Weird Spark Plug Color

[dwcdds93]

I put a 370 jet in my yz 125 and it seemed to run better than with the 360 (stock setting). It still seemed to be running rough on the top end though. I pulled the plug and it was white on one half, and black on the other. I always thought it should be white OR black, not both. Also, the electrodes seemed pretty black for just about 15 minutes on a new plug. Does anybody have an idea of what this is?

 

[reelrazor]

Okay, white on one SIDE of the electrode and black on the other SIDE?

White white? Or tan? Black black? Sooty or shiny?

Depending on the type and timing of the ports in a particular engine you can get a plug like that. One side gets hit with a dense cold charge, and the other is exposed to the exhaust.

It could be that the plug heat range is wrong and that the mixture is wrong, but together they mask each other's symptoms.(say too hot a plug, but too rich a mix-you don't foul the plug, and the plug doesn't cause detonation).

There is a thing called 'indexing' the spark plug. This is where you make a note(or mark the outside of the plug) as to which direction the ground electrode points. They make shims to use under the plug to get the electrode pointing in the direction you need. Most often this means that the ground electrode 'points' at the exhaust port.