End of the spark plug insulator skirt cracking or breakage.The use of low-grade gasoline or cause premature ignition timing.End of the spark plug insulator skirt cracking or breakage.The use of low-grade gasoline or cause premature ignition timing.End of the spark plug insulator skirt cracking or breakage.The use of low-grade gasoline or cause premature ignition timing.End of the spark plug insulator skirt cracking or breakage.The use of low-grade gasoline or cause premature ignition timing.