The owner's manual for most marine engines recommends winterizing the engine by fogging with oil before storing the boat for the winter, and replacing the spark plugs after a certain number of hours or years. Many people (and marinas) have adopted the habit of changing spark plugs early each spring, replacing the fogged (oily) spark plugs with fresh ones for the new boating season.
Others replace plugs only "as needed," such as when the engine doesn't run as smoothly anymore, top RPM drops, or the lugs appear to be fouled. A few hints to make plugs last longer:
Use ethanol-free gasoline if you can
Always use anti-seize to keep plugs from sticking in the engine
Change the oil at recommended intervals (e.g. 50 hours) and don't overfill with oil
Standard plugs are OK if you change every year; iridium or other premium plugs may last longer if you change every 2-3 years (i.e. if you boat fewer than 100 hours/year)
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