How to Test the Spark From a Motorcycle Spark Plug

by Chris Gilliland
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The spark provided by your motorcycle's spark plug is responsible for igniting the flammable mixture of air and fuel within the engine. A failing or damaged spark plug could weaken the engine, limiting its performance or keeping it from starting altogether. Checking your spark plugs often and testing their ability to produce a spark can prevent problems from occurring on the road. The test requires little more than a basic tool set and a working understanding of how to access and remove the spark plugs from your motorcycle.

Step 1

Park your motorcycle on its side stand, or place it on a service stand, and allow the engine to cool completely.

Step 2

Remove the seat, fuel tank and body panels from the motorcycle to provide access to the engine cylinder head and spark plugs, using an Allen wrench or socket wrench.

Step 3

Pull the spark plug cable cap gently off the spark plug, by hand. Lay the spark plug cable. Unscrew the spark plug from the cylinder head, using a spark plug socket and a socket wrench.

Step 4

Inspect the condition of the spark plug insulator -- the ceramic portion of the spark plug and the electrodes at the tip of the spark plug. Look specifically for cracks in the face of the insulator and the insulator tip near the electrodes, as well as bent or melted electrodes. Replace the spark plug, if there is any damage to either the insulator or electrodes.

Step 5

Look at the general coloration of the spark plug's insulator tip, near the electrodes. Ideally, the insulator should have a light tan color, indicating that the engine and fuel systems are operating properly. A wet-looking, dark brown insulator tip is an indication that the fuel system is running rich, a condition that adds more fuel into the cylinder's combustion chamber, or that the spark plug is not firing. Alternatively, an ashy white insulator indicates a lean condition, or more air than fuel, causing the cylinder to overheat. Do not discard the spark plug at this time.

Step 6

Measure the gap, or the distance between the side and center electrodes at the spark plug's tips, using the appropriately-sized feeler gauge, as directed by your motorcycle's factory service manual. Slide the feeler gauge between the electrodes. You should feel a slight drag as the gauge moves through the gap. Adjust the gap to the factory-specified clearance, using needle-nose pliers, if the feeler moves freely through the electrodes or cannot fit within the gap.

Step 7

Place the spark plug into the spark plug cable cap. Hold the spark plug by the cable cap and press the electrodes gently against the engine cylinder head. Start the engine and watch the gap between the electrodes for a spark. The spark plug is considered good if it is undamaged and a spark appears between the electrode and the cylinder head. Move the spark plug to a different location and try again, if a spark does not appear. If the spark plug does not produce a spark, the ignition coil may be causing the problem.

Step 8

Remove the spark plug from the cable cap. Place an in-line spark plug tester tool into the spark plug cable cap, in lieu of the spark plug. Attach the tool's grounding clamp to the engine cylinder head. Start the engine and watch the tool for a spark within the tool's body. The ignition coil is good if the tool produces a spark. Replace the ignition coil if a spark is not produced within the tool. Replace the spark plug if the ignition coil produces a spark.

Step 9

Screw the spark plug into the engine cylinder, by hand, if the spark plug is in good working condition and produces a spark. Tighten the spark plug to no more than 12 foot-pounds with a torque wrench, unless directed otherwise by your motorcycle's factory service manual.

Step 10

Test the remaining spark plugs, if any, using the method described above. Replace any damaged or faulty spark plugs. Reinstall the motorcycle's body panels, seat and fuel tank, using an Allen wrench or socket wrench.

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