Coil power on outside of wire/spark boot?!?!

1981 XJ750 SECA.

Started running like crap, stumbling and losing power. Idling weird; sometimes low, sometimes high. (this was effected by my tweaking the idle adjust so it would stay lit at lights.) Sometimes would lose power on highway, and then power would kick back in. To be clear, engine stayed running, electricity all stayed on; just felt like it went from a 750 to a 300 motor, max speed 55, no acceleration. Then would kick back in like normal, leap ahead, happily go faster than I'm willing.

roadside evaluation showed that one pipe was not hot. (#2 from left) At idle, when I pulled the cable off that spark plug, zero difference in sound of engine. Replaced spark plug, just to get the low hanging fruit. In the process of replacing spark plug, found that when the bike is running, there is ignition current on the outside of the cable/plug boot. (had not noticed before because I was wearing riding gloves) Not just a buzz, substantial current; made me jump.

Looks like the coils from my parts bike (82 XJ550 SECA) are a match, so I'll probably start with replacing the coil. Wondering if anyone has experience with this, or if you wiser wrenches might read this and say "Ah, your whatzit relay is bad!" or whatever.

Full disclosure, I just recently had a short that was backfeeding power from my hot ignition switch wire into my turn switch. This resulted in me tearing down the main wiring harness, replacing a bunch of wires, and reinstalling everything. Also tore down, cleaned, and rebuilt ignition switch, turn switch, and fuse box.

 

Once everything is dry get a spray bottle full of water. Wait until it's dark out (a dark garage is best), start the bike, and psray down the wires and coils. You will easily see where the insulation has broken down and is allowing the magic pixies to dance outside of their usual home.

The insulation on the wires can begin cracking and aquire pinholes. The coils bodies can also crack, but that is less common.
Unfortunately the only way to replace the plug wires is to cut them short and splice in new ones, dig into the coils to replace them, or or get coils that have replaceable wires.

Cutting back the wires and splicing in new, and repairing cracks in the coil bodies:
ignition-wires.112190

Replacing the wires in stock coils:
spark-plug-wires.29868

This is a low cost coil replacement, but there are better coils available from Chacal.
cb-750-coils.88468

If it's just the wires that are shorting to ground, you can wrap the wires with self-amalgamating tape as a temporary measure (will last for a few years if need be, but is not a permanant solution)

 

huh. Fascinating. Just so I'm sure I'm understanding correctly; if the ignition wires crack, they can leak power to the outside of the wire? And this would lead to the cylinder not firing because the electricity intended to fire the spark plug is grounding out to the motor body?

 

This sounds like the most fun test I've heard of for a mechanical problem. So I'll see sparks where the electrical short is?

 

Yes. A real lightshow.
The point of this exercise is to figure out if the problem is just with the plug wires, or if the coils are also cracked. You already found out that there is a high-voltage short to ground.

If you have it in your budget you can skip the test and just replace the coils and wires with new.

 

original coil:
12v CM12-09
4H7
208

Parts bike coil:
12v CM12-10
4G0
105

Can anyone confirm that these are interchangeable?

 

If the wire is arcing to the block, a usual suspect is the plug caps.
Also really dirty, oily or salty(if you live in Canada salt dust from winter on roads) wires will cause a short, clean em well.
Unscrew the caps from the wire and thest resistance 5k ohm +- 10%
Also check to be sure it was screwed center into the wire, trimming 1/4 inch of corroded wire off is good idead too.

 

I replaced the coil, and was getting identical symptoms. Did as you suggested, arcing problem totally resolved, great spark on plug to motor. Cylinder still isn’t firing, but that problem was totally resolved by your suggestion, thanks!

Now I’m wondering if something’s up with the #2 carb.

 

While you may have remedied the problem of not getting spark at the plug, you didn't do anything to remedy the underlying problem of insulation that has failed.
As long as everything stays dry there won't be a short to ground, but as soon as things get damp again....

 

Good to know. This is the coil off the parts bike, so it may be fine, but i'll try your spray test and see if I get a fireworks show.

 

One thing to check is the plug cap ,unthread number 2 and swap with number 3 see if number 2 still cold or if number 3 is now cold. As to coils 81 to 83 Honda CB750 coils will swap right in and have replaceable plug wires. And can still be bought brand new, been running these for 3 years now. Cycle Recycle part 2 Carrie's these 44.95$ ea and wires and caps .

 

That's a good idea, thanks!