AFAIK, it is from what Russell says. Gunk from breakin prematurely fouling the plugs. I hope Randy Smith chimes in on this one. I just do what I'm told most of the time.
![Wink ;)](https://stunthanger.com/smf/Smileys/classic/wink.gif)
I'm good at learning and remembering. I don't theorise, or investigate! The really smart guys do all that for me.
![Grin ;D](https://stunthanger.com/smf/Smileys/classic/grin.gif)
EDIT: Well, as I was typing, Randy DID chime in!