2002 Yamaha YFM660 Grizzly factory service and repair manual, Page 99Get this manual
CHECKING THE SPARK PLUG CHECKING THE IGNITION TIMING
CHECKING THE SPARK PLUG 1.Remove: Spark plug 2.Check: Spark plug type Incorrect ReplaceStandard spark plug: DPR8EA-9NGK 3.Check: Electrode Weardamage ReplaceInsulator Abnormal color ReplaceNormal color is medium-to-light tan color4.Clean the spark plug with spark plug cleaner or wire brush