- A loose or worn belt shows up as a squeal, a weaker drive and a slower blade, and it is one of the quietest ways a mower that still starts wears itself out.
- Wobbly wheels, a clogged deck, a dull blade and a tired spark plug each give a sign you can see or hear, and catching them early helps you avoid a costly repair.
- Over one in 10 mowers in a 2023 Which? survey of 5,387 owners did not reach the seven-year mark without a fault, so a short check in the shed beats waiting for a breakdown.
A Mower That Still Starts Can Still Be Wearing Itself Out
Your mower starts first pull and it cuts. But it squeals when the blade engages, or it pushes like a barrow full of wet sand, or the lawn looks ragged an hour after you finish.
Switch the mower off first. On a petrol mower, pull the spark plug wire off the plug so it cannot fire by accident; on an electric mower, remove the power source. Then work through six checks in this order: the drive belt, the wheels, the underside of the deck, the blade, the spark plug, and the fuel and oil. Each one has a sign that tells you whether the part needs cleaning, adjusting or replacing.
A mower needs the same attention as a car, as some of these faults end in a breakdown, not just a poor cut. With proper care a mower should last at least a decade, yet in the 2023 survey of 5,387 owners, over one in 10 mowers did not make it past seven years without some fault.
The Squeal and the Slow Blade: A Loose or Worn Belt
The signs of a belt problem are a less powerful drive, a blade that spins slower than it should and a squeaking sound while the mower runs.
The squeal is slip. Rubber that has hardened or glazed from heat loses its grip on the pulley and slides against it under load, so the belt cannot pass the engine’s full power on. A glazed belt, one with a shiny, smooth inner face, will not get its grip back, and tightening it does not cure it. Which belt it is depends on what brings the noise on. A squeal when the blades engage points to the deck belt that spins them, found on riding mowers. A squeal on a self-propelled push mower only when the drive is engaged points to the small belt that runs from the engine to the rear wheels.
With the engine off, look at the belt. If it is loose, tighten it. If it is cracked or worn, replace it. A worn belt is easy to spot once you know what to look for: the edges should be smooth and even along the whole length, and an edge that looks shiny, glazed or burned means the belt needs replacing. A brittle belt with cracks is at risk of breaking, and shallow cracks count. A hot rubber smell while you mow means a belt is slipping or rubbing, so stop, switch off and let everything cool for 10 to 15 minutes before you look.
If a new belt wears out quickly, look at what it runs on. A faulty belt or a seized pulley can make belts fail early, and so can a burr on a pulley or something rubbing along the belt path. Black rubber dust piled around one pulley under the deck points at that pulley or its bearing, so sort it out before the new belt goes on.
Replacing a belt can be complex, so call a professional if you are unsure. If your self-propelled mower has stopped pulling itself, the walk-through on replacing the belt yourself shows what the job involves before you decide.
The Wobble and the Heavy Push: Worn Wheels and a Clogged Deck
Worn wheels are hard to push, cut unevenly and move with a wobble. The wheels of a walk-behind mower are usually held on by a shoulder bolt and a bushing, the metal tube in the middle of the wheel, and both wear with use until the wheel moves about.
Take the wheel off and look. If the bolt and bushing are worn, replace both and keep the wheel. Some mowers have no bushing, and there the bolt wears the plastic of the wheel itself, so the wheel needs replacing. Wheel sets are not universal, so check the manual before you buy one.
A heavy push can also come from under the mower. A clogged deck shows as cutting that gets worse, clumps of grass left on the lawn, hard work to push and excessive vibration. Clippings, dirt and debris build up under the deck and cut the mower’s efficiency, and the build-up can spread lawn disease.
Clean under the deck after each use, especially after wet grass. If that is not realistic, clean it at least once a month. With the power source removed, scrape and spray away what is stuck on with a wire brush, a damp cloth or compressed air. Never use a garden hose on an electric mower: water can get in and damage the electronic components. The full routine is in the post on how to clean a mower deck.
