You wouldn’t expect to run a 10-mile race after spending all winter on the couch drinking beer and eating potato chips. Likewise, too many of us neglect the health of our gas-powered lawn mowers then curse them come spring when they just say no. Here are six ways guaranteed to shorten the life of your mower from Consumer Reports' mowing expert Peter Sawchuk who is just back from conducting our mower tests in Florida.

Consider your mower’s fuel tank the perfect place to store old gasoline. Even without additives, stabilized gasoline will eventually gum up, clog fuel lines, and ruin your carburetor. And gas containing ethanol ages even more quickly. “It’s not so much the ethanol but that it absorbs a lot of water,” says Kris Kiser, president and CEO of the Outdoor Power Equipment Institute, a trade group. “Water in the engine is a killer.”
Smart move: At the end of the season, take 10 minutes to drain the tank or run the engine dry.

Never change or add oil. Moving metal parts need oil, and an engine that’s denied clean oil, and enough of it, will overheat for sure.
Smart move: The best time to do change the oil in a walk-behind mower is when it’s drained of gas—since you’ll need to flip the mower to remove the old oil.

The blades still turn, so forget about sharpening them. Ideally, you should sharpen your mower’s blades monthly—and even more often in the fall if you mulch leaves using your mower. This keeps the engine from working harder and less efficiently than it needs to, which affects its lifespan. But if you indeed forget, the brown-tipped grass will remind you before it dies altogether.
Smart move: You’ll need: A piece of 2x4 to wedge in the blade to keep it from turning (shown in photo) as you remove it. Sharpening costs about $10 if you don’t do it yourself.

Leave in your original air filter year after year. An engine’s carburetor needs to mix the gasoline with filtered air for smooth running, but the bigger hazard in not cleaning or replacing the filter annually is that a dirty or torn filter can allow dirt into the engine, which will accelerate wear and shorten its life.
Smart move: It costs about five dollars for the part and 30 seconds to put it in.

Ignore the engine’s cooling fins, even if they’re clogged with clippings. The cooling fins help distribute heat from the engine, which matters most on hot days. Let grass clippings and dust accumulate on them, and the engine could overheat.
Smart move: Running a whisk broom back and forth over the fins for a few seconds.

Don’t check the lawn for stationary hazards before mowing. Even if you know the location of every metal stake, sprinkler head, or tree stump, you could come upon a thick branch or jutting rock suddenly and hit it with the mower. Doing this with a walk-behind mower can bend the crankshaft. “If you bend the shaft, typically because of the price point of the product, it’s dead," says Kiser. “You’re just not going to get it repaired.”
Smart move: Take short walk and make a visual inspection before you mow.

There’s a seventh way to kill your mower as shown by Milwaukee homeowner Keith Walendowski when he took his shotgun to his Lawn-Boy a few years ago. But technically the mower was already gone—or at least needed a little TLC.

Source: http://news.consumerreports.org/home/2012/03/six-ways-to-kill-a-mower-and-what-to-do-instead.html