Your gas-powered lawn mower operates the same as any gasoline engine. Fuel burns in a chamber spinning different gears to generate the rotation of the blade. The burning of fuel, plus the dust and debris from the air intake system, causes the internal parts of the engine to get dirty. You can take the engine apart to clean it, or take the lawn mower to a professional. A cheaper option to keep your lawn mower's engine running smooth and clean is using a fuel additive.
Pour half a can of standard fuel injector cleaner into a 5-gallon gas can.
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Fill the rest of the gas can with high-octane gasoline.
Shake the can thoroughly. Unscrew the gas tank lid on the lawnmower. Pour the mixture directly into the gas tank. Screw on the gas tank lid.
Remove the mower's air filter, usually located on the side of push and self-propelled lawn mowers. Unfasten the flat-head screw that secures the filter, using a screwdriver.
Shake off dirt and debris from the air filter. Reinstall the air filter.
Start the mower and use it. The cleaner in the fuel will remove a majority of dirt inside the engine.