If you notice your weed eater losing power when you use it, you may have a problem with carbon buildup on the muffler. This problem has become less common, as Stihl continues to improve both their trimmer design and the burning quality of their oil, which is known for its ability to burn clean and leave little, if any, residue on the machine. However, if you've used the wrong type of oil, or if you've put too much oil into your Stihl machine, you may encounter some carbon buildup on your muffler. It's a sticky black residue, but fortunately, getting it off isn't a complicated process.
Turn the engine off and let it cool completely before handling any part of the muffler or engine on the Stihl trimmer.
Set the Stihl equipment on hard, level ground, so that the right side (on your right when you are using the trimmer) is up and the left side is on the ground.
Locate the muffler. In all Stihl trimmers manufactured in 2000 or after, the muffler will be on that right side, which is facing up.
Remove the screws holding the muffler and shielding in place with a screwdriver. There will either be two or four T-27 Torx screws, which are star-shaped, depending on which model you have. Remove any shielding. In newer models, the entire mufflers will slide off of the machine for easy cleaning.
Remove the screen inside the muffler. It may be pressed into place or may have a threaded screw holding it into place. If the screen is excessively dirty or worn, remove it.
Soak the muffler in a heavy-duty degreaser for about two hours. Remove the muffler and rinse well.
Dry the muffler off with a shop rag or paper towel before putting in a new screen.
Put the muffler back in place and tighten down the screws.