1984-1986 Mercury Force 4HP Outboards Service Manual, Page 36Get this manual
SPARK PLUGS DeecrlptlonA spark plug consists 01 two electrodes separated from each other by an air gapThe side
electrode is connected to the shell of the spark plugThe center electrode is completely Insulated from the shell
High voltage, produced in the secondary coil windingis applied to the center electrode and causes spark to jump the gap to the side electrodeThis spark ignites the fuel-air mixture and starts the combustion process the cylinder
of the type of ignition troublePlugs fail lor various reasonsOften the porcelain insulator cracks or becomes coated with 011carbon or other depositsThis can cause the high voltage ig nition to pass from the center electrode to ground without jumpi ng the spark gapAs an engine operates the electrodes are gradually burned or worn awayIn time, the gap becomes so wide that the available Ignition voltage cannot jump the gap and the engine misses