinders 2 & 3 to set the fuel mix, using a colortune. #2 set up fine, then #3. It was firing erratically, ie missing every so often. Put the timing light on the ignition cable (clip on type) and the pulses were very regular, so not ignition. Scratched my head some and decided that I had a leaking manifold gasket or similar problem to do with the carby. Then remembered that #3 and #4 were siamesed and any leaks affecting one would also affect the other. So I put the colortune into #4 and lo and behold smooth firing. Back to #3 and still erratic.

At this point I set the mix on #4 and went away to think, and to write this.

My thoughts could only come up with a valve problem, as they are the only things unique to #3.

My questions now are can it be anything else? Do I have a sticking valve, and how do I check? If I do have a sticking valve how do I fix it?

Other observations:
No discernable backfiring, popping or pinking.
On the road the car runs well.
Idle speed can vary from about 700 to 900.
Long time running on idle the temp sits at N, running on the road it drops to a bit below N.
Oil cooler fitted
New 123 GB 4RV distributor on curve B, new coil, new ignition wires
At idle the timing wanders a bit
Harmonic balancer timing mark as spot on as I can determine

TIA

Herb
Herb Adler

Sticking valve - check the clearances?
Art Pearse

Herb. I agree with your original diagnosis--air leak near the rear of the intake manifold or near the rear carb. Note that number three cylinder, sharing an exhaust port with number two cylinder, commonly runs slightly leaner than cylinder four which has its own, dedicated, exhaust port. A slight air leak will cause a "lean miss" in number three (or number two when it is the front carb system) cylinder which results in the symptoms you describe. Also check the end of the intake manifold for a loose welch plug on the end and the top for any loose plugs.

Les
Les Bengtson

Herb -
All of that, plus bad plug, bad dist cap.
If it is a lean misfire it should go away if you set it a couple flats rich.
Sometimes you can see a sticking valve if you run the engine and use a timing light to watch the valves; try different leads to hook the light to so you see the valve in question illuminated when closing. Fuel injector cleaner squirted on the valve stem while running sometimes frees them up, but not likely if you have modern seals fitted. If it has bronze guides fitted they may be too tight.
We had a whole big thing on the MGA board a few months - year ago, all about distributor problems on Steve Gyles' car. But your new dizzy shouldn't have those problems.

FRM
Fletcher R Millmore

A thing I would check is the carburettor balance. You may be drawing slightly more air through the front carb weakening the rear cylinders. Three and four will still run as they will be supplemented by the balance tube. The effect it can have on the rear cylinders may be a little different three as to four because four follows three in quick succession. (If all else fails worth a look)
Also you mentioned a bit of spark wander in your post. I find with my dial back timing light I get a bit wander if my inductive pickup gets too close to other leads.
Denis4

Herb
As with any tuning problem cover the basics first.
You have established good spark at the leed--try another sparkplug
Check valve clearances-also are your rockers in good condition-specially the face that contacts the valvestem.
Having said this I've got my money on carburettor balance-- ie- The front one sounds like it is doing all the work leaving the rear to misbehave.
With your timing-most MGB's are already starting to get some mechanical advance at idle and will float around a tiddle the ideal is to set it statically. If it is really bad it may have damaged or rusted out springs in the dizzy-- wouldn't be the first one to have that happen to. Willy
WilliamRevit

A valve sticking open should definitely cause a distinctive popping i.e. the sound of combustion in exhaust or intake, quite different from the noise a simple non-firing cylinder makes which is more an absence of a exhaust pulse.

An incorrectly firing plug, either from an HT problem or a mixture problem, should show on the plug, being black or oily for the former and burnt or white for the latter.

With a 123 problems with springs and weights won't be an issue, but unlike mechanical distributors they don't have a smooth advance curve but operate in a series of steps when each rev point is reached. therefore if your idle is close to one of the step points it could well be seen to be flipping back and fore between two distinct values.

It's not clear which lead or leads you clipped the timing light onto, you need to compare 3 and 4 as a minimum, if not all four plus the coil lead. Even with regular and consistent flashes on all leads it could still be a problem with a plug, so swap those around and see if the problem moves.

But if you are saying it ran smoothly with the Colortune in 4, i.e. in place of the plug, but you got the misfire when the Colortune in 3, then that indicates No.4 plug is faulty.

