Yamaha RHINO 660 YXR660FAT Manuel Du Propriétaire page 294

Inspection
The spark plug is an important engine component
and is easy to inspect. The condition of the spark
plug can indicate the condition of the engine.
The ideal color of the porcelain insulator around
the center electrode is a medium-to-light tan for a
vehicle that is being ridden normally.
Do not attempt to diagnose such problems your-
self.
Instead, take the vehicle 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)
8-55