Rob Scanlan is a Certified & Accredited Master Marine Surveyor
covering Maine to Long Island, New York.
781-595-6225 (OFFICE) firstname.lastname@example.org
MARINE SPARK PLUGS ?
By Rob Scanlan, CMS/MMS/ACMS
Accredited & Certified Marine Surveyor
What makes a spark plug a marine spark plug? There is no difference between spark plugs intended for inboard or outboard engines and car engines. There is a vast difference between spark plugs and it's most often measured in heat range. The term "heat range" refers to a spark plug's ability to dissipate combustion heat from its firing end into the cylinder head. A spark plug must run cold enough at wide open throttle to avoid pre-ignition, and hot enough at idle and low rpm to burn off the conductive deposits that short-circuit the ignition pulse and misfire engines. It is imperative to have the correct heat range spark plug installed in your inboard marine engines and outboard engines.
During my compression testing of inboard and outboard engines. I remove and examine all the spark plugs. In general, if an engine logs lots of hours trolling, and the plugs are black, it's a safe bet the plug isn't "hot" enough. If the engine logs most of its hours running at wide-open throttle, and the insulators are blistered, I recommend a colder spark plug.
You are probably aware of the recent arrival of the copper core spark plugs to the marine engines. Although a newcomer to automotive and marine power plants, they've been standard fare in aviation engines since the 1930's."Copper Plus" spark plugs derive their name from the copper nucleus encapsulated inside the center electrode. Copper dissipates heat faster than the normally nickel-alloy electrode, the heat range is broadened.
Copper core plugs feature a longer than normal insulator nose. Thanks to the longer nose, and the inherently longer fouling path, it takes longer for carbon to foul a cold engine. With copper, low speed fouling the bane of the trolling fisherman, is reduced. Also, that maddening engine bog on full throttle, after extended idling at the dock, disappears.
A bonus for parts room managers is that with the extended heat range, fewer model plugs are needed to outfit the maze of marine engines and horsepower ratings. For you the boat owner, it means the parts place is less likely to be out of stock.
Regardless of the manufacturer, or type, spark plugs are easier to remove and replace dockside rather than 20 miles from port with a heavy sea. It would be a time like this you're liable to discover you don't even have a spark plug wrench on board, let alone the spare set of plugs that probably would have gotten you home. Spark plugs replaced at spring commissioning are "get home under power" insurance. While there are some do-it-yourselfers (I call them asphalt admirals or dockside diplomats) who would disagree with my advice, few will deny that in spite of recent technological advances, spark plugs are still a weak link in the ignition system.
I rely on my computer diagnostics engine analyzer during the test running engines to pinpoint which plugs are firing and which ones are dead. It also shows me how well the ignition system is sparking, and how well the cables are shooting the juice to the spark plugs.
Early on I talked about examining the spark plug's firing surface and how the color of the deposit gives clues as to the condition inside the combustion chamber. Normally, a plug insulator will be a light tan to gray with few deposits. If I see aluminum deposits adhering to the electrodes it means pre-ignition is melting aluminum alloy off the pistons; and I will address the problem.
Damp, or wet black carbon fouling is caused by a too cold of a heat range plug, or by prolonged trolling. Other possibilities are the carburetor is adjusted too rich, or you're pouring too much oil in with the gasoline. Weak spark could also be the culprit. If the plug insulator is blistered white or gray, the spark plug heat range is likely to be too hot. But also check for over advanced ignition timing, the wrong propeller or defective cooling system. Finally, the fuel octane might be too low for the engine, or the gas stale.
When the time comes to install new plugs, I screw them in finger tight. If there's a gasket, I tighten an additional one-quarter turn; I will feel the gasket squish as the plug snugs up tight. Another variation, the tapered seat plugs, the ones without a gasket, are tightened one sixteenth of a turn past hand tightening. Regardless of the plug, I never over tighten.
DIAGNOSTIC & COMPRESSION TESTS ON GASOLINE & DIESEL ENGINES AND ALL MAKE/MODEL OUTBOARDS.