How To Service A Petrol Lawnmower
A yearly service keeps a petrol mower starting first pull and cutting cleanly and you can do the whole job at home with basic tools.
The short answer
To service a petrol lawnmower, pull the HT lead off the spark plug, then work through the air filter, oil, spark plug, controls, blade and fuel in one session. Fit a new filter, drain and refill the oil to the dipstick mark, fit a new plug, sharpen the blade, scrape out the deck and refill with fresh petrol. Do it once a year before the first spring cut.
A petrol mower that spends the winter in the shed with old oil, a tired plug and a deck full of dried grass is the one that refuses to start on the first warm Saturday of spring. Servicing it yourself needs only a few basic tools, and the steps are much the same whichever brand you own.
The photos in this guide come from a service we carried out on our own red petrol mower, which runs a Briggs & Stratton 300 series engine. Anyone who's changed the oil on a car or a motorbike will find the job familiar, and if you haven't, we take it one step at a time below.
Tools And Parts For A Lawnmower Service
Gather everything before you start so the mower isn't left on its side while you hunt for a socket:
- Four-stroke lawnmower oil in the grade your handbook lists
- An oil drain pan or an oil extractor pump
- A replacement air filter to suit your engine
- A new spark plug, plus a long-reach plug socket and wrench
- A screwdriver for the filter cover
- A flat file or angle grinder for the blade
- Fresh unleaded petrol in a clean fuel can
- Kitchen roll, a stiff brush and a pair of gloves
Before you touch anything, pull the HT lead off the spark plug. With the lead off, the engine can't fire if the blade is turned by hand while you're working underneath, and it's a habit worth keeping for every job on the mower. If you're still deciding which machine to buy, our guide to the best petrol lawn mowers for every garden size is a good place to start.
How To Service A Petrol Lawnmower Step By Step
1. Replace The Air Filter
The air filter is the easiest place to begin. It sits in a plastic or metal housing on the engine, held on by a screw or a pair of clips. Undo the cover, lift it away and pull the old filter out.
Paper filters are replaced when they're clogged, while some foam filters can be washed and re-oiled, so check which type your engine takes. Push the new filter fully into its seat before refitting the cover. A filter sitting at an angle lets dust straight past it into the carburettor.
2. Change The Oil
Oil that has been in the engine for a full season or longer is due a change. Mower engines take a dedicated four-stroke lawnmower oil, and the right grade for yours is printed in the handbook. The bottle in our photos is Alco oil, but the brand matters less than matching the grade your own handbook gives.
Warm oil drains more freely, so run the engine briefly first if it will start. Then either draw the old oil out through the filler with an extractor pump or tip the mower onto its side and let it drain into a pan, keeping the air filter and carburettor facing upwards so oil doesn't run into them.
Pour the new oil in a little at a time, checking the dipstick as you go, and stop once the level reaches the full mark. An overfilled engine smokes and runs roughly.
3. Fit A New Spark Plug
The spark plug is the part most people leave alone because they assume it's fiddly. It's one of the quickest jobs on the list. On our mower the plug sits at the front of the engine, and you can see the HT lead running across the OHV cover to the rubber cap that pushes over it.
Pull the cap off, fit a long-reach plug socket and turn it anti-clockwise to remove the old plug. Thread the new plug in by hand first so it can't cross-thread, then nip it up with the wrench and push the HT cap back on firmly.
4. Free Up The Controls
The safety bar you hold against the handle has to move freely and spring back as soon as you let go, because that's what stops the engine and blade. Self-propelled mowers have a second lever for the drive, and it needs the same check.
Work each lever back and forth while watching the cables. Any stiffness is cured with a squirt of WD-40 or light oil on the pivots and cable ends. A frayed or kinked cable needs replacing rather than lubricating.
5. Sharpen The Blade And Clear The Deck
Even the best mower tears grass with a blunt blade, leaving ragged tips that turn brown a day or two after cutting. With the plug lead still off, tip the mower over with the filter side up and check the blade edges for nicks, rounding and bends.
A flat file brings back a dull edge, and an angle grinder for cutting and grinding metal speeds things up on a badly worn blade. Keep to the original cutting angle and take the same amount off each end so the blade stays balanced. A bent or cracked blade should be replaced, and if you'd rather not sharpen it yourself, a mower repair shop will do it for a small fee.
