It's just plain smart to tune up your lawn mower every spring. According to manufacturer Briggs & Stratton, a tune-up can increase fuel economy by a third, extend the life of the engine, reduce repair costs and decrease emissions by up to 50 percent.
The three main components involved in a mower tune-up are the air filter, spark plug and oil, all of which must be changed. The needed parts and supplies can be found at home centers and hardware stores.
Start by changing the oil. Run the engine for a few minutes to warm up the oil so it'll drain better. Stop the engine, remove the drain plug and empty the old oil (tilt the mower back to get it all out, if necessary). Replace the drain plug and fill the mower with oil until it's visible through the oil fill hole, or check the level on the dipstick, if so equipped.
A clean air filter helps maintain the proper fuel/air ratio, allowing the mower to burn less gas. Swap out the old air filter (Photo 1).
Pull off the spark plug wire and remove the old plug with a spark plug wrench or deep socket (13/16 in. or 3/4 in., typically). Hand-turn the new plug until the threads catch. Ratchet the plug down until it stops, then turn it another quarter turn (cranking down too hard can break the plug or render it nearly impossible to remove).
Next, remove and replace the spark plug (Photo 2). If the plug's rusted tight, spray it with a penetrating lubricant like Liquid Wrench or JB80. Let it soak in for 10 minutes before trying to loosen the plug again.
The spark plug will be factory gapped, but make sure there is a gap between the electrode and overhanging arm. If you drop the plug and the overhanging arm bends tight to the electrode, the mower won't start.