But then again if you are saying that when in No.3 the Colortune was erratic, but showing the correct colour when it *did* fire, then it can't really be a vacuum leak, I can't really see that coming and going like that, and only affecting one cylinder to boot. That's more like an HT problem.

Try weakening and richening the mixture, just to see what to see what happens.
Paul Hunt 2010

Herb: A quick test for a vacuum leak is to spray some carb cleaner around the base of the carbs and intake manifold with the engine idling. If there is a brief change in engine idle, it is a good indication of a vacuum leak. If there is no change, then you can pretty much eliminate a vacuum leak issue.

cheers

Gary
79 mgb
gary hansen

Hi Guys,
Thanks for your quick replies. Unfortunately I won't be able to do anything for a week or so, off to the hospital for eye surgery tomorrow.

I won't address each reply separately, but just some general comments.

I don't see how siamesed exhaust ports could possibly affect the mixture of #3 cylinder, as when #2 is exhausting #3 is on its compression stroke, ie the exhaust valves don't overlap. The only effect that I could envisage is a scavenging effect, but this would reduce the size of the charge in #3, not the mix.

The erratic firing in #3 was discovered with the colortune, some flashes, then no flashes then some more flashes etc. #2 and #4 showed consistent flashing, so unlikely to be plugs. Will now check #1.

I had tried enriching the mix, as part of the tuning, ie set to rich, by the colortune, then back off till you get the bunsen burner blue. #3 was erratic the whole time.

The timing light was only clipped to #3 lead, as this was my first test to determine if it was an ignition problem. Timing light flashed consistently, whilst the colortune was erratic, both being used together, hence I ruled out ignition.

As I won't be able to do anything for a couple of weeks , I can't try out any of the above, but, please, if anyone has any other ideas please post them.

With the 123 dizzy, the advance is 10* between 500 and 1000 rpm, so unlikely to be caused by a step change in the curve, as my idle is set somewhere around 800.

The carbs should be balanced, as I use a flowmeter for this, because I can't discern any difference with the hissing sound using a tube.

One other symptom that might have a bearing is that the valves are set to 15 thou, but one valve is noisy when the engine is running, this could indicate a sticky valve, being "slow" to close. Will check this out further.

Thanks

Herb
Herb Adler

Herb -
A slow closing valve will reduce compression, possibly to the point it won't fire, and especially at slow speed. If the valve is sticking only at a near closed position, it will have little obvious effect at higher speeds, other than being a bit clackety. If I am not mixed up, #3 ex should be closing about the time #1 is firing, so any irregularity of #3 ex ought to show up using #1 lead for illumination.
You can also identify which valve is noisy by putting a feeler gauge in that one with the engine running.

FRM
Fletcher R Millmore

Herb Its not the Siamese exhaust ports that effect anything in low speed tune it is the Siamese intake ports and the pulses they produce in the intake ports effecting cylinder combustion. It is firing and developing power on number three or you would notice a miss, just altering the mixture enough to effect the color tune. Denis
Denis4


Does the ticking rocker tick all the time or only when the missfire is happening-- If it is only when it misses it could well be a sticking valve - Has the head been rebuilt with stainless valves by any chance as they do need a little extra stem clearance
OR the surface of a cam follower might have worn out of true.
If it is a camfollower it will idle ticking and then go quiet and miss then tick and run ok then quiet and miss as the follower spins in it's bore and touches on irregular surface points.
Cheers Willy
WilliamRevit

Herb, only because I had a miss firing issue similar to yours I'm mentioning this. Check and make sure you don't have a slight scratch or and I know this'll sound funny, a cobweb or finger print, inside the distributor cap around the proper terminal. A finger print will attract and hold moisture for a short period of time. Just a thought. Sometimes the simplest things can cause the hardest problems to solve. PJ
Paul Jennings

"The timing light was only clipped to #3 lead, as this was my first test to determine if it was an ignition problem. Timing light flashed consistently, whilst the colortune was erratic, both being used together, hence I ruled out ignition."
Paul Hunt 2010

Hi,

Just a short update.
Had a quick look at the valves and #6 (#3 inlet) was a little bit clacky, but so was #2. Colortune showed more consistent firing than previously.
I couldn't see any sticking of #3's valves, but then a very small "opening" wouldn't be very apparent.
I will wait till I pull the engine / gearbox to replace the gearbox and then also remove the head and check all valves.

Herb
Herb Adler

This thread was discussed between 28/03/2010 and 05/05/2010

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