While the mower is on its side, scrape and brush out the packed grass under the deck. A stiff brush and a little water shifts most of it. Clippings left to build up hold damp against the steel and choke the airflow that lifts grass into the box.
6. Refill With Fresh Petrol
Petrol goes stale when it sits in a tank for months, and stale fuel is behind plenty of mowers that won't start or keep misfiring. If yours has been stored over winter, drain the tank into a suitable container and refill it with fresh unleaded. Take the old fuel to a household waste site that accepts it rather than pouring it away.
7. Give The Mower A Clean
You can skip this step, but grass and leaves work their way into every gap on a mower, and some of those gaps matter. Debris around the petrol filler cap can drop into the tank the next time it's opened, and anything packed around the recoil cover and cooling fins traps heat around the engine.
A brush and a damp cloth are all it takes, and with a season's muck cleared off, the mower goes back in the shed looking cared for.
When To Service A Petrol Mower
Once a year is enough for most home lawns. Late winter or early spring is the best time, so the mower comes out of storage with clean oil, a fresh filter and a sharp blade ready for the first cut. A large lawn mown every week through a long season works the engine harder, so check the oil level and give the air filter a clean partway through summer as well.
A sharp blade matters most on young grass, which tears easily. If you've just laid turf or sown seed, our guide on when to cut new grass for the first time explains how high to set the mower.
After The Service
With the work done, push the HT cap back on, check the oil one last time and start the engine. Our guide on how to start a petrol lawnmower runs through priming and the pull cord, along with the common reasons a mower won't fire.
At the end of the season, run the tank low or drain it, clean under the deck and store the mower somewhere dry. If yours now costs more to keep going than it's worth, we've compared petrol and electric lawn mowers to help you decide what to replace it with.
What People Also Ask Us
How often should you service a petrol lawnmower?
Service a petrol lawnmower once a year, ideally in late winter or early spring before the first cut. That annual service covers the oil, air filter, spark plug, blade and fuel. If you mow a large lawn every week through a long season, check the oil level and clean the air filter partway through summer as well.
Which way do you tip a petrol lawnmower?
Tip a petrol lawnmower with the air filter and carburettor facing upwards. Tipping it the other way lets oil and fuel run into the filter and carburettor, which causes heavy smoke and a mower that won't start. Pull the HT lead off the spark plug first so the engine can't fire if the blade turns.
What oil does a petrol lawnmower take?
A petrol lawnmower takes four-stroke lawnmower oil, and the exact grade is listed in the engine handbook. SAE 30 is a common grade for walk-behind mowers, but go by what your handbook says. Never use two-stroke oil or mix oil into the petrol, as four-stroke engines keep the two separate.
Can you leave petrol in a lawnmower over winter?
Leaving petrol in a lawnmower over winter is a common reason it won't start in spring, because the fuel goes stale in the tank. Run the tank dry at the end of the season or drain it, then refill with fresh unleaded before the first cut. Store the mower somewhere dry and away from heat sources.
Should I warm the engine before changing the oil?
Yes, if the mower will start, give it a short run first. Warm oil is thinner and drains out more completely, so less of the old oil is left behind in the engine. When you refill, add the new oil in stages and check the dipstick each time, because overfilling makes the engine smoke and run roughly.
Can I sharpen the blade with a file or do I need a grinder?
A flat file is enough for a blade that's just dull. Save the angle grinder for one that's badly worn, where it saves a lot of elbow grease. Whichever you use, follow the original angle and take the same off both ends to keep the blade balanced. Bent or cracked blades need replacing, not sharpening.
Why pull the HT lead off before starting work?
Turning the blade by hand can turn the engine over, and with the HT lead still on the plug it can fire while your hands are under the deck. Pulling the lead off removes that risk completely. Keep it off for the whole service, including the blade and deck work, and only push it back on when you're ready to start the mower.
Which part of a mower service is easiest to start with?
The air filter is the gentlest start. Its cover comes off with a screw or a couple of clips, the old filter lifts out and the new one pushes in, so you'll have one job ticked off in minutes. Just check whether yours is paper, which gets replaced, or foam, as some foam filters can be washed and re-oiled.
Archie is our gardening specialist. He started out as an apprentice at a gardening firm, built up his own knowledge from there and now writes most of our garden content.