Petrol-powered lawn mowers, like any machine driven by an internal combustion engine, require maintenance over time to keep running smoothly. Here are the five things you should check to give your mower a tune up. In this video from This Old House, Roger Cook shows us how to keep the mower running happily. If you're familiar with engines, you've probably done most of this before — but personally, I need some guidance. First of all you should check the oil level (ideally before every use) and change it if it's dirty. Then you should check the air filter, as a dirty, clogged filter will just make the engine work harder and less efficiently. Next up is the spark plug; Cook suggests you change it "every season" (that is, a few times a year).
Of course, you should also check that the blade hasn't dulled too much and sharpen it if necessary.
What I didn't know is that old petrol can go "stale" and you should only buy a small quantity (say a month's worth) at a time. Otherwise the ageing ethanol in the petrol can damage the engine. You can add a fuel stabiliser to extend the life of the petrol and prevent it from damaging the engine. Get those five items in check, then all you need is the lawn.
How to Tune Up Your Lawn Mower [This Old House]