weird spark plug situation

[johnyrude200]

I have a ’75 25hp evinrude where I can see the spark jumping from the boot cap, out from under the end of it, to the outside of the spark plug.

This plug boot is particularly long, and I have tried 3 times to make sure the plug wire terminal is seated on the end of the plug. I tried switching plugs too, and it is an intermitent problem that goes back and forth.

Of course, when the spark starts jumping outside the plug, I’m loosing that cylinder. Is this a case where the plug boot is just too long and needs to be trimmed, where the terminal is working it’s way off the end of the plug and the spark is finding something to jump to?

First time I’ve seen this one, and I just accidentally caught it when I was inspecting the water jacket for any leaks while the motor was running.

[frankr]

Try some dielectric grease in the boot. If it still arcs out, replace the boot. It’s probably carbon tracked from the arcing. Actually, the short boots have been superseded by the long ones.

[johnyrude200]

I trimmed the boot back 1/4 inch and the issue went away. I suspect the normal vibrations of the motor were causing the terminal, which may not have been fully seated on the plug head, to vibrate off and cause the arcing.

And these ’75 models run like crap compared to the earlier ones…as we’ve all discussed…again. (First one of this season, forgot just how much they snort/stall when cold).

[r-c]

Appears that there a crack on the inside of the boot. You could try to cut it back some. It may not repair the problem. Replace the boot. It may be just too cracked to function correctly. 2016 – 1975 is 41 years. Rubber gets old.
Remember that voltage goes to the path of least resistance. Maybe also consider cutting the spark plug cap down some so it goes to the electors and not else where.

[johnyrude200]

It was a brand new coil from sierra.

[joecb]

What is the source of the boot? OEM or something from China? If you have a good ohm meter check the resistance of the rubber. You may be surprised at what you see. I had a couple of ignition issues that I tracked to black rubber parts that were conductive at the high voltages encountered in ignition systems. Apparently the rubber composition had enough carbon black in it to conduct the current.
Joe B

[Mumbles]

just in case.

[fleetwin]

After misfiring like that, a carbon track may have formed on the spark plug porcelain as well, you might want to replace those plugs "just in case".

[jeff-register]

Please remember Everything will conduct an electrical force, even air will conduct when given enough force (electric voltage & air = lightening) It has a force scale & rated as per it’s "dielectric" capacity. Even good copper wire has a dielectric place on the sliding scale. All it takes is a microscopic crack for the energy to travel on as Fleetwin has brought to our attention for educational purposes. Just remember a dielectric constant for referencing resistance.

[jeff-register]

JR,
I ran across a Mallory ign site. You would like the plug boots they have, Worth checking out.

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