A lawn mower features a simple combustion engine and parts that are highly durable. This is a tool of the house that takes a beating every time it cuts the grass and even when it sits outside in the elements. When your lawn mower won't start, the problem may be easier to diagnose and fix the problem than you think.
Gas It Up
It may sound obvious, but the simplest of things often go unchecked. Open the gas tank make sure there is gas in the tank. If there is, you may need syphon it out and add new fuel or—at the very least—top it off with fresh fuel.
All lawn mowers have a filter for the engine, usually right beside the fuel tank. You may have to remove the fuel line in order to get to it to remove it. A dirty fuel filter will cause fuel to not properly circulate. A small pair of pliers or wrench will allow you access to the filter. Remove it and wash it or remove debris with a can of air. If the filter is not able to be saved you can pick up a generic filter at the local hardware store.
Another filter often left unchecked is the air filter. Not all lawn mowers have one of these. If your lawn mower does it will be located close to the exhaust in a container. Remove it like you would a fuel filter and clean it the same way.
Over time spark plugs can become so dirty that it no longer is able to spark. Spark plugs are usually visible on most lawn mowers. Remove the hose and attachment covering the end of the spark plug. A wrench may needed to remove the spark plug. Look at the end of the spark plug and if it is dirty clean it up with brake cleaner and a wire brush. If this does not get the job done then purchase a new spark plug. Keep in mind that a spark plug can be tightened too much causing it to not fire. You can also wheel in a jump starting mower to see if yours will kick over. If it does then the spark plug most likely needs cleaning and not replacement.
If your lawn mower has been sitting out then old gasoline may be the cause of your troubles as they remain in the carburetor. You can purchase a cleaner specifically for the carburetor. Clean it out well and try to start it up again. If this doesn't work then it may be the air and gas mixture inside the carburetor. There are screws found on the carburetor that can be adjusted to offer more air flow or gas mixing.