The engine on a weed eater is a two-cycle engine, meaning that oil and gasoline is mixed together as fuel, but also lubrication for the engine. A two-cycle also runs at twice the rpm as a four-cycle engine, which puts more stress and heat on the ignition coil. When your weed eater starts to run rough or not start at all, testing the coil is a great place to start. The procedure only takes 10 minutes to complete.
Set the weed eater flat on a workbench.
Remove the spark plug wire from the spark plug located at the front of the engine. Remove the spark plug with the spark plug wrench and install a new spark plug (the new plug is pre-gapped), to ensure proper testing.
Insert the spark plug wire lead into the spark tester. Attach the alligator clip to the metal end at the tip of the spark plug. Turn the ignition switch to the ON position.
Pull the starter cord rapidly and watch the window in the spark tester. If the spark is strong and blue, the coil is operating properly. If the spark is weak or non-existent, the coil needs replacing.