The Stihl FS 66 line trimmer has a two-cycle engine that uses a mixture of gas and oil. This mixture is not only used as fuel, but also as the lubricant for the internal-engine components. The fuel enters the engine's cylinder, and the spark plug ignites the fuel, causing combustion. The spark is created by the ignition coil. If the ignition coil malfunctions, the trimmer will run poorly or not start. Testing the coil is easy and can be done in a few short steps.

Place the weed trimmer face up on a level work bench.

Remove the air-filter cover. The cover is located on the right side of the engine, directly over the carburettor and is held in place by a twist screw. Slide the air filter out. If the air filter is dirty, replace it with a new one. If it is not dirty, reinstall the air filter. Reattach the air filter cover.

Remove the sparkplug wire from the spark plug located on top of the engine. Plug in one side of the spark tester into the sparkplug wire that was just removed. Plug in the other side of the spark tester into the spark plug.

Pull the starting coil rapidly while watching the spark tester; if there's a strong blue spark, the coil is fine. If the spark is weak or non existent, the coil needs replacing.