its not that they "go bad" your bike still runs great im sure. its a thing that happens slowly over time.

one thing that commonly happens is that the insulators themselves dry out by exposure to UV light and heat and the wires start to leak voltage.

My dyna is 3 years old and I shocked myself while ridding over the summer! I ran my hand near the wires while i turned the petcock to reserve! first tell tale sign.

These modern ignitions have alot more power than point type ignitions and are harder on wires that an old shovel.

For an eye opening dissertation on sparkplug cables and ignitions see: Magnecor.com

 

I replace my wires/coil about once a month when Im racing every week.

I run the Accel wires on mine ( a friend worked there so I got aton of wire in bulk).
Good spark is everything on hi comp engines.

Now on your low compression (compared to mine) every day street bike I would replace them every season along with the plugs.

 

Well, it sounds like I might as well replace them.
Thanks guys, I learned alot.

 

The Parts guy at H-D said not to run anything but the stock wires on my 07 FDX. Can't remember the reason, but what you said makes sense.

 

Nevermind

 

You don't need to keep the stock wires, but you do need to watch what you use. The Indy guy that I go to ordered me a set of Accel 8 mm Silicone Ferro Spiral race wires. He did tell me that if the bike didn't run right to put the stock wires back on, but he didn't expect anything and they worked fine. I did not notice any change after changing them, but my wires were still "new".

 

BECAUSE THAT IS WHAT THEY SALE!!

 

Actually, not sure if the wires matter that much- its more the suppresion, they are looking at ionic current to check for misfires (much like a Mercedes V12 does). In the car's example, it is actually sent back to the ECU from the coils, however bad resistors will screw it up. To watch it on a scope is really kind of weird, you see minute bits of 'misfire' (can't feel it at all), then the computer starts adjusting- if it doesn't correct, the cylinder gets shut off. I'm guessing Harley uses it due to the crank pulses, so the crank sensor wouldn't be accurate enough.

 

Man, that sounds pretty sophisticated.