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Q: How do I know when I have a bad spark plug?

Q: How do I know when I have a bad spark plug?

A: Diagnosing and reading signs of bad spark plugs can be quite a science, and one could write a book on the subject (not me, thanks). But sometimes you might think you have a problem and can’t get professional help, so you must quickly look for some gross signs to give you a clue as to whether the problem is a bad plug and which one. I’m talking about the typical spark plug with a center positive electrode surrounded by white insulation and a ground electrode extending at a right angle from the base, which is screwed into the block. There are other configurations.

Usually you’ll wonder about your plugs when the engine seems to lose power, is difficult to start or is running roughly. Unfortunately, all these symptoms could be caused by various things, but it’s often easiest to check and perhaps replace spark plugs first.

Obviously, turn off the engine then check that the wire leads are firmly on the plug terminals. Sometimes vibration will shake them loose, causing a poor connection. This may manifest in rpm fluctuation, although this symptom could also indicate other problems. Inspect the plug wires for cracks in the insulation or signs of overheating or other abnormalities.

Next, unscrew the plugs, making note of which came from which cylinder. If any one looks substantially different from the others, it may be the culprit. Even though plugs have no moving parts, components can become damaged by the intense heating and cooling, concussion and vibration. Things to look for include cracks; scorching; blow-by around the threads (it may not have been tightened enough); a wider gap between the electrodes; carbon deposits — particularly around the center positive electrode or the insulation; oily deposits on and around the electrodes in 2-strokes (4-strokes don’t mix oil in the gas, but poor combustion can leave excessive carbon on them); and pitting or uneven surfaces on either electrode or the insulation around the positive electrode.

It may help to compare against new plugs. Plugs firing well should be relatively free of deposits but may develop an even, light brown color on the white insulation around the center electrode. Black, oily deposits on all plugs in a 2-stroke may indicate that too much oil is being injected or has been mixed in the tank gas, or a more serious ignition problem.

Check the gaps of the plugs with a feeler gauge. Your engine manual should have the specs. All plugs will suffer a gap increase with usage, but a plug with a much broader gap than the others may indicate a problem. Sometimes even a new plug will have an incorrect gap.

If one of your plugs is different from the others, replace it and see what happens. All the plugs may need replacing if the electrodes are burnt back — this can be from normal wear — and the gaps no longer within spec. In a pinch you can bend the negative electrode to attain the proper gap, but this isn’t as good a solution as replacing the plug. If all the plugs are OK, you may have a problem with your coil pack. On modern outboards these are the black boxes to which the plug wires are connected.

Some mechanics test by removing the plug, plugging on the wire lead, grounding the plug’s threaded base against the block, and cranking the engine. In theory, if the plug is firing, you’ll see a spark jump between electrodes. If there is no spark, the plug may be defective. If you connect a new plug and there’s still no spark, the coil pack, wire or something else is probably bad. However, you often can’t see the spark in daylight unless you darken the area around it or put a black surface behind the plug. And just because you see a spark doesn’t mean the plug is firing as it should.

Also, the voltage coming through the wire to the plug is very high and can give a dangerous shock, even though you’re holding the rubber cap that’s connected to the plug. Heavy rubber gloves and a properly insulated tool to hold the plug are important.

Further, when you crank the engine it sprays a mist of gas into the chamber, which is open because the plug is out. Therefore, there’s a significant risk of explosion. This method normally should not be used except in extreme circumstances and by someone who really knows what he’s doing and is willing to take the risk. Normally it’s better to do with two people. It’s better to use a qualified professional, though this may not be an option if you’re out on the water, and it may be safer to try to find a remedy yourself.

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