What makes a 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, there really isn't any difference 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 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 preignition, and hot enough at idle and low rpm to burn off the conductive deposits that 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 power plant.

If you want to know how well yours are matched-up to the Merc250's, 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 an engine, it's a good idea to check the its numbers to see if the correct type is installed. In your case, Mr. Garwig, I consulted the Champion Master Application guide, and the RBL8 is recommended for some 250 engines, but you need to reference the serial number to be sure. The J2J also finds a home with some 250powerplants. 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 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 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 plug brags twice the carbon-fight ability ofts 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, fewer model plugs are needed to outfit the maze of engines and horsepower ratings. For you the consumer, it means the parts place is less likely to be out of stock when you come a calling.

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 rig. 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 "get home under power" insurance. 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 log looks good, it might be dead.

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'll 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 Autolites.

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. If you see aluminum deposits adhering to the electrodes it means preigniton is melting aluminum alloy off the pistons; immediately consult a mechanic to find 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, 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 on the water.