Your plug isnt hot enough..
According to NGK:
Lower than the self-cleaning temperature of 400°c-450°c (750°f-850°f). Air-fuel mixture richer than 8:1 to 10:1.
A Simple but interesting read is this NGK site..
If you are running 18mm plugs you can always get 14mm conversion inserts and have access to a wider range of modern replacements.