Spark plug debate.

[nickdun]

I've been zooming around in my Victor Special after fitting a new spark plug.  After an engine rebuild (and complete restoration) I fitted it with an NGK B7ES (replacement for Champion N4) which the parts book, and most owners, recommend.  The plug failed after 1100 miles and I bought an NGK BP8ES which is a replacement for a Champion N6Y which is the plug recommended in the BSA workshop manual for Victor Specials from '68 on.  The plug is cooler than an N4 but the bike starts much easier and seems to run better.  Anyone have any thoughts on this?

[Spaceman]

As far as I can see, an N4 plug was listed in the VS parts book for 68 and 69 and also in the owner's manual. This also agrees with RR for 1969. I honestly wouldn't expect there to be a need for a hotter plug in a VS. I've never come across a failed spark plug before but, if there was a problem, I would check the spark plug has a good brown colour and you are not running with a weak mixture.

[nickdun]

The failed plug was the correct browny colour, maybe even a bit on the dark side, which isn't a bad thing with ethanol fuel.  It could of course be that the bike is easier to start with a new, working, plug.  The BSA Workshop Manual definitely lists a Champion N6Y for a 68 on VS, but I agree the Owner's manual and Parts book list a N4.  Bike does seem easier to start with a cooler plug though.....

[Spaceman]

That colour would imply the N4 plug has the correct temperature rating. However, I see what you mean about the Workshop Manual but that doesn't agree with the parts book or the owner's manual! Normally, you would go with the latter.

[nickdun]

Also 1960/70s plug recommendations don't take account of modern E10 or E5 fuel.  Wish I understood it more.....

Anyone?

[rhyatt]

The N4 and ngk BP are fundamental different plugs.
The P is a projected nose sticking further into the combustion area.
There have been dodgy ngk plugs around for a while.
.
Could the failure be down to a dodgy plug cap  ?
Check it's resistance , no more than 5 k possibly 10k.

[Beesy]

Not sure if modern fuels might make a difference. Ultimately if the bike is running fine and the plug is telling you everything s all good, it must be a good outcome.

[nickdun]

I have checked the plug cap and it is fine.  It's a modern repro of the bakelite original, and the terminal screws straight into the HT lead.  I think I'll give the new plug a couple of hundred miles and check the colour then make a decision.  As you say it's unusual for a plug to die, perhaps the better starting/running is because the bike now has a better spark from a good plug.

Thanks for your thoughts, and sorry if this is raking over old ground.

[LightningAndStarfire]

Ethanol has about 30% lower energy content than petrol, E5 would thus give 5%x30%=1.5% leaner mixture and E10=3%. This implies a hotter running engine, on the other hand ethanol has a higher heat requirement for evaporation thus a cooling effect. Probably not a need for spark plug change due to E5 or E10.

[rhyatt]

When you say the plug cap is fine  is it the correct resistance measured with an ohmmeter?
I hope it is a solid copper ht lead as well, not suppressed.

[nickdun]

I unplugged the HT lead at the coil and measured the resistance to the plug terminal.  The lead is copper wire and the terminal the old fashioned type that screws straight into the HT lead.

Are NGK plugs reliable?  I use them as that's what the car spares shop at the top of my street sells.  Would another type be more acceptable?

[rhyatt]

And the resistance should be no more than 10k ohms, possibly 5k ohms.
Ngk are historically a slightly wider Heat range, but there are reports of dodgy ngk plugs on the market.
Ultimately, if its running well, with a good colour then all is well

[nickdun]

Thanks for your help.