Just like in a bike, with the bad spark plug, or a running bike can be stopped due to its bad spark plug.  Sometimes our bikes stoped in running position. Then we try to start it, but as a result, after many trying, we can begin our bike due to its spark plug. If in the spark plug are some dust, or t's damaged so it will never begin again.  In the lawnmower, the same spark plug is used. Spark plug generates typically electric power from the battery. And the power is taken by the engine. So the machine starts up. If the spark plug is full of dust or is damaged, so the mower never starts again. Just like in a bike, with the bad spark plug, or a running bike can be stopped due to its bad spark plug.  Sometimes our bikes stoped in running position. Then we try to start it, but as a result, after many trying, we can begin our bike due to its spark plug. If in the spark plug are some dust, or t's damaged so it will never begin again.  In the lawnmower, the same spark plug is used. Spark plug generates typically electric power from the battery. And the power is taken by the engine. So the machine starts up. If the spark plug is full of dust or is damaged, so the mower never starts again. So If a Self-propelled mower won't start, so firstly check your mower spark plug. If it is full of dust so clean it. Then try to start it. Yet, it does not start, so replace the spark plug. Gasoline If you store your mower in the last winter and unfortunately left some gas in the mower tank. Now this season, while you were unboxing your lawnmower, you saw there is some gasoline in the tank. If the gas emits a bad smell, so you need to remove all the gasoline from the tank, replace the air filter and the gas filter too. Put fresh petrol in the tank. Then check all mower part; if all part is good. So pull your mower and get prepare for mowing. Air Filter and Oil Filter After unboxing your mower from the storage; secondly, check your mower air filter and oil filter. If both filters are dirty and full of dust so must be clean. If you consider your mower long-lasting life, so you must replace both filters in new. Carburetor Cleaning a carburetor is technical and complex work. But it is not impossible to work. Check the mower guideline books; they provide guidelines on how to clean the carburetor to the customer. If you can not clean it, so must take your mower to customer support or the mechanic.