How to service a lawn mower

Video by Proper DIY. This guide is based on the video "How to Service a Petrol Lawn Mower" from the Proper DIY YouTube channel. It runs through the whole annual service routine clearly and is particularly good on the spark plug gap check and how to drain old fuel safely — worth fifteen minutes of your time before you get started.

1. Drain or run out the old fuel first

Old petrol is one of the most common reasons a mower will not start after winter. Petrol left in the tank and carburettor for more than a few months goes stale, leaves gummy deposits, and can block the carburettor jets. Before you do anything else, either run the engine until it stops from fuel starvation or drain the tank with a hand siphon into a proper fuel container.

Do this outside with the mower on a flat surface, away from anything that can catch a spark. Dispose of old petrol responsibly — most local authority household waste recycling centres accept small quantities of fuel in a sealed container. Do not pour it down the drain or onto the ground.

2. Change the engine oil

Most domestic petrol mowers use SAE 30 or 10W-30 four-stroke engine oil. Check your mower’s handbook to confirm the right grade. With the engine cold and sitting on level ground, locate the oil drain plug — usually underneath the engine block — or, on smaller mowers, tilt the mower to drain oil out through the filler cap into a container. Have a rag handy; this is invariably a bit messy.

Once the old oil has drained, replace the drain plug and refill with fresh oil to the mark on the dipstick. Do not overfill — too much oil can work its way into the air filter and cause the engine to smoke and run badly. A small funnel helps if the filler neck is awkward to reach.

3. Replace the spark plug

Spark plugs are cheap and a new one every season takes the guesswork out of why the engine is hard to start. Remove the plug with a spark plug socket — most small engines take a 5/8-inch socket — and check the old one. A light grey or tan electrode means the engine has been running well; black and sooty suggests it has been running rich; oily or wet points to oil getting into the combustion chamber, which wants further investigation.

Install the new plug hand-tight first, then give it a quarter-turn with the socket. Overtightening can strip the thread in the aluminium head, which is an expensive repair. Reconnect the spark plug lead, making sure it clicks firmly onto the plug terminal.

4. Clean or replace the air filter

The air filter stops dust and grass debris from getting into the engine. A clogged filter makes the engine run rich, use more fuel, and lose power. Most domestic mowers have either a foam filter or a paper pleated element. Foam filters can be washed in warm soapy water, rinsed, dried thoroughly, and lightly oiled with engine oil before refitting. Paper elements should just be replaced — trying to clean them often damages the pleats.

Hold the filter up to the light to check how blocked it is. If you can barely see light through a paper element, it needs replacing. Filters are usually available from the mower manufacturer or a local garden machinery dealer for a few pounds. It depends on the model, but most are simple to swap in two minutes without any tools.

5. Check and sharpen or replace the blade

Disconnect the spark plug lead before you go anywhere near the underside of the mower — this is not optional. Tip the mower onto its side with the air filter and carburettor facing upward (this stops oil running into the air filter). Inspect the blade for nicks, bends, and dullness. A blade that has hit stones will have chips along the cutting edge; a bent blade will cause vibration that can shake the mower apart over time.

A mildly blunt blade can be sharpened with a flat file, working at the same angle as the original edge — roughly 30 to 45 degrees. Keep the same number of strokes on each end to maintain balance. A badly damaged or heavily nicked blade is better replaced than sharpened. After refitting, check the blade bolt is tight; a loose blade is genuinely dangerous.

6. Clean the deck and check the wheels

Grass clippings build up on the underside of the cutting deck and, if left, can restrict airflow and cause the engine to work harder. With the plug lead disconnected, scrape off compacted grass with a plastic scraper or an old kitchen knife — nothing sharp that will nick the deck. A hose and stiff brush gets the rest.

Check the wheels spin freely and that the height adjusters move between positions without sticking. Oil any pivot points with a drop of light machine oil. If the mower has a self-propelled drive, make sure the drive cable engages smoothly when you squeeze the handle. Top up the fuel with fresh petrol, reconnect the plug lead, and give the starter cord a pull — a well-serviced mower should fire up within two or three pulls.

When to call a handyman

Call Richard if the mower has a more serious fault — fuel leaking from the carburettor, persistent white or blue smoke, or a pull cord that has snapped inside the housing. These want a bit more mechanical knowledge and possibly some parts sourcing. If you are not confident working around petrol engines, or simply do not have time to do the service before the season gets going, it is also worth knowing that lawn mowing is something Richard covers around Sandwich and nearby East Kent villages.