now that i have your attention. i have a 2004 z400. just put in a k&n with a an outerwear, filter adapter, no lid, and rejetted it dyno jet 146. according to the plug chart found on this site my plug should be ok. the firing end is light brown. the rest is kinda dark. i also have a billet end cap fo my exhaust(eliminates spark arrestor). i dont have it in right now cause its so dang loud. am i gonna have to rejet to use that billet tip?

thanx.

 

the firing end is very light brown.

 

Here, try this. This should shed some light on it. Jetting1 How_It_All_Works
I'm by no means an authority on this, but if memory serves correctly; If tip is black or oily your too rich, if it's white or very light colored your too lean, if it's brown your just right. But call someone that gets paid to do this before you go trusting my advice.