Rapid plug failure in Chummy

[Ruairidh Dunford]

I have found NGK B5HS to be a good option on high compression heads.

[John Cornforth]

Hi

All those plugs look very oily, so maybe it's a bit unfair to blame the plugs themselves. Few of our engines get hot enough for long enough to get the insulators up to the self-cleaning temperature. I too have had good results with NGK B5HS, the 5 rather than 6 heat rating helps in this respect.

I have recovered fouled plugs in the past by heating the tips up with a gas flame till they are dull red and allowing the oil/carbon mix to burn off. A garage would never do this as it's not economic at £100 an hour, but if your time is free there's nothing to lose. Beware of wire brushing or scraping as this can just embed particles of metal into the insulator ceramic which is the last thing you want.

I suggest you double check the rest of the ignition system, as a weakened spark is less likely to fire a marginally fouled plug.

If you test plug insulation resistance with a multimeter, any reading of less than 1 Megohm should be treated with suspicion. Anything less than about 200 kilohms and it probably won't fire. However, this is not definitive, as testing at a few volts in this way won't show up a plug which only fails at high voltage. By contrast, I have checked good plugs at 1000 volts DC with an insulation tester and the resistance was above 5000 Megohms.

[Ian Williams]

I agree with Rory and the other's on the point that the plugs are fouled either through oil and or over rich mixture. It is also well known that modern plugs do not have glazed ceramic insulators which, as has been pointed out, make them far more susceptible to fouling. However I don't agree that they can not be recovered. I have successfully on many occasions cleaned plugs in such a condition with brake cleaner and then blasting the insulator with a very fine media. Not always 100% successful but this method works for me most of the time. Obviously you need to fix the root cause or you will be quickly back to square one, and if using sand or glass bead absolute cleanliness is ESSENTIAL, Soda is less of a concern on this point but always successful on really bad plugs.

[John Cornforth]

Re: the glazed or unglazed ceramic

I was curious about this oft-repeated theory. Despite a lot of searching, I am still to find any definitive information.

[Ruairidh Dunford]

It’s practice that has been shared above, not theory.

[Ruairidh Dunford]

I have successfully on many occasions cleaned plugs in such a condition with brake cleaner and then blasting the insulator with a very fine media. Not always 100% successful but this method works for me most of the time.

I will give this a go, Ian, I had only tried cleaner and wire wheeling them previously and without success.

Also, I’d read that they could not be recovered, so did not attempt further methods.

Thank you.

[Nick Turley]

I have found that the only thing that will clean oil fouled plugs is oven cleaner.
Such a nuisance to do I dont think it's worthwhile.
As has been suggested hotter plugs is the solution looking at those photographs I would try NGK B4HS have used them in an oily austin seven sucessfully.
Do get genuine plugs.

[Charles P]

This is a problem that I’ve had with a blown engine that effectively runs on two stroke.
I’ve found that blasting with baking soda and cleaning with brake cleaner works ok, but it’s a faff.
Decent old stock plugs are generally better.

[Chris Garner]

Ah, baking soda!!
As a complete aside some time back I went to my local Sainsbury's to buy said item. Enquiring from a young assistant where I might find it, I was asked whether I wanted the organic variety or not. One despairs sometimes.

[Ian Williams]

Haha, you are obviously more wealthy than us in NZ Charles, genuine plugs can be quite expensive here and we are tight!. I also always buy up stocks of new old plugs where I can find them, they seem more reliable added to which genuine old stock plugs generally look more authentic in our engines than modern alternatives