Grounding spark plugs

[wagemaker]

I have heard/read that one should ground plug wires while testing compression or otherwise turning over a motor or some motors. Why? Any particular motors? What happens if I don’t?
I have been very inconsistent about this but then, my motors don’t always run as well as I would like.

Also can I ground the wire end without a plug, or do I need to leave a spark plug attached and ground the threads?

[rudderless]

Internal arcing between windings is not a good thing to do to a coil. Any method to bleed off energy to ground is fine. Either through the plug or by grounding the plug wire itself.

[jw-in-dixie]

Ground the plug wire so that when the wheel is turned the voltage will pass to ground readily. Leaving the wire ungrounded allows the high voltage created no place to go – might damage the coil . Okay to attach to the plug as long as you ground the plug threads, as with a wire.

[Mumbles]

Here’s some simple homemade jumper wires used to ground the system. They were cheap to make but nowhere near expensive as a new coil.

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[jcrigan]

Just like what the others here said. In addition let me add that a spark can ignite any fuel or solvents nearby. This could be near a spark plug hole and cause that cylinder to fire.

[vintin]

My understand is that it is not that the voltage has no place to go or that the voltage builds up…..neither is the real cause of the problem. The problem is that with no load the voltage in the secondary winding runs much higher than it does when loaded by the air gap in a spark and this high voltage can ‘punch’ through the insulation on the coper wire in the secondary winding. Once there is a path through the insulation the voltage will follow the path of least resistance and the coil is shot.

[frankr]

quote VinTin:

My understand is that it is not that the voltage has no place to go or that the voltage builds up…..neither is the real cause of the problem. The problem is that with no load the voltage in the secondary winding runs much higher than it does when loaded by the air gap in a spark and this high voltage can ‘punch’ through the insulation on the coper wire in the secondary winding. Once there is a path through the insulation the voltage will follow the path of least resistance and the coil is shot.

Well you are correct of course. Yes, the problem is that the no-load voltage runs much higher than normal. But the reason it runs much higher is because it has nowhere to go. When time to fire, the secondary voltage rises over time until it reaches its maximum capability or is released by jumping a gap, whichever comes first. The time lapse is almost instantly, but still takes time. As always, electricity follows its fundamental rule–it follows the path of least resistance, whether the path be arcing across a spark plug gap or arcing through the insulation around the copper coil windings. So everybody’s right.

[PugetSoundBoater]

VinTin and FrankR- i always ground the plug wires when comp testing. I wondered what exactly the purpose for doing it is,thought the electrical flow getting constipated is not good.
Thanks for the explanation.

"Some people want to know how a watch works, others just want to know what time it is"
Robbie Robertson

[The Boat House]

My experience is that while true the risk is exaggerate.
Ever developed a miss to find a plug wire has come off?
I cant say how many times that has happened
to me but it never ruined a coil so I would suspect
the likelihood of ruining one from just pulling a motor
over is pretty slim on these old magneto ignitions.
I have a motorcycle that I can switch from battery
ignition to a magneto. As there is only 1 spark plug
per cylinder I would have to remove the plug wires
from the mag. I never grounded them. The last time
I tried to use the coil in the mag it was dead. Took
awhile but I’m sure that is the reason.

[rudderless]

I have seen park terminals for the plug wires and they were grounded. Can’t remember where though…

I suspect the modern ignition uses higher voltage than the older point system. The higher voltage might be harder on the coil insulation.

When I got my omc`15 it was in a box all apart. The coils had burn marks right through the outer plastic cover. I tore one down just to see. Could not believe the damage. I checked for spark on the coil before autopsy and it did spark fine.

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