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. You can try cleaning the carburetor 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 can make the engine run poorly. 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.