Yamaha Motor Ultramatic GRIZZLY 660 Manuel Du Propriétaire page 342

Do not attempt to diagnose such problems your-
self.
Instead, take the ATV to a Yamaha dealer. You
should periodically remove and inspect the spark
plug because heat and deposits will cause the
spark plug to slowly break down and erode. If elec-
trode erosion becomes excessive, or if carbon and
other deposits are excessive, you should replace
the spark plug with the specified plug.
Specified spark plug:
DPR8EA-9 (NGK)
8-73