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How to test an ignition coil
Most people think you can check a small engine’s ignition
system by removing the spark plug and grounding
it while cranking the engine and watching for a spark.
Nope. That just confirms that the spark plug fires in
open air. It doesn’t mean the ignition coil is strong
enough to fire the plug when under full compression.
Here’s a better way to check. Buy an inexpensive
ignition system tester (Briggs & Stratton No. 19368; about $27
at amazon.com). Then connect the
tester between the spark plug boot and the engine.
Watch for a spark in the tester window as you pull the
starter cord. To test the operation of both the ignition
coil and the spark plug, connect the tester between the
spark plug boot and the spark plug. Then start the
engine and watch the tester window. A spark plug misfire
will immediately be evident in the tester window.
You can use the tester on any small engine (mower,
snow blower, chain saw, string trimmer, etc.).