1999 Mercruiser HI-Performance GM 377 EFI Engine Service Manual, Page 47Get this manual

1999 Mercruiser HI-Performance GM 377 EFI Engine Service Manual, Page 47

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TROUBLESHOOTING
Chipped Insulator
Chipped insulator usually results from careless plug regappingUnder certain conditions, severe detonation also can split insulator firing endsReplace spar