The magneto, or ignition coil, in a Weed Eater trimmer provides the strong electrical output needed to fire the spark plug, which ignites the fuel in the cylinder. When the magneto malfunctions, the Weed Eater will lose energy or stop running. Checking the magneto will help you determine whether it is the source of a Weed Eater malfunction, and it is a simple task that takes only a few minutes to complete.
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Turn off the Weed Eater. Lay it on flat, dry ground outdoors or in a well-lighted indoor work space. Allow the engine to cool completely before checking the magneto; you run the risk of burns if the internal components are still hot when exposed.
Remove the cylinder cover, which is on the engine block just beneath the main handle. Look for a removable portion of plastic held on by two to four screws, depending on the Weed Eater model.
Locate the spark plug, and disconnect the spark plug wire. Examine the spark plug electrode for signs of corrosion. Keep the spark plug if you don't see any corrosion.
Adjust the spark plug gap to 0.7 millimeters, the industry standard spark plug gap specification. Return the spark plug to its housing. Adjust the gap if the spark plug does not feel secure in the housing.
Attach the main wire attachment for the spark plug into the receiving end of a spark plug tester. Attach the metal alligator-style clip of the spark plug tester onto the electrode at the base of the spark plug.
Turn the Weed Eater on. Activate the recoil starter of the unit. Proceed to rev the engine to 850 rpm (revolutions per minute). Monitor the test window of the spark plug tester for a strong blue spark arcing between the two electrodes.
Take your Weed Eater to a repairman certified to work under the limited warranty of Weed Eater brand string trimmers if the spark is very weak or there is no spark; the magneto will need to be replaced.