If the engine of your Stihl weed eater struggles to generate power, the spark-arresting screen on the muffler could be the problem. If it is damaged, you'll need to replace it. If it's dirty, you can often improve the performance of the engine by cleaning the screen.
Things You'll Need
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Loosen the spark plug screw and pull off the spark plug boot.
Remove the mounting screws from the shroud and lift it off.
Remove the self-tapping screws above and below the baffle and spark-arresting screen.
Remove the baffle and screen.
Clean the spark-arresting screen with a rag dampened with gasoline. Let the screen dry.
Reassemble the unit and test it.