The residue that results from old fuel being left in the tank can create a restriction in the carburetor. This is why you should always use fresh fuel when filling the tank and a fuel stabilizer to help maintain the quality of the fuel. For better stability, you can use a pre-mixed fuel and oil product that is ethanol free. You can try cleaning the carb with carburetor cleaner to eliminate the clog. If cleaning is not effective, you can often purchase a carburetor repair kit to replace some of the components or install a new carburetor altogether.
Due to carbon buildup and a weakened electrode, the spark plug will degrade over time which could result in the string trimmer running rough. You should inspect the spark plug for damage or wear and you can also use an ignition tester to help determine if the spark plug is defective. Generally, we recommend replacing the spark plug once a year.
The spark arrestor is a small screen that keeps sparks emitted by the engine from exiting the muffler. Over time, the arrestor can become clogged with soot which may cause the engine to run rough, you can try cleaning the arrestor with a wire brush, but you may need to replace it with a new one.