Due to carbon buildup and a weakened electrode the spark plug will degrade over time which can prevent this string trimmer from starting. 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 residue that results from old 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 pre-mixed fuel and oil product that is ethanol free. You can try cleaning to eliminate the clog, if cleaning is not effective, you can often purchase a carburetor repair kit to replace some of the components or installed a new carburetor altogether.
A dirty air filter might prevent the string trimmer from starting as well by restricting the air coming into the carburetor. As with the spark plug, the air filter should be replaced annually or whenever it is visibly soiled.
If the starter’s recoil spring is broken, the rope won’t be able to recoil onto the pulley, resulting in the trimmer not starting. Many recoil springs can be replaced individually but it may be better to replace the whole recoil starter assembly.
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 prevent the trimmer edge from starting. You can try cleaning the arrestor with a wire brush but you may need to replace it with a new one.