A lawnmower is meant to make your yard work easier, but without regular maintenance of the mower, you may find yard work just that little more difficult. Dirt in the system as well as bad fluids will make the mower perform inefficiently. Sputtering of the engine while mowing is often an indication of bad mower maintenance, not a fault with the machine. Most repairs to stop sputter are cheap and part of your regular mower maintenance.
Remove the cap of the gas tank and look in at the gas. Inspect the gas for signs of water. Water will make the mower sputter. Remove the old gas by tipping the machine over and catching the gas in a plastic tub. Replace the gas.
Turn the mower over so the oil case is sitting under the carburetor. Set a drain pan under the oil plug. Remove the oil plug using a wrench and drain all the oil out of the machine. Refill the crankcase with the oil type specified in your manual.
Remove the screw holding the air filter cover in place. Pull out the air filter and inspect it for signs of dirt. Replace a paper filter if dirty. Wash a foam filter using a drop of liquid dish soap and warm water. Squeeze the foam filter dry and allow it to sit for a while to dry out completely.
Check the underside of the unit for caked on grass. Excessive grass under the deck will make the mower sputter. Remove the spark plug using a wrench to prevent the mower from turning on while working underneath. Scrape the grass off using a paint scraper.