A man fixing a lawn mower part.

14 Common Lawn Mower Faults And How To Fix Them

11 minutes

Knowledge of lawn mower maintenance is essential for do-it-yourself lawn caretakers who wish to keep their lawns looking immaculate. So, to help, here are 14 common faults you may have with your lawn mower and the related maintenance advice and tips that will help you fix them.

Common lawn mower faults that need a fix relate to the engine either not starting or not running smoothly. The 14 common ones are the lawn mower failing to start, smoking, going slow, a hard-to-pull starter cord, stuck starter cord, the lawn mower overheating, not cutting the grass, blocked or flooded carburetor, the lawn mower failing to bag or discharge properly, not mulching, vibrating badly, cutting the grass unevenly, and failing to turn off. Proper maintenance will prevent these problems, and you can always call in the professionals too.

14 Common Lawn Mower Faults And How To Fix Them

A man performing maintenance on his lawn mower.
Good ol’ fashioned hard work right here.

Knowing the 14 most common law mower faults and how to fix them is an essential part of proper lawn mower maintenance. So, let’s start with when the lawn mower fails to start.

1. Lawn Mower Fails To Start

A lawn mower that fails to start is the most common problem with lawn mowers. This is especially true if you store it over the winter or don’t use it for a long time.

There are a handful of things you should examine if your lawn mower doesn’t start, starting with issues with the fuel.

Fuel Problems

The most obvious cause is fuel starvation, as your lawn mower won’t operate if the fuel tank is empty. 

Similarly, dispose of any fuel that is more than 30-60 days old before cleaning the carburetor. Old fuel may be contaminated with water and, depending on the type of container it was stored in and the temperature it was stored at, can deteriorate very quickly.

Also, your gas tank may be leaking. If this is the case, you should replace the tank or replace any hoses that may have perished or become cracked or damaged. Check the hose clamps on the fuel line and the tubing on either side. That is most likely where the problem lies.

Battery Problems

Lawn mowers, like most small vehicles, run on batteries. The batteries will eventually lose their ability to retain or carry a charge due to deterioration. If this is the case, you will need to replace them. 

Batteries on small engines are either 6 volts or 12 volts. Replacements are available from your lawn mower dealer, online, or from an auto-parts store.

If you have a multimeter, you can test your battery to see if it is flat. On 12-volt batteries, a reading of fewer than 12.2 volts indicates the battery needs a charge.

To charge your battery, hook it up to a trickle charger. The battery should be ready for use within 24 hours.

Air Filter Issues

Dust and grime clog air filters. This impedes the airflow and makes it difficult or impossible for your lawn mower to start

Simply remove the air filter to clean it and get rid of the accumulated dirt. If it is severely damaged, it might be better to replace it entirely.

Spark Plug Failure

Your lawn mower may not be able to operate due to loose, filthy, or disconnected spark plugs. Before you try to start your machine, make sure the spark plugs are secure, clean, and tight. Spark plugs can last a long time, but are prone to wear and tear, so replace any damaged or worn ones.

When checking your spark plug, check the condition of the electrode and the spark gap. If the tip of the plug is black and oily, you may have a badly worn engine that needs an overhaul.

If the tip is very wet from fuel, then your carburetor may be letting in too much fuel, which will prevent the engine from starting. A spark plug in good working condition will be a light to tan color with minimal buildup of soot or carbon on the electrode.

2. Smoke Coming From The Lawn Mower

While smoke coming from your lawn mower can be quite worrying, it is usually not too serious. For example, the oil reservoir may be overfilled or the oil cap may be loose. If oil leaks out onto the muffler or any part of the engine while it is hot, it will cause billowing white smoke.

If you see the smoke, switch off your lawn mower and wait for the engine to cool. Then check the oil cap to see if it is secure. Also, check the oil level to look for overfilling. If it is too full, remove some of the oil and you should have no further problems.

If an irregular or rough idle accompanies the white smoke or runs very rough or vibrates excessively, then you may have a damaged engine that requires professional maintenance or repair.

 3. Lawn Mower Speed Very Slow

If you have a self-driven mower, then a very slow speed is indicative of a drive belt that has become loose. The drive belt is located inside the engine case. You can consult your manual to check for its exact location. 

Inspect the belt with the lawn mower switched off. Check to see if it is damaged, worn, or just loose. 

You can re-fit a loose belt and then adjust the tension or, if the belt is worn or damaged (it can become stretched over time), then exchange it for a suitable replacement.

 4. Starter Cord Hard To Pull

You’ll find it difficult to pull the starter cord if there is debris stuck in the blades, if the starter mechanism is jammed, or if the recoil spring is damaged.

With the lawn mower switched off, check to see if there are grass clippings or branches stuck in the blades. Removing the obstruction will solve the problem.

While you are checking the blades, ensure the blades are intact and the bolts are tight. If the blade is loose or missing, the starter cord will be hard to pull as the weight of the blades acts like a flywheel, which helps to pull the engine through its compression stroke.

Also, make sure to raise the cutting deck when pulling the starter cord. The blades may be impacting the ground or thick grass, making it difficult for the engine to turn over.

5. Starter Cord Stuck

You may find that the starter cord jammed inside the starter mechanism due to coming loose inside or because the recoil spring is broken or damaged. 

If this is the case, then you won’t be able to pull the starter cord at all. You will have to disassemble the starter cord assembly and replace the spring to solve the problem.

6. Lawn Mower Overheating

The signs of an overheated lawn mower engine can be rough idle, smoke rising off the chassis, uneven revs while running at high speed, and failing to restart after being switched off for a short time.

Lawn mower engines are mostly air-cooled, so make sure the fins around the engine are kept clean. Also, make sure there is no buildup of grass clippings or dirt around the exhaust and underneath the chassis.

