How to Tune Up a Lawn Mower

Your manual wins. These are general practices. When your tool's manual says something different, follow the manual.

Before you start

• Pull the spark plug wire (or remove the battery on electric mowers). This prevents an accidental start while your hands are near the blade.
• When tipping a gas mower, keep the air filter and carburetor side up so oil and fuel don't flood them.

Parts and tools

New spark plug, oil (usually SAE 30 or 10W-30, per the manual), air filter, fuel stabilizer, socket set, spark plug socket, gap gauge, file or grinder, blade balancer, scraper.

Steps

1. Spark plug. Remove the old plug with a spark plug socket. Check the gap on the new plug with a gap gauge (the gap is in the manual; many mower plugs use 0.030 in). Thread it in by hand, then snug it with the socket.
2. Oil change. Run the engine a few minutes so the oil flows, then drain through the drain plug or by tipping (carburetor side up). Refill to the dipstick's full mark. Don't overfill.
3. Air filter. Paper filters get replaced. Foam filters get washed in soapy water, dried, and lightly oiled if the manual says so.
4. Blade. Block the blade with a piece of wood, remove the bolt (most turn counterclockwise to loosen; check the manual), and mark which side faces the grass. Sharpen the cutting edges at the original angle. Hang the blade on a balancer or a nail: if one side drops, file more off that side. An unbalanced blade shakes the engine apart.
5. Deck. Scrape caked grass from under the deck. It blocks airflow and causes rust.
6. Fuel. Old gas causes most hard starts. Drain gas that has sat for more than a month, refill with fresh gas, and add stabilizer if it will sit.
7. Check the rest. Drive belt cracks (self-propelled), wheel height adjusters, cables, and the safety bail that stops the blade.

When it still won't start

Fresh fuel, a new plug and a clean filter fix most mowers. If it still won't run, the carburetor is likely varnished from old fuel and needs cleaning or replacing.