How to Test a Spark Plug in 3 Steps

When you’re diagnosing ignition problems on a motorcycle, ATV, or UTV, it’s a good idea to rule out the simple stuff first, like a bad spark plug.  

How to test a spark plug

Before you dig deeper into your vehicle’s electrical system or engine, there are some simple tests you can do to make sure that power is getting to the spark plug, and that the plug itself is producing a spark. Here’s how to test a spark plug in 3 easy steps.

Tools Needed - Spark Plug Testing

Buy spark plug tester

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NOTE: Before performing any of these tests, make sure there isn’t any fuel on or around the engine or anywhere nearby, as a spark can ignite fumes and start a fire. 

Spark Plug Testing Steps

Step 1. Disconnect the spark plug wire and connect the plug tester to the wire. Next, connect the other end of the tester to an unpainted metal area on the engine and attempt to turn the motor over. 

Spark plug tester

A spark in the tester confirms that you’re getting electricity through the plug wire. Otherwise, the problem isn’t the plug and you’ll need to dig deeper into your vehicle’s electrical system. If you have a spark, move on to testing the spark plug itself.

Step 2. Use a deep-well spark plug socket to remove the spark plug and inspect it. 

Spark plug testing socket wrench

A quick inspection of the spark plug can give you some clues about what’s going on inside the engine’s cylinder. 

Oil on the end of the plug can mean too much oil in the mix on a two-stroke engine, and it can mean ring, gasket, or valve seal issues on a four-stroke.

Oily spark plug

Carbon on the electrode can mean the engine is running too rich, which can be a result of too much gas or not enough air. 

Spark plug testing carbon

And finally, a whitish corroded electrode can indicate the motor is running too lean, which means not enough gas and a motor that can run hot.

Corroded spark plug

Step 3. Reconnect the spark plug to the spark plug wire, then touch the end of the plug to something metal on the engine and attempt to turn the engine over. 

Motorcycle spark plug test

A spark in the electrode indicates the plug is carrying electricity, while no spark indicates a bad plug.

If you’re not getting a spark, you need to replace the plug. But even if you get a spark and it’s yellow or orange instead of the blue-white of a good spark, replace the plug anyway.

Want to see a motorcycle spark plug test in action? Watch the video above to see how to test the spark plug on a Kawasaki motorcycle.


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