Pencil leading

Against my better judgement, I am forced to the opinion that rubbing a pencil point across the spark plug gap makes it easier for the current to jump the air gap at low RPM, thus frequently persuading an engine to run that has otherwise proved difficult to start.

Before I am further persuaded to take up arts involving Summoning, please can anyone come up with a cogent, scientific reason why this works.


Kim Siddorn,

Reply to
J K Siddorn
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Pencil lead is graphite mixed with clay.

Graphite is conductive to electricity, and thus sparks.

Rubbing the pencil across the points gap deposits flakes of graphite which does two things:

1) It reduces the gap slightly 2) It provides a number of points for the spark to jump across (against only the smallest point/shortest path on the original plug gap)

Encrustation of the point gap insulates the surface of the points, but the graphite deposits spreads the surface available out again over the top of the insulating layer.

That is why 'cleaning only' is not as effective.


-- Peter & Rita Forbes Engine pages for preservation info:

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Reply to
Peter A Forbes

As I said to Kim off-list, My dad told me that one, apparently regularly used to start recalcitrant Stuart (P5?) engines used to drive the pump in airfield fuel bowsers on a cold damp morning.

I also mentioned it in a letter to SEM some years ago, suggesting that others might like to write in with their 'Dodgy Dodges', but for whatever reason the letter was never published.

Reply to
Nick H

Kim, I would have thought you would invoke the power of Thor, the God of Lightning! Martin Percy is pretty powerful as well :-).

I've never started an engine with a pencil yet. An Ohmmeter across the HT section of the coil usually finds the problem on a rather less mystical way. I was surprised to hear you say that you had so few problems with Wico A mags recently. Of the ten or so I've fiddled with only one had a coil which wasn't open circuit on the HT side, admittedly the current can jump the break, but this is a short term effect and a lot of spark energy is lost. Also, the condensers in my mags if original have all been duff. I know about those because I can test them at work. Oil impregnated paper capacitors are useless after thirty years plus, because oxidation of the oil produces all sorts of conduction where you don't want it.

My feeling is if you're stuck, maybe the pencil helps a bit, but if the magneto has problems you're going to be winding that handle plenty!

Regards, Arthur G

Reply to
Arthur Griffin

Tried heating a bit of soft pencil lead in a hot flame? Burns very well and hot. Maybe that little bit of extra umph to the spark is enough to get the cylinder to fire.

Reply to
Dave Liquorice

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