7 Signs Your Mower Needs a Professional Repair
Most of what goes wrong with a mower is genuinely fixable at home. Blocked jets, dull blades, blocked air filters and flat batteries account for the large majority of failures, and none of them need a workshop.
But there is a line, and the expensive mistake is not crossing it. It is carrying on past it. These are the seven signs that the machine needs someone with a bench and a torque wrench.
Metallic Knocking From Inside the Engine
Not a rattle from a loose panel, and not the clatter of a blade hitting the deck. A rhythmic metallic knock that rises and falls with engine speed is coming from inside the block, and it usually means a bearing or the connecting rod.
Stop immediately. Every additional minute of running is doing measurable damage to a part that is currently repairable and will shortly not be. This is the one item on the list where the difference between stopping now and stopping in ten minutes is the difference between a repair and a new engine.
Blue Smoke That Does Not Clear
Blue or grey smoke means oil is reaching the combustion chamber and being burned along with the fuel.
There is an innocent version. If you tipped the machine the wrong way while cleaning the deck, oil has flooded the cylinder and the smoke will clear within a few minutes of running. Check the oil level too, because overfilling causes the same symptom and is easily corrected.
If the level is correct and the smoke persists on every start, the piston rings or valve seals are worn. That is an engine strip and it is not a driveway job.
A Blade That Will Not Stay Balanced
A blade that vibrates immediately after a careful sharpening and balance usually is not the problem. The problem is what it is bolted to.
A bent crankshaft, almost always the result of hitting something solid, throws the blade out of true no matter how well the blade itself is balanced. You can sometimes see it by removing the blade and watching the shaft as it turns slowly by hand.
If the machine has recently hit a rock or a tree root and has vibrated ever since, assume the shaft rather than the blade. The correct blade procedure is in our guide on preparing a mower for storage, which is when most people deal with it.
Fuel Leaking From Anywhere
A visible fuel leak is a fire risk sitting next to a hot exhaust, and it is not something to monitor and see how it goes.
Some causes are simple, such as a perished fuel line or a loose clip, and those are worth checking yourself. But a leak from the carburettor body, from the tank seam, or one you simply cannot locate means the machine should not be run until someone has found it.
| Where it leaks | What to do |
|---|---|
| Fuel line or clip | Replace the line, straightforward at home |
| Carburettor bowl gasket | Possible at home with a rebuild kit |
| Tank seam or moulding | Replace the tank, do not attempt a seal |
| Cannot find the source | Stop running the machine and get it looked at |
The Starter Cord Will Not Move
A cord that pulls normally but does not retract is a recoil spring, and recoil assemblies are inexpensive and replaceable as a unit.
A cord that will not pull at all is a different matter. Do not force it. Remove the spark plug and try again gently. If it moves freely with the plug out, the cylinder was hydrolocked with oil or fuel and you have found your answer. If it still will not turn with the plug removed, something inside the engine is seized and forcing it will turn a repair into scrap.
Cracks in the Deck or the Blade
This one is about safety rather than cost.
A mower blade spins at roughly three thousand revolutions per minute. A cracked blade at that speed does not simply stop working, it comes apart, and the pieces leave the deck with real energy behind them. There is no acceptable repair for a cracked blade. Replace it.
Deck cracks matter for the same reason. The deck is the containment around the blade, and a cracked deck no longer reliably performs that job. Welding a mower deck is possible but it is a job for someone who understands where the stress paths run.
It Runs Correctly Then Cuts Out When Hot
A machine that starts perfectly, runs for fifteen or twenty minutes and then dies, only to restart once it has cooled, is showing one of the more genuinely diagnostic symptoms in small engines.
The common cause is an ignition coil breaking down as it warms up. Coils can develop internal faults that only appear above a certain temperature, which is why the fault vanishes every time you go looking for it cold. Testing this properly needs the coil measured at temperature.
Before assuming the coil, rule out the simple version. A blocked vent hole in the fuel cap creates a vacuum in the tank as fuel is consumed, and produces almost exactly the same symptom. Loosen the cap and run the machine again. If it keeps going, you have found a two minute fix rather than a workshop visit. That trick and others like it are covered in our guide on small engine servicing basics.
Where the Line Sits
The pattern across all seven is straightforward. Fuel, air, spark and consumables are yours. Anything involving the crankshaft, the bearings, the rings or a structural crack belongs to someone with the right equipment.
The good news is that the preventable failures vastly outnumber the serious ones, and almost all of them come down to how the machine is stored and serviced. Our guides on what ethanol fuel does to a small engine and storing a generator correctly cover the habits that keep machines out of workshops in the first place.
Frequently Asked Questions
Is it worth repairing an old lawn mower?
As a rough guide, if the repair costs more than half the price of a comparable replacement, replacing usually makes more sense. The exception is a well built machine with a simple fault, since a quality mower with a fresh carburettor can outlast a new budget model by many years.
Why does my mower cut out when it gets hot?
The two usual causes are an ignition coil breaking down at temperature and a blocked vent hole in the fuel cap creating a vacuum in the tank. Test the cap first by loosening it and running the machine again, since that costs nothing and takes two minutes.
Can a bent crankshaft be repaired?
Straightening is technically possible but rarely economic on a domestic mower, because the labour involved usually approaches the value of the machine. On a ride on or a commercial mower it is more often worth doing. Either way it is workshop work, not a home repair.