What the Blade and the Spark Plug Tell You
You do not need a tool to read the blade. Roger Cook, a contractor at This Old House, says: “You let the lawn tell you when to sharpen. You take a piece of grass after you cut it and you look at it. If it’s a nice clean cut on the top of a piece of grass, then your blade is great. If it’s all ragged on the edge, that’s not good.”
A sharp blade cuts the grass cleanly. A dull blade tears it, which leaves a ragged edge, a patchy look and a lawn that takes longer to mow. Torn tips can give the lawn a brown cast two or three days after mowing and make the grass prone to disease.
To sharpen, disconnect the spark plug wire and tilt the mower with the air filter facing up. Remove the blade, then file each end, following the original bevel angle. An angle grinder is faster, but wear eye protection, ear plugs and gloves. Balance the blade before it goes back on: an unbalanced blade strains the engine. If you would rather not do it, a repair or hardware shop will. The steps to sharpen and balance the blade are laid out in full on the site.
On a petrol mower the spark plug is the next check. A plug that is wearing out shows as hard starting, frequent stalling and a drop in power. If it is damaged, corroded or covered in deposits, replace it. If it is only dirty, clean it and reset the electrode gap with a gap tool. Manufacturers recommend a new plug every 25 to 50 hours of use, at the start of each season or when the engine plays up, whichever comes first. Thread the new plug in by hand to avoid cross-threading, and do not over-tighten.
Fuel, Oil and Air: The Damage You Cannot See
Most petrol contains ethanol, and ethanol absorbs moisture from the air. Over time that causes water to condense inside the tank, and as the fuel moves through the system the components deteriorate and the engine can gum up. Fuel left over winter degrades and deposits sink to the bottom, which is why a mower struggles to start in spring.
Craig Murray, owner of Liffco Power Equipment, puts the fix in plain terms: “Don’t leave cans of gasoline sitting around, because the fuel will absorb water, which makes motors run rough. Funnel whatever’s left into your car’s tank, then buy fresh gas come springtime.” Buy only the fuel you will use in a month. If the mower will sit for more than 30 days, add a fuel stabiliser, which slows evaporation and keeps the fuel fresh. A lower-ethanol petrol absorbs less moisture, and ethanol-free is better if you can get it. For long storage, run the tank dry or siphon it empty.
Oil needs changing at the start of each mowing season or after 50 hours of use, whichever comes first. Check it on level ground with the engine cold. Cook explains why: “When the engine’s cold, all the oil is drained down. You’re getting a proper reading. If you check it when it’s hot, some of that oil could still be up in the engine. You could actually overfill the oil.” Dirty or darker than usual means change it. Cook says the worst scenario is running an engine with low oil: “You could actually seize the engine, blow it up.”
Check the air filter too. “The engine has to breathe,” says Lee, a small-engine repair expert featured on This Old House. A new filter looks clean and white, and after a season or two, Lee says, “it’s gonna be gray, it’s gonna be dirt.” His advice: “Just blow it out, you could, if you want it, or you could just get a new one.” Replace the filter at least once a year, and more often in dusty conditions.
How to Stop It Coming Back, and When to Call Someone
Make the six checks your end-of-season routine. Clean the deck, sharpen and balance the blade, change the oil, fit a new plug and filter, and empty or stabilise the fuel. The mower then goes into the shed with nothing left to gum up or go dull over winter.
A cordless mower needs the same attention, aimed at the battery. If the battery is not old but run time drops or charging takes longer, keep the mower and battery clean and dry, and check and clean the battery connections. A lithium-ion battery is sensitive to extreme temperatures, so store it indoors and away from hot greenhouses and sheds. Recharge it a few times over winter. A battery left at 0 percent can lose the ability to hold any charge at all. On a corded mower, do not use it with a visibly damaged cable until the cord has been replaced or repaired as the manual says.
Call someone when the job is bigger than the sign. A belt you are unsure about is the clearest case, and so is a belt that keeps jumping off its pulleys or wearing out fast with no cause you can find. A blade you have no safe way to grind goes to a repair or hardware shop. Staying on top of this kind of maintenance can help you avoid costly repairs or buying a new mower earlier than you expected.
