Stihl does not recommend owner servicing of all muffler types on its weed trimmer and brush trimmer machines. Some models include emission control devices housed within the muffler and should not require service unless deformed or damaged. Stihl recommends replacement by a qualified technician rather than home repair. Other models without emission control mufflers may include spark arrestor screens that can be removed and cleaned or replaced by owners. Check operator's manuals for specific maintenance suggestions for your machine.
Make sure the ignition switch is in the the off position. Let the machine cool completely before servicing the muffler. Mufflers contain internal metal parts that stay hot long after the muffler housing cools. Thirty minutes of cool-down should be plenty. Test by holding a hand near the top of the engine but do not touch. Wait until no heat radiates from the machine.
Locate three mounting screws that hold the engine shroud or top cover in place. Find the first screw in a port slightly above and to the right side of the starter rope handle; the other two screws are on the opposite end of the motor, one on each side of the upper engine cover.
Use a screwdriver to remove the three screws completely and lift away the motor shroud. Clear away any obvious debris from the top of the engine and muffler.
Find the spark arrestor screen at the exhaust port of the muffler. In most Stihl two-cycle engines, a single screw holds the screen in place. Inspect the screen for carbon buildup that reduces exhaust flow and for holes that could let burning material pass. If the screen is clear and intact, replace the motor cover.
Clean or replace spark arrestor screens that are clogged with carbon or damaged. Remove the single retention screw. Lift and pull the spark arresting screen out of the muffler. Brush carefully with a brass wire brush to loosen carbon. Carbon deposits in the muffler itself indicate problems with the fuel mix. Scoop out any severe buildup with the screwdriver tip.
Slide cleaned or new screens into place over the muffler exhaust port and fasten with the retention screw. Replace the engine cover and reinstall the cover screws.