Clean your machine.
To keep debris out of your oil and air filter, hose off all that old dried grass and dirt before you begin.
Change the air filter.
A clean air filter helps your lawn mower’s engine run more efficiently and use less gas. On most walk-behind mowers, the air filter can be easily accessed using only a screwdriver. If you’re not sure what filter to buy, take your old filter with you to the hardware store.
Change the oil.
Consult your owner’s manual to locate the oil drain plug. Then start your mower and let it run for a few moments to warm up the oil so it drains more easily—but don’t let it get too hot. Being careful to avoid hot surfaces like the muffler, remove the drain plug and drain the oil. Replace the plug, and refill the oil to the appropriate level using the oil specified in your owner’s manual. Recycle the oil at your local recycling facility.
Replace the spark plug.
Remove the spark plug wire from your mower’s spark plug. Using a spark plug wrench or deep-socket ratchet (commonly a 13/16-in. or ¾-in), remove the old spark plug by turning it counter-clockwise. Measure the electrode gap on the old plug, or consult your owner’s manual for the correct gap measurement to make sure the gap on your new plug matches. Then, put a dab of dielectric grease on the tip of the new spark plug to prevent corrosion, and screw it into place with your wrench, being careful not to over-tighten. Replace the spark plug wire, and try to start your mower.