Spark plugs

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Spark plugs

gilesorton
There has been a fair bit said about spark plugs in the recent thread but I thought perhaps worth starting a thread specifically on this as I have got myself rather confused.

My Silk (138) has been running very happily on NGK B8 HS. But I need to buy in some spares and what should I go for?

Arthur recommends Champion L4G gold palladium and I could pick up a box of 10 NOS from USA for £70 on eBay (by the time the post and duty is paid) if anyone wanted to share.

But my handbook says Champion L10 (for sustained low speed use) or NGK B8 HC. (How does that differ from NGK B8 HS?)

Is there a definitive view as to best plug to use and where it might be sourced?
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Re: Spark plugs

hendred
The answer to your last question is probably not.  Owners will have their favourites just like which oil to use.  The plugs listed in the handbook are all obsolete now.  I can't find an old NGK catalogue to find out what the C in B8HC stands for.  It could be copper cored.  Looking at plug cross references B8HS is the modern replacement for B8HC.  The S stands for a standard copper core electrode.
The problem as I see it is how modern plugs stand up to fouling.  NGK plugs have a reputation for not being very good if heavily fouled.  If the engine is set up so that it isn't over oiling and the air fuel mixture is ok then they seem to work ok.  Which is your experience I believe.  NGK  B7HS or B8HS work ok in my Silk as long as I put a new set in at the beginning of the riding year.  If left in the engine over winter  they start ok but fail after the first run.  I believe it is something to do with combustion salts on the insulator and/or oil.  
I am currently trying NGK BP8HS plugs, not because of problems with B8HS, but because there is a theoretical advantage by moving the spark further into the combustion chamber.  It has reduced the four stroking at small throttle openings but not cured it.  Probably need a bit more cutaway on the slide.  
Fine wire electrode plugs spark at a lower voltage which can be an advantage and some can withstand more fouling.  I am using Champion L3G , which is readily available in the UK and has the same heat rating as B8HS plugs, in my MZ.  The reason for doing so is that under certain conditions the ignition system voltage drops and the lower spark voltage of the fine wire electrode overcomes the problem.  The engine runs very well on the plug which is added bonus.
In conclusion, if the NGK B8HS works well for you why change?  They are readily available and cheap enough to have two spare sets stored in the side panel pockets.
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Re: Spark plugs

gilesorton
Many thanks indeed for that magisterial and very helpful response.

I will stick with the current plugs for the time being - the Silk was running beautifully on them today. 100 miles round Derbyshire - Matlock Bath, Buxton, Cat & Fiddle, Staffordshire Roaches, Manifold Valley. Glorious! Just an annoying problem with the radiator but I will start another thread if I cannot find the answer elsewhere on here first.

Interested to note you still run an MZ. A 1973 ES250 Trophy was my first proper motorbike. In 1977 it cost me £110, but after the engine broke I took it apart and then gave it away. And they seem to fetch £2000 in top nick now, though that seems to me rather a lot for what it is.