Harley Davidson spark plug situation

[Cor66Vette]

This is regarding a 1968 Harley D Electraglide. The rear cylinder spark plug won't fire when the ignition wire is snapped tightly onto the plug, but fires fine when the wire barely makes contact with the plug. The plug fires fine when the spark from the wire end is allowed to jump across to the plug. The spark is strong -at least in appearance. I changed plugs, I changed wires, and for the heck of it I swapped the coil with one from another bike. Nothing seems to matter.

What are your thoughts about a possible course of action? Thanks.

[Rich Z]

Sounds to me that when the wire is connected to the spark plug, maybe not enough voltage is being generated in order to cause a spark across the gap. When you pull the wire off of the plug, that produces an air gap for voltage to build to produce a spark. It sounds like the plug grounding strap isn't grounded for some reason. But if that were the case, then you wouldn't get a spark from the end of the wire to the tip of the spark plug, neither.

Hmm.... You changed the plug, so that rules out a plug that is broken and internally grounding itself. Is there anything in the cylinder that could ground out the spark plug electrode itself?

Does the spark plug fire if pulled from the cylinder? Is the spark gap too wide?

Try the spark tests in the dark to see if you can see sparks flying places they shouldn't be.

Heck, I know NOTHING at all about motorcycles........ Sorry...

[Cor66Vette]

Originally Posted by Rich Z

Sounds to me that when the wire is connected to the spark plug, maybe not enough voltage is being generated in order to cause a spark across the gap. When you pull the wire off of the plug, that produces an air gap for voltage to build to produce a spark. It sounds like the plug grounding strap isn't grounded for some reason. But if that were the case, then you wouldn't get a spark from the end of the wire to the tip of the spark plug, neither.

Hmm.... You changed the plug, so that rules out a plug that is broken and internally grounding itself. Is there anything in the cylinder that could ground out the spark plug electrode itself?

Not really. With my testing of the plug, it is installed in the cylinder, so if there was a problem with the plug grounding out to something, it shouldn't matter whther the spark fires the plug when it jumps and doesn't when the wire is snug, right?

Does the spark plug fire if pulled from the cylinder? Is the spark gap too wide?

I didn't try the plug out of the bike but I tried it in the front cylinder and it sparks fine. Seems like the rear cylinder AND allowing the spark to jump are a factor.

Try the spark tests in the dark to see if you can see sparks flying places they shouldn't be.

Haven't tried that yet. On the list.

Heck, I know NOTHING at all about motorcycles........ Sorry...

I was hoping that even though this is a car forum that the symptoms I'm experiencing could be generic to older basic gasoline engines- mc or car.

Well, tomorrow's another day. Thanks, Rich.