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using a lead pencil on a spark plug

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using a lead pencil on a spark plug

Post by brooko on Wed Aug 08 2012, 21:12

Can someone explain to me why and how this works.
On the weekend at south cerney show my engine would not fire but one of the older genertion said to coulour the end of the spark plug witha pencil i put the spark plug back in and the old girl fired on the first crank can someone please explain thanks

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Re: using a lead pencil on a spark plug

Post by matt86 on Wed Aug 08 2012, 21:32

brooko wrote:Can someone explain to me why and how this works.
On the weekend at south cerney show my engine would not fire but one of the older genertion said to coulour the end of the spark plug witha pencil i put the spark plug back in and the old girl fired on the first crank can someone please explain thanks

never done it myself but all i can assume is that lead is a conductor so its making the spark jump the gap more easy kinda like a magnet will attack in this case jump the gap easier (better) and once its going has enough umph to keep going .... sounds like you have a weak spark ...

and in modern pencils its graphite not lead .

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Re: using a lead pencil on a spark plug

Post by pauldg on Wed Aug 08 2012, 21:53

To detract just slightly - pencils have never actually contained lead in the lead, it's always been graphite... (Or so my visit to the pencil museum told me Laughing )

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Re: using a lead pencil on a spark plug

Post by Foden on Wed Aug 08 2012, 22:25

The only way my PT will start is with a well pencilled plug, goes first time then, otherwise I could be winding all blinking day! That is with a rewound magneto as well, yet if I put the mag on another engine it will crack up straight away.
It's a bit like getting an awkward oiled up two stroke to fire, remove the plug lead and hold it about 1/4" from the plug and 90% of the time it will fire due to the magneto having to provide more oomph to jump the gap. The old 'button trick' on plug leads had the same effect, in fact many years ago accessory firms sold a gadget that you fitted in the plug leads that actually made a false gap in the cable, again the spark was intensified and gave a better start especially with an oily plug. Wink Dont think Halfrauds sell them though so I wouldnt bother trying! Smile

Pete.

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Re: using a lead pencil on a spark plug

Post by nutgone on Sun Sep 23 2012, 10:02

I've been told the reason the engine fires up is due to the way the graphite burns in the cylinder. Graphite will also conduct electricity, but not very well (you can use a pencil like a resistor, & like a variable resistor if you really need/want to), so I'm guessing the graphite, being closest to the spark, ignites well in the compressed space, burns for longer than the spark itself & helps to complete the ignition of the fuel/air mixture.

If your plug needs this treatment a lot it may not be the magneto at fault, it's probably the plug itself.

Also, I don't like the idea of giving magnetos an extra gap to jump, this will only serve to wreck your mag coil, in time.
They are designed to jump only a gap of around .018" for extended periods of time (although I've recently found out some flywheel mags have higher voltages & can use bigger plug gaps). The button trick sounds like a mag-killer to me, I've always found the best way to make a spark more intense is to make the gap a bit smaller.

Many things sold for coil ignition systems aren't suitable for magnetos, including all modern plugs which should be re-gapped to around .015" to .018". Suppressor caps & resistance HT leads are other things to avoid on magneto engines.

Sorry, went a bit :offtopic[1]: there....

As far as "pencilling the plug" goes, that's the best explanation I can give. If your engine needs it all the time though, there's something wrong. But then it's an old engine, there's bound to be something wrong with most of them! Hell! If it works for you, keep doing it! Wink

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Re: using a lead pencil on a spark plug

Post by jackieboi on Mon Dec 17 2012, 00:03

im to tight to go buy new plugs so i rekon i will have to get some pencils Smile

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Re: using a lead pencil on a spark plug

Post by kevjhnsn on Mon Dec 17 2012, 00:20

jackieboi wrote:im to tight to go buy new plugs so i rekon i will have to get some pencils Smile
GRAPHITE IS A GREAT CONDUCTOR
motor and alternators use it for there brushes ,and they a static and the coils/armatures are spinning over a great speeds
i always use it for the first fire up on any engine that has stood for many yrs or is hard to start
petrol used to have low amounts of a lead type in it and some plugs wouldnt work after unleaded was run in them
so what it is scratch scratch but it works Very Happy
kev

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Re: using a lead pencil on a spark plug

Post by fowlerfan on Mon Dec 17 2012, 12:46

New D16's and D21's can be got for only a few quid and well worth the investment. 'Leading' the plug is a pain and suggests faulty/dirty plug or poor spark.

If you have a dying mag you can help it for a while by reducing the gap. In fact I generally try and use a smallish gap on any mag as it puts less stress on the coil.

A good mag on an engine with a good plug is half the battle and saves a lot or problems. Over the years I have saved up and got a few of my mags rewound, it really is worth it.

Cheers
Dave

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Re: using a lead pencil on a spark plug

Post by nutgone on Mon Dec 17 2012, 16:50

fowlerfan wrote:
If you have a dying mag you can help it for a while by reducing the gap. In fact I generally try and use a smallish gap on any mag as it puts less stress on the coil.

Cheers
Dave

Good advice indeed Dave:
Mag specialists say a general maximum plug gap of .016"-.018" for normal magnetos. The recommended gap for motorcyclists is .015" allowing for erosion of the electrode over time.

For some reason, flywheel mags specify a larger gap, many of them saying .025", which just seems too wide to me.

Anyone choosing to ignore this advice does so at their peril, as a wider plug gap will eventually burn out your mag coil, resulting in the need for a re-wind, costing a minimum of £80 for the most common coils.

All my plugs are gapped to .017", as I generally clean them very often & always re-gap when I've done so. (I prefer old detachable plugs myself, but always test the electrode resistance, as Champion 8 COM's tend to lose the connection from the terminal to the electrode. This can be repaired though).


I would still like to know the science behind penciling plugs. I think it has to do with the graphite burning the way it does, causing a longer spark/burn, therefore a better chance at combustion. Just my guess.

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Re: using a lead pencil on a spark plug

Post by jay on Mon Dec 17 2012, 17:10

you will also find new type plugs the ceramic isnt as thick or as hard as on older plugs,modern fuel injection engines only squirt so much fuel in at cold start up, a old engine like ours are easy to flood and make the plug wet which takes more time for it to dry out,the pencil trick drys the plug tips and wipes away the oily residue...Try another known good plug in the engine as if the mag is ok on the other engine your engine either has a poor plug or lead or compression is poor and even the carb may have a air leak

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