A lawn mower that has been sitting for a prolonged period of time can have a number of mechanical issues that would prevent it from easily turning over and starting. One easy way to isolate the problem is to work through common potential causes, one by one.
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Over time, the motor oil in your lawn mower can thicken. This is especially true if your lawn mower has not been run for awhile. A good starting point when trying to get a lawn mower running again is to change the oil. Purchase the necessary amount and weight of oil noted in your user manual and drain the crankcase of the old oil. Attempt to start the mower once you have completed the oil change.
Gas that is left in lawn mower can become stale after about 30 days. Additionally, condensation inside the gas tank mixes with the gas and waters down the fuel. Remove your gas tank and empty it of any old gas. Run the engine, if it will turn over, to burn off any old gas still in the carburetor or fuel lines. Reattach and refill the tank and attempt to start the mower.
Clogged Intake Filter
The intake filter sits on the intake side of the engine and filters air before it is pulled in to the engine to be mixed with fuel and combusted. If this filter is clogged, the engine effectively is suffocated and combustion can not occur. This causes the engine to run very briefly, or not at all. Remove the intake filter case and clean or replace the filter.
Bad Spark Plug
The spark plug ignites the fuel and causes the engine to turn over. If the plug has been sitting, or has gotten wet, it may have corroded or become clogged with insect debris. Remove the spark plug using a spark plug socket and replace it with a plug that is designed for your model number. Reattach the cable and attempt to turn the engine over once more.