Spark Plugs and Spark Checking
Spark Plugs are important. They’re also cheap and easy to change. The spark plug is the first thing you look at when a motor won’t start. Spark, compression, and fuel are the three main ingredients. Spark plugs come in different sizes and styles. See Parts Department to learn what sizes and styles there are.
Reading spark plugs: The condition of the spark plug tells you about the condition of the engine. A normal spark plug has a light brown coating on the white porcelain insulator, and sharp corners on the center electrode. A high mileage spark plug in a healthy engine looks the same but the electrode corners are rounded. Sparks like to jump from pointy things. A rounded center electrode or way too big a gap might make the engine hard to start.
Checking for Spark: Remove the spark plug, connect the cap to it, and lay it on the engine so the metal shell touches the engine. Turn the engine over either by pedal starting or by flicking the flywheel with your hand. Little momentary light blue sparks should be jumping the gap, making a “snap” sound each time the piston goes up and down. Modern CDI ignitions are hard to see in bright sunlight, so check for spark in the shade. A bright white, pink, or yellowish spark is bad. That usually indicates a fouled plug. Dim and blue is good, especially if the snap snap snap sound is loud.