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Sparkplug question

Sparkplug question

Sparkplug question

I realise it is a bad practice, but once in a while it does happen, that, while checking to see if a sparkplug is sparking, (with the sparkplug removed and the sparkplug body resting on engine ground), that the sparkplug slips and ends up hanging in free air on the end of the sparkplug lead.

I've always been mystified why on certain occasions that the spark continues to jump anyway, considering the sparkplug body has no longer any contact with engine ground. Can anyone explain that please. I have even had it happen with the sparkplug lead connected directly to the sparkplug without a sparkplug cap, when I don't see any spark path upon the sparkplug ceramic insulator surface, so I presume the conduction path is not along the surface of the sparkplug lead.

Any ideas why this can happen?

I do realise it is bad to let this happen and that the coil insulation can be damaged, but that is not really what I'm concerned about right now. I'm just baffled why the spark jumps to plug body even though the body is attached to nothing.


RE: Sparkplug question

IF there's an arc at the plug gap,
there has to be a second path, from the sparkplug body to a ground.
Via an arc in air, or via your hand if you are holding the plug.

Experiments on modern spark systems are not good for them, or you.

Mike Halloran
Pembroke Pines, FL, USA

RE: Sparkplug question

I still don't understand.

I am not holding it in my hand.

The sparkplug is just dangling on the end of the plug lead, and there is no visible arc through the air or along the surface of the porcelain.

I'm still mystified.

RE: Sparkplug question

Greg, it isn't even on an engine. It is a single cylinder ignition coil connected to a battery and I am manually opening and closing the points. I still get a spark at the plug while dangling in the air. I know this is not a good practice and I can, (and probably will), destroy my components but I don't mind as they are spare parts and I want to learn how this can occur.

RE: Sparkplug question

Try doing it in the dark. Try cleaning the plug and the lead and the cap. I'd guess a second spark between the porcelain and the cap, I do remember seeing some peculiar marks on an old two stroke's plug cap that could have been tracks from a current.


Greg Locock

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RE: Sparkplug question

The current is returning as a sheet along the surface of the plug to the boot to the wire jacket and down the wire jacket to the coil returning to the primary terminals.

Do the test over with a brand new wire and it will be a different result.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Sparkplug question

Thanks. I will do it in the dark and see if that throws any light on it smile

RE: Sparkplug question

Great idea! Make it VERY dark and wait while your eyes adjust.

There is also a small contribution of capacitance to the sparkplug body. I'd guess maybe 1/4 of the spark goes to charging that capacitance that then discharges down the surfaces too.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Sparkplug question


The darkness did the trick.

The sparks are running up over the porcelain and the outside of the lead!

Seeing is believing.

I did try to photograph it for you but it was too difficult in the dark, while manually operating the points etc.

Thanks again.

RE: Sparkplug question

Dang, that would've been a nice shot to see. Need to lock open the shutter and then spark away.

Yes, you can confirm all this by holding the insulated wire in the same scenario as you will become part of the path too.

This is why you see the pros always holding the plug down on the engine block with a screwdriver - they've all done it once before holding the boot or wire.

I have frequently checked wire status by driving out of town with the missing-engine vehicle and up to the woods, in the dark, under the trees, on a turnout. Popped the hood and it looked like a large city from space with flickering light all over the wires. It really needs to be black dark though, any moon and you can't see it. You can however feel it if you lay a hand on the wires. You'll only do THAT only once though. :/

Keith Cress
kcress - http://www.flaminsystems.com

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