Forum

[Presto]

This type of spark plug failure seems to occur fairly often but is a complete mystery to me. There is nothing in a plug to ‘fail’ unless it is damaged physically. <
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A plug can become contaminated with carbon and this creates the vicious circle of current tracking along the ceramic allowing the tip to cool and attract further carbon build up. This problem is normally is the result of a plug of the wrong heat range being used – a plug that is too hard and never attains its (very high) working temperature. <
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As for replacing plugs – in 50 yrs of riding (yawn!) I have very very rarely needed to change a plug – even if I have changed them for ‘fun’.

[PeteF]

Presto, minute cracks in the insulator can cause failure but it's difficult to see this. The spark takes the path of least resistance through the crack (especially if fouled with carbon) instead of jumping the gap. This often only happens during compression so you get a good looking spark when testing outside the engine.

[Presto]

I agree Pete - but I guess that comes under the heading of 'physical damage' that I referred to. But how those cracks or other such damage can be caused by a fitted plug is a mystery.

[PeteF]

Yes, I suppose it is physical damage. I guess it's caused by repeated heating/cooling.

[Graham43]

...yes but! I have had other bikes by other makers, none expensive bikes all messed about by me, and they don't need an emergency supply of new plugs. This Electra X needs at least two a year on a lowish mileage bike. Did 'British' Enfields have the same problem?

[Dennis C]

"did British Enfield's have the same problem?." Mine doesn't.

[Beezabryan]

Oil, in quite small quantities, will fark a modern NGK. Something to do with the pot insulator I'm told

[Davie Hall]

My electra x ate plugs when I first bought it. An iridium plug lasted about a week before it started misfiring. Eventually I realised that if I put ths choke off within a couple of hundred yards of starting it it was fine. The basic ngk plug has now been in for 7000 ish miles. No matter what I tried I couldn't get the old ones to work again

[Graham43]

Thank you all, a lot of interesting comments. On the positive side at least if it stops first thing I do is to change the plug. Dave, same with me I could not make the duff one work again.

[apparently lucky eddie]

It seems that every other NGK is a snide nowdays, so be aware. After endless NGK woes I now use Champions in my (old) bikes and they don't miss a beat. That said, my little car is now coming up towards 80,000 on the original ie. genuine, fitted NGK's, which have never been out of the engine.