Spark plug issue

[carrierdave]

Hi all,
Has anyone experienced issues with their spark plugs, whereby one or two plugs seem to stop functioning?
I have had a set of iridium plugs in for a couple of months and then the other day when I started the car it was only running on three cylinders. I changed the plug over for a normal one and she was fine.
I've just been out to run the engine and found the standard one I put in no longer functions and another plug has failed.
I've now put another set of new plugs in and she runs perfectly. What I have noticed is that if I pull the lead just off the plug you can hear it spark and the engine seems to pick up.

Any ideas?

David

[Chris]

Hi David.
Can't give a definite cause or cure, but starting from cold, if your car is having to turn over until your carbs are primed, then this is sufficient time for your plugs to become wet with fuel, oil or both. I had a similar problem some time ago, and the cure was an electric fuel pump which ensured the carbs were primed before the engine turned. Also, if not fitted, a cold start coil will give a fatter spark.
Good luck
Chris

[carrierdave]

Many thanks for your thoughts Chris. One other item I didn't mention, when I first had the problem with the iridium plug, I removed it from the engine, put in a replacement in to keep the engine stable and then connected the plug lead to the iridium plug and earthed it on the engine. I then started the car and observed no spark!
I put the lead onto the new plug in the engine and she ran fine. I also checked the other leads on this plug and nothing.

I haven't tried the other plugs tonight, but will have a look at them tomorrow and let you know.
When I took the plug out it was wet with petrol, so your idea of the wetting of the plugs when priming the carbs maybe part of the problem.

David

[Panda]

Hi David,
The plug is definitely fuel or oil fouling. The trick of pulling the ht lead off and creating a small air gap allows the coil to build up a higher voltage which is often enough to fire across the plug gap rather than dissipating down the insulator. Many years ago when I used to fix lawnmowers, I had a test wire with a small gap which used to work well when wired temporarily into the circuit. Try a hotter spark plug if you don't drive the car hard and everything else is ok. Don't let it idle excessively and give it a bit of full throttle up the nearest hill as often as possible to burn off soft carbon deposits.
Alan P

[Pistacchio sprint 72]

What is the tip of your spark plug looking like?

[AHM]

Have you tried the plug that doesn't work on the lead for one that does.

[carrierdave]

Hi Alan - many thanks for the information it is most helpful. Could I ask, what spark plugs do you use? Or what ones would you suggest?

I have tried the cleaned plug on another ht lead/cylinder and there appears to be no spark at all.

I think I will try a different coil and spark plug.

David

[carrierdave]

Sorry forgot to say, the iridium have a funny pointed electrode and the new set of spark plugs are just the standard BP6 ES and haven't been in long enough to show any signs of wear.

David

[Chris]

It's a long shot, but take a look at your rotor arm. There are some with a resistor between the central pick up and the lobe. This caused a failure for me, when the resistor burned out giving lack of electrical continuity. This was preceded by a period of bad running.
Chris

[carrierdave]

Thanks Chris I'll have a look.

David

[TonyJ]

Which cylinder did the spark plug come from?

I found my number 4 (nearest the windscreen) spark plug either caused a misfire or packed up relatively quickly. The engine would run rough until hot. Probably because of the breather being connected to this side of the air box and so oil was being sucked into the cylinder and fouling the plug. I now have a Cliveboy breather that doesn't connect to the air box and this trouble has gone away.

Another benefit of the new breather is that the airbox is oil free and doesn't drip oil messing up my clean engine bay . There seemed to be lots of oil in the airbox but I never noticed any significant reduction on the dipstick. I have no idea whether this is a normal occurrance or not.

Tony

[au-yt]

Which cylinder did the spark plug come from?

I found my number 4 (nearest the windscreen) spark plug either caused a misfire or packed up relatively quickly. The engine would run rough until hot. Probably because of the breather being connected to this side of the air box and so oil was being sucked into the cylinder and fouling the plug. I now have a Cliveboy breather that doesn't connect to the air box and this trouble has gone away.

Another benefit of the new breather is that the airbox is oil free and doesn't drip oil messing up my clean engine bay . There seemed to be lots of oil in the airbox but I never noticed any significant reduction on the dipstick. I have no idea whether this is a normal occurrance or not.

Tony

Tony
Interesting tip on the breather I will look into that ,

I have found that the O rings on my car had an air-leak. This causes a weak mixture in the idle circuit that comes good post transition to the main circuit. nipped the nut on that choke a 1/4 and the mixture screw was responsive. sorted


Spark Plugs a recent experience with a 70s Maserati fitted with iridium plugs, they work well with tightly controlled fuel air mixture, not a V8 with 4 DCNFs, a change to a hotter classic NGKBP5ES transformed the tune. I will concede part of this could be just new plugs.

Graeme

[rgh0]

The exotic metal tipped plugs ( iridium, platinum etc) are designed for long life in modern cars to meet emissions regulations that require plugs to last a much longer distance ( 100000 kms plus). This life is possible with these plugs as the mixture control is so much better in a modern fuel injected engine and the exotic metals avoid tip erosion over a much longer distance of driving.

Unfortunately the will fail from insulator wetting and fouling just as easily as a standard plug in a carburetted twin cam but at 5 times the cost you get reluctant to replace them. I buy standard NGK BP6ES and just swap them when ever I have a issue.

cheers
Rohan