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MG MGA - Failing spark plugs

Here is an intriguing problem with my MkII 1600 coupe which has me completely stumped. 500 miles ago spark plug no.2 failed and the engine ran on 3 cylinders. Soon rectified by replacing all the plugs but now, 5oo miles later the same thing has happened, no.2 plug has failed. I've replaced it and the engine runs fine. I have electronic ignition. All thoughts on this baffling problem are welcome, thanks.
J Houlgate

I have never seen a spark plug just give up the ghost and "fail", so the chances that you have 2 spark plugs on your #2 cylinder that have suffered from premature death are very slim.

Did you change spark plug wires? Or just the plug?

My guess is something in the #2 spark plug lead is failing, whether it is the connector at the plug, connector at the cap, wire itself, or bad #2 terminal in the cap.

When you handle the wire to replace the plug, you are briefly clearing the open/high resistance condition.

The other explanation is that you are actually Bruce Banner, and when you get to the second spark plug you "hulk out" and crack the bastard.
Mark J Michalak

What kind of spark plugs? Resistor plugs can sometimes be a problem. Champion resistor plugs in particular used to have about 10% failure rate right out of the box (just a few years ago) due to too high resistance. If the resistance is too high they might work for a while, then fail when the resistance goes higher with some use. A little additional resistance in the HT wire or connectors can aggravate the situation.

If you are using resistor plugs or resistor or resistor wires or suppressor connectors, check them all with a ohm meter. Resistor plugs should be 3000-5000 ohms down the center conductor. Resistor wires should be about 5000 ohms per foot of length. Resistor connectors may be 3000-5000 ohms like resistor plugs.

MGs like non-resistor plugs, solid wires, no resistance at all. The only good reason for using resistor parts in the ignition HT circuit is for suppression of RF noise, and most modern radios are RF tolerant. The suppressor parts may be needed to keep a vintage radio happy.
Barney Gaylord

Some plugs have air gaps instead of resistors - they measure infinity all the time. I once had a plug with the bottom end of the center electrode not connected to anything - if you held the plug electrodes up, as you do usually to check it, the gap was right. But if you turned it over, the electrode fell down to zero gap, and of course the internal gap increased to compensate. Took about three tries to find that one!

Air gap plugs are used because the air gap becomes the limiting factor in spark voltage; as the voltage builds, a partially fouled plug will bleed the voltage off and you get no spark. The gap forces the voltage to rise to the set point, and when the spark jumps the internal gap it gets to the real gap and jumps it no problem, with no time to bleed off. Net result is that while normal firing voltage might be 8-15Kv, these make it 20 Kv all the time, a very nice spark for leaded fuel or lean mixtures or oil fouling etc. Some of the "anti fouling" gizmos they used to sell were just auxiliary gaps you put in the wire between connector and plug.

FR Millmore

Some reasonable stuff there however the best thing to do to start with is say what plugs you have fitted, and just to ensure things are as they seem.

How do you know it is number 2 that is failing what test are you doing?

Why do you not simply change number 2 for example?

Have ou tested simply swapping number 2 with number 3 and see what happens?
Robert (Bob) Midget Turbo

How failed? Gap closed up? Black and sooty or covered with oil and fouled? Just stopped working?

If just not working, I'd look at the plug wire - esp at the connection at the plug boot and the one in the cap.

If you have carbon-core leads, they can be flaky where the little sharp set screw goes through them to make the connection in the cap.

AJ Mail

Thank you everybody, I am digesting your replies though I think to start with I shall renew all the plug leads anyway. Any opinions about carbon leads versus old-fashioned copper ones, carbon are on there at the moment? In answer to Robert, I have tested the plug by holding it against the engine block and not seeing a spark, whereas I do get a spark on the others. The plugs are NGK. Thanks from John.
J Houlgate

My local auto parts store doesn't even have copper lead plug wires any more - all silicone or carbon core.

I bought mine from Moss, I believe, and whatever they are they seem to work fine...don't think they are copper though!

AJ Mail


My dial-in-advance timing light (the type that has a snap over clamp around the No.1 plug lead) only works correctly with the carbon/silicone leads. With copper core leads the timing light shows about half a dozen jittering timing marks.

Steve Gyles

OK John the point I wanted then to make was did you then put that plug into a different lead, thus simply checking the plug at fault or the lead.

If the plugs are NGK and are of the correct heat rating BP6ES hen I would suggest it is almost impossible for 2 to fail so qhickly. I have been using NGK for more than 30 years and have never had 1 fail let alone 2?
Robert (Bob) Midget Turbo

John, Jim asked the question "How failed? Gap closed up? Black and sooty or covered with oil and fouled? Just stopped working?" Are you looking at a clean plug or is it fouled? Picture please.

Lindsay Sampford

Information I came across regarding fouling & shorting out problems with modern plugs

Could this be the problem?

R A Evans

While I can't say there never were glazed insulator plugs, I don't think I ever met one. It used to be standard practice to sandblast them, and every shop, gas station, or parts store had the uni to do so.
In the late 1960s, when long reach 10mm plugs for my Bristol were not available, and they would lead foul, I spent a lot of time cleaning them and examining them under magnification. KLG and Lodge I had, some of them came apart. I finally settled on Nitric acid, because it eats lead glaze.
Now I routinely clean fouled plugs by heating the business end with a propane torch. Never a problem.

But, that is a cool place to remember!

FR Millmore

Gentlemen, I have read all the comments and links, thankyou.
Further playing about with this reveals that when the failed plug is held adjacent to but not touching earth it sparks strongly across its own electrodes and across a 10mm+ air gap. But when held touching the engine block the spark between the electrodes collapses and becomes a rippling spark around the central electrode which is something I have never seen before. This behaviour holds good for the failed plug on all leads. The failed plug is wet and sooty (to be expected if not firing) whilst the others are pretty much OK, if a little on the sooty side. I am forced to conclude that something in the ignition circuit of no.2 cylinder is causing plugs to fail, but I certainly don't know what. In the meantime I have ordered a new set of leads and distributor cap and, whilst I shall be relieved if they solve the problem, I dislike not really understanding why.
J Houlgate

You have just demonstrated what I said in my post near the top about spark gaps.
It is not necessarily or even probably the HT circuit for #2, as that is pretty much the same as for all other plugs. In any event, your spark gap result shows that the circuit is fine.
You have a combustion anomaly in #2 causing low level fouling of the plug. Once it is fouled, it behaves the same on all leads, but: The plug failure is the consequence, not the cause.
Check compression, valve clearance, signs of HG coolant leaks, Oil in cylinder from broken rings, or really bad (broken) guides.

FR Millmore

John, make sure you haven't got those awful, heavy-felt air filters. They caused me no end of trouble a couple of years back with symtoms exactly the same as yours. If you have, fit K&Ns and your car will be transformed!
Lindsay Sampford

This is what my plugs looked like. No.2 on the left, No.1 on the right. Plugs 3 and 4 looked the same as No.1 and the car ran like a 3 legged dog!

Lindsay Sampford

Thanks for your comments. My plugs are pretty similar to yours. Re. K&N filters, I already have them. I have replaced plugs, leads and distributor cap on the basis of easy things first and I already had them on order. I tested compression and rockers as Mr.Millmore from Pennsylvania suggested and these are fine. For now, we are running nicely and without that pulsing noise when pulling hard. It remains to be seen if it lasts but am going on holiday with the car next week so fingers crossed.
Regards from John
J Houlgate

This thread was discussed between 16/06/2012 and 21/06/2012

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