Brown line on plug insulator

[scott45acp]

I looked at my plugs.  They are ngk with about 300 miles on them.  The front plug has a thin brown line on it almost like a pen line.    Bike is a 09 883 sportster.  Stock engine with a SE aircleaner.  Tuned with powervision using S&S tear drop AC map as a starting point.  Any thoughts what on the cause of at line?  Will post pics tomorrow.

[redmtrckl]

Sounds like it is getting ready to grenade on you. Just load it on a truck and send it to me for safe disposal.
Oh, and don't forget to pay the freight. No need to thank me.