Running a lawn mower that is overheating will eventually lead to engine failure and a hefty repair bill.

7. Lawn Mower Doesn’t Cut Grass

There are a variety of reasons why your lawn mower may not cut your grass effectively.

Grass should always be cut dry. So, avoid cutting your grass if it has just rained, especially if the grass is long. Wet grass places undue stress on your lawn mower. It will wear out the blade’s cutting edge more quickly than if it is dry.

Raise the level of your cutting deck if the grass is long and take two or three passes. Lower the deck each time until the grass is at the required height.

Also, your blades may be dull if you have not looked after them regularly. Sharpening intervals vary depending on how often you use your lawn mower and the size of the lawns you mow.

A good rule of thumb is to sharpen the blades at the beginning of the season and then again in the middle of the season. If you are cutting large areas more often, then you may have to sharpen your blades every 4 to 6 weeks.

8. Lawn Mower Carburetor Blocked

There are a few signs your carburetor is dirty and has a blockage, which are the following:

  • You’ll find it hard to start the lawn mower and when the engine does run, it will stall while you are mowing the grass.
  • While you’re mowing, the lawn mower will run faster and produce a harsher exhaust note than usual.
  • You may notice black smoke coming out of the exhaust when the engine runs rough.
  • There will also be a noticeable increase in fuel consumption.

Solving the problem requires you to remove the air filter to clean the carburetor and then remove and disassemble the carburetor.

Unless you have experience working on small engines, this is a job best left to a qualified technician. If not, you can damage the carburetor if you don’t take care when you put it together.

9. Lawn Mower Carburetor Flooded

If you flood the carburetor, then your lawn mower won’t start, and there will be an overwhelming smell of fuel.

The most common cause of flooding is prolonged use of the choke or a stuck choke when attempting to start the lawn mower. 

If your lawn mower has an automatic choke, restarting the engine immediately after switching it off can cause the carburetor to flood.

To solve the problem, disengage the choke and wait for 5-10 minutes for the fuel to evaporate.

If you remove the spark plug, look to see if it’s very damp. If so, this is confirmation of a flooded engine.

Also, if someone didn’t set the carburetor up correctly, you can turn the mixture screw counter-clockwise a quarter turn and then slowly turn it clockwise until you get a correct mixture.

10. Lawn Mower Fails To Bag Or Discharge Properly

Common grass catcher issues, such as clumping, clogging, uncut grass, and an empty grass bag, are brought on by a variety of reasons.

If the grass is too tall or wet or the cutting deck is set too low, the grass will get clogged in the chute and not discharge properly.

As a general rule, don’t cut the grass if it is too wet. Use several passes to cut long grass. Raise the deck first then lower it after each pass.

Trying to cut too much grass too quickly causes grass to build up on the blades. Using a throttle setting that is too low or if your lawn mower engine is not performing at its peak level will also cause the grass to not discharge efficiently.

If you try to cut the grass too quickly, you’ll cut more grass than it can eject, causing a blockage.

Also, if your lawn mower experiences a belt slip or power take-off (PTO) slip, then grass can get stuck in the chute. 

To avoid most of these issues, make sure to keep your cutting deck clean, especially if the grass you’ve been cutting is slightly damp. Also, make sure you sharpen your blades, as this will ensure the grass clippings are the right size. This also helps prevent the grass from clumping together and forming a mat that is difficult to expel.

11. Lawn Mower Not Mulching

A lawn mower that doesn’t mulch most likely has dull or damaged blades. Trying to cut wet grass or tall grass with the cutting deck set too low will also prevent the lawn mower from mulching efficiently.

Check the condition of your blades and replace or sharpen blades that are not in good shape. Also, ensure the chute and cutting deck are kept free of old, dry grass and that your lawn mower will mulch properly.

12. Lawn Mower Vibrating Badly

If you find that your lawn mower vibrates excessively, then the most likely cause is a broken blade or one that is out of alignment. 

The uneven weight distribution at each end of the spinning blade is what causes a vibration similar to a washing machine that knocks when the load is not evenly distributed.

You must replace the damaged blade. You must also make sure to properly align and tighten the replacement to eliminate the uneven weight distribution. 

13. Lawn Mower Cuts Grass Unevenly

If your lawn mower is not cutting level with the ground, then it is usually due to a broken or damaged wheel or loose, crooked cutting deck.

You may also just need to tighten the bracket holding the wheel in place or replace the damaged wheel.

If your blades are dull, the cut will be uneven. Sharpening the blades easily remedies this.

If the blade has made contact with a large root, branch, or boulder that stopped the motor, it may have bent the crankshaft. This will require professional assistance to repair.

Also, if you don’t unclog the cutting deck and blades, then it can cause uneven cutting as well.

14. Lawn Mower Fails To Turn Off

While a lawn mower that won’t start is irritating, it can be just as irritating if it won’t turn off.

The cause is usually a damaged ignition (kill) switch or disconnected ground wire. If the ignition switch is faulty, then replacement is your best option.

If the ground wire is damaged or not properly connected, replacing the ground wire and terminals will solve the problem.

In Conclusion: 14 Common Lawn Mower Faults And How To Fix Them

You can avoid most of the common lawn mower faults with proper maintenance and care of the equipment as per the manufacturer’s instructions. This is especially true when it comes to how to fix these common lawn mower faults.

Keeping the blades sharp and not trying to cut too much grass too quickly will also reduce the wear and tear on your lawn mower and prolong its useful life.

John Mulder
Lawn Mowers Enthusiast
John has years of experience maintaining and servicing small engine equipment dating back to 1982. While not out flying or sailing, you’ll find him tinkering with lawn mowers, weed whackers, chainsaws, and ATVs. He loves looking after his landscaped gardens, and perfect lawn, and tending his fruit trees.