The ignition systems on Stihl weed eaters operate with breakerless magneto ignition system. These ignition systems house an electric coil, which builds, stores and fires off the high voltage charge. This charge travels to the spark plug where it ignites the fuel. If this process is off anywhere in the ignition system, the trimmer will have no spark.
Check Spark Plug
When operating on the ignition system of a Stihl weed eater, take special precautions as the voltage involved can be lethal. Wear leather work gloves and always set the trimmer on the ground or on something directly touching the ground. Move the ignition switch into the "Off" position and unhook the rubber boot from the tip of the spark plug. Unscrew the spark plug with a socket wrench and inspect the metal tip. If the tip looks black, bent or broken in any way, replace the spark plug.
Test For Spark
Refit the spark plug back into the rubber boot, with the metal tip pointing out. With the trimmer on the ground, brace your knee onto the shaft or get a friend to help hold the trimmer steady. Switch the ignition back to the "On" setting and hold the ignition wire just below the rubber boot--wearing work gloves. Position the tip of the plug about 3cm away from a metal point on the engine block that is touching the ground. Pull sharply on the starter rope and check for a spark.
If there's no spark during the test, check all of the ignition system wires for damage or loose connections. Check the inside of the rubber boot to make sure dust or other debris isn't messing up the connection. Make sure the rubber boot isn't loose or hanging off the HT lead wire. Check the short-circuit wire and lead wire running from the ignition module to the starter switch. Make sure these wires aren't pinched or touching any metal on the engine block, such as the crankcase. Replace any damaged or broken wires.
Replacing Ignition Module
After replacing any damaged wires, repeat the test for spark. If there's still no spark present, the flywheel and the ignition module will need servicing. The flywheel needs to spin evenly and at a high enough speed to generate the proper charge. Check the magnets on the end of the flywheel to make sure they aren't discolored. Check the flywheel to make sure it spins evenly on the crankshaft. If there's still no spark, replace the ignition module.