Ever wonder exactly what makes a marine spark plug a marine spark plug? There is a vast difference between spark plugs and it's most often measured in heat range. Per se, other than corrosion resistance, there really isn't any big differences between spark plugs intended for inboard or outboard engines and car engines. But it might be helpful to cover a few of the essentials.
The term heat range refers to a marine spark plug's ability to dissipate combustion heat from its firing end into the cylinder head and cooling system. 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 would otherwise short-circuit the ignition pulse and misfire the engine. So you can see it's important to have the correct heat range plug installed in your marine engine.
If you want to know how well your spark plugs are matched-up, remove and examine the 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. On the other end of the spectrum, if the engine logs most of its hours running at wide-open throttle, and the insulators are blistered, try a colder plug.
Further, because so often the wrong plugs end up on a marine engine, it's a good idea to check the its numbers to see if the correct type is installed. For example, consulting the Champion Master Application guide, we see that the RBL8 is recommended for some 250 different engines, but you need to reference the serial number to be sure. Another plug, the J2J, also finds a home with some 250 marine engines. As an interesting aside, through the years the venerable J series Champion plug, has seen service in outboard motors, inboards and even Chevy small block full race engines.
Undoubtedly, due to the media blitz, you're probably aware of the recent arrival of the copper core spark plug to the marine world. Although a relative 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. And because copper dissipates heat faster than the normally nickel-alloy electrode, the heat range is broadened.
Copper core spark plugs feature a longer than normal insulator nose. Thanks to the longer nose, and the inherently longer fouling path, it takes longer to carbon foul a still cold engine. In fact, this breed of spark plug brags twice the carbon-fighting ability of its everyday cousins. 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, often disappears. A bonus for parts room managers is that with the extended heat range of Copper Core Spark Plugs, fewer models of spark plugs are needed, which in turn means the parts place is less likely to be out of stock when you come a calling.
Platinum spark plugs are renowned for their long life due to a high melting point. A thin wafer of platinum is bonded at the firing point of the center electrode so it doesn't wear as quickly as a traditional spark plug. These are the plugs you typically found in low-maintenance engines.
Iridium Spark Plugs: Iridium is a precious metal six times harder and eight times stronger than platinum and boasting a 1,200 degrees (F) higher melting point. It also conducts electricity better. So the center electrode can be a smaller diameter without sacrificing service life. The ignition spark is more concentrated, improving ignition for better fuel economy, stronger acceleration and sharper throttle response.
Regardless of the manufacturer, or type, spark plugs are easier to remove and replace dockside rather than 20 miles from port with a heavy swell rolling your boat. It's times like these 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. The finale of this sermon is that spark plugs replaced at spring commissioning are cheap while get home under power insurance isn't. While there are some do-it-yourselfers who would disagree with this sage advice, few will deny that in spite of recent technological advances, spark plugs are still a weak link in the ignition system. Remember, even though a spark plug looks good, it might be as dead as door nail. (By the way, what is a door nail?).
Without a doubt, pro-mechanics don't have to play the guess and by golly game do-it-yourselfers do. That's because they've rely on the shop's electronic engine analyzer. When test running an engine, the oscilloscope pinpoints which plugs are firing and which ones are dead. It also shows how well the ignition system is sparking, and how well the cables are shooting the juice to the spark plugs.
With such an tester, you don't suffer the expense of blind remove and replacement. You should also know there's more to changing spark plugs than you might think. For instance, consider the handy-dandy cross reference charts that tell you an Autolite XYZ is the same as a Champion QRS. The problem is the manufacturers don't always agree on plug design specifications and a chart might not recommend the correct plug for your power plant. Don't use cross reference charts, instead use a Champion chart to recommend a Champion brand plug, an an AC chart for AC plugs, and Autolite for Autolite and NGK for NGK.
Early on we talked about examining the spark plug's firing surface and how the color of the deposit gives clues as to the general state of affairs inside the combustion chamber. Normally, a plug insulator will be a light tan to gray with few deposits.
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, screw them in finger tight. If there's a gasket, tighten an additional one-quarter turn; you'll 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, don't over tighten. If the threads strip, it means an expensive repair bill and a rig tied up in the shop instead of out on the water.