Sparkplug Schedule

Posted: March 1, 2014

By: Grid Michal

My question for the experts is one I can never get a straight answer on. Is it myth or maintenance to change the sparkplugs once a season or after several seasons? I have 2001 Mercury 125 two-stroke carbed outboard. The NKG plugs have gone through two foggings and three full boating seasons (approximately 100 hours). The engine runs great, starts good and pushes the 18-foot bowrider to 43 mph nicely.
Here’s an approach for you: Remove one plug wire and insert an old NGK plug in it. Leave the plug already in the block where it is. Jam the electrode of the old plug into a sirloin tip and start the engine. Momentarily, the sirloin will be cooked enough to eat, as long as you like it medium-rare. Your modern-day ignition system (anything built in the last three decades) will fire a spark plug under virtually any circumstances, short of salt water. I’ve seen plugs used until they became “surface-gap” plugs, with no electrode. The engine ran fine, for all intents. Was it efficient? No. Was it safe? No. Talk about detonation waiting to happen! Check the sparkplug gap on a new plug: You should be able to slide a matchbook cover through it. If you can slide several thicknesses through the gap of a used plug, you’re ready to replace them. One-hundred hours, no matter how you achieve it, is probably a good time to replace the plugs.