If the string trimmer engine runs poorly the air filter might be clogged. Remove the air filter and inspect. If restricted replace with the correct filter.
Common
02 - Spark Arrestor
The spark arrestor is a small screen that prevents the engine from emitting sparks. Over time, the spark arrestor can become clogged with soot. If the spark arrestor is clogged, the engine may run roughly. To unclog the spark arrestor, remove it and clean it with a wire brush. You may also replace the spark arrestor.