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.).