Spark Plug heat range change...
 

91 N/A: I just changed the plugs and noticed that the leadings were fairly heavily carboned. The trailings showed normal wear. Should I use a hotter plug in the leadings?

Allright then, let me pose a different question but in reference to the same issue. Would the leadings be carboned up if the timing was not advanced enough? Or, would advancing the timing eliminate the problem of carbon buildup on the leadings?