Due to carbon buildup and a weakened electrode, the spark plug will degrade over time which can prevent the engine 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 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.
The ignition coil sends voltage to the spark plug when the engine is running. A defective ignition coil may prevent the engine from starting. As with the spark plug you can use an ignition tester to test the coil to determine if the components have failed.
The recoil starter engages the crankshaft to turn over the engine. A malfunctioning recoil starter may not engage the crankshaft correctly. When the rope is pulled, tabs extending from the pulley and cam should grab the hub of the engine, causing it to turn. When the rope is released, the tab should retract as the rope rewinds onto the pulley. If the assembly appears damaged or does not function properly, you should replace it with a new one.
The flywheel key is a small metal piece that fits into the crankshaft and engages with the flywheel. If the engine stops suddenly, the flywheel key may break in half to prevent damage to the engine. If this happens, you will need to replace the broken flywheel key with a new one before the engine can be restarted.