The Ultimate Guide: Troubleshooting Your Lawn Mower When It Won’t Start

Introduction

An engine is a powerful machine that needs three essential things to work: fuel, air, and a spark. The fuel, often petrol or diesel, provides the energy, while the air provides the oxygen necessary to burn the fuel efficiently. The spark, created by a spark plug, ignites the fuel and air mixture, causing a small explosion that makes the engine’s parts move. If any one of these three elements is missing – fuel, air, or spark – the lawn mower won’t start and run. These three components are equally necessary to start the engine and keep it running.

An engine is a carefully orchestrated machine, where all the parts must work together seamlessly for it to function properly. Clogs and blockages in the finely tuned moving parts can prevent the engine from working, just like a small pebble can jam a complex clock’s gears. The other parts of an engine have important roles too. Some create high voltage needed to make the spark needed to ignite the fuel, while others filter the air and fuel to ensure a smooth and efficient burn. There are also safety components that protect the user from potential harm. All these parts must be set correctly and work together harmoniously to keep the engine running and the user safe. Regular maintenance of your lawn mower will ensure your lawn mower starts properly every time. This guide will cover 99% of the problems you might encounter when starting your lawn mower. If it still doesn’t work, maybe you should consider a new model – here you’ll find the top 5 petrol lawn mowers?

Operator Presence Control (OPC) also known as “Dead man’s switch” or “Dead man’s handle”

The Operator Presence Control (OPC), also known as “Dead man’s switch”, “Dead man’s handle” or “engine flywheel brake”, is a safety feature found on many lawnmowers. Its purpose is to ensure the lawn mower’s engine and cutting blades stop running if the operator releases their grip on the handle. This safety mechanism helps prevent accidents and injuries, especially when the operator loses control of the mower or needs to stop the machine quickly. To troubleshoot the OPC, first, make sure you have moved it to and are holding it in the “run” position. Next, check for any visible damage or wear on the switch or handle. Ensure the cable connecting the switch to the engine is properly attached and not broken. The OPC is a crucial safety component that ensures the safe operation of a lawn mower, protecting both the user and those around them, and it should not be bypassed.

Fuel

Fuel is a critical component of a petrol lawnmower’s operation, and several issues can arise if it is not managed correctly. One common problem is that fuel may not reach the engine due to a clogged air/fuel filter or a malfunctioning carburettor. To solve this, tap on the carburettor to let the fuel flow through to the engine and check for clogged filters.

If the fuel filter or carburettor becomes clogged, it may prevent fuel from reaching the engine. In this case, the old fuel from the float bowl should be drained, and the carburettor should be thoroughly cleaned. If these steps do not work, the carburettor may need replacing.

Sometimes, the fuel cap can also cause problems. Without a vent, a partial vacuum forms in the fuel tank that eventually stops the flow of fuel. If the mower sputters and dies after a few passes, it could be due to the fuel cap. You should remove the gas cap to break the vacuum and then reattach it. If the mower does not start or cuts off again after a few minutes, a new gas cap may be necessary.

You should always make sure you purchase the correct fuel for your lawn mower.

Old or stale fuel

One of the most common reasons why petrol lawnmowers won’t start initially is due to old or stale fuel. Petrol is an organic hydrocarbon and like all organic substances, it breaks down naturally over time. Gasoline has an ideal shelf life of about 30 days due to the ethanol it contains. If left inside the lawnmower for longer than this period, it can cause corrosion inside the machine and even clog the carburettor. Most people forget this and assume all is well when they see leftover fuel in the petrol tank. This can lead to unnecessary checks and changes.

To avoid problems with stale fuel, remember to replace the petrol inside the lawnmower every 30 days or so. You can also use a fuel stabilizer to keep petrol fresh for up to 3 years. However, fuel stabilizers cannot revive stale fuel. If the fuel in your mower is more than 10 to 12 weeks old, drain it and replace it with new, fresh fuel. Do not refill it with fuel from a jerry can because it is probably as old as the fuel in the mower itself. Instead, replace the whole lot with new fuel from the pump and only buy fuel you will use within a few weeks of purchase.

Check the Air Filter

A common issue that can prevent a petrol lawnmower from starting is a dirty or blocked air filter. The air filter guards the carburettor and engine against debris such as grass clippings and dirt. Over time, these particles can clog the filter and prevent the engine from starting.

To check the air filter, open the filter cover and inspect the sponge or paper filter for blockages. If the filter is dirty or torn, it must be cleaned or replaced. Depending on the type of filter, it may be sufficient to tap it regularly and apply some air filter oil.

Cleaning the filter involves washing foam filters in hot water and detergent to loosen grime, allowing it to dry completely, and wiping fresh motor oil over the filter before replacing it in its housing. On the other hand, paper filters can only be replaced.

A clean air filter is essential for the smooth operation of the lawnmower. Without it, the carburettor may be starved of oxygen, causing poor starting, poor fuel economy, and potential engine damage. To keep your mower in good condition, replacing paper filters or cleaning or replacing foam filters after every 25 hours of engine use is recommended.

If your air filter is dirty, cleaning or replacing it can be a straightforward solution. In addition to grass clippings and dirt, some air filters can get clogged with debris, such as leaves. Regular air filter inspection and cleaning can help prevent these blockages and keep your lawnmower running smoothly.

Blade Issues

Blade issues can cause a lawnmower to fail to start. If the blades are jammed or obstructed, the lawnmower won’t gain enough momentum to start. It’s important to check for any blockages around the blades.

Grass clippings can clog the mower deck, particularly when mowing a wet lawn, preventing the blade from turning. If the starter rope seems stuck or is difficult to pull, it’s likely due to a clogged deck. It’s recommended to turn the mower safely off and tip it over onto its side to examine the underbelly for large clumps of cut grass that can be removed using a trowel.

When inspecting the cutting blade, remember to turn the mower over correctly and according to the manual. You should always remove the spark plug cap before working on or near the cutting blade to avoid unexpected starts. Winter grass is tough and can quickly blunt a blade, so it’s essential to keep the cutting blade sharp. A blunt blade will give a poor cut, labour the engine causing engine wear, and increase fuel consumption. Cutting blades spin at high speeds and always need to be balanced. Debris can damage the cutting blade, causing engine strain and leading to costly repairs.

Mower cutting blades can be sharpened and balanced, and owners can refer to the owner’s manual for details or call their local dealer or mower service centre for assistance.

Choke / Flooded engine

The choke is a feature on some lawn mowers designed to assist with starting the engine, especially when it’s cold. To use the choke, pull it out before starting the engine and then push it back in once the engine is running smoothly. Be cautious not to leave the choke on for too long, as this could cause the engine to flood. However, many newer lawn mower models have replaced the choke with a primer. To use a primer, press it a couple of times before starting a cold engine. Be careful not to press it too many times, as overdoing it can also lead to a flooded engine. If you encounter issues starting your lawn mower, ensure the choke or primer is functioning properly and not causing any flooding.

If you suspect that your engine is flooded, don’t worry – there are steps you can take to resolve the issue. A flooded engine occurs when there is too much fuel and not enough air in the combustion chamber, making it difficult for the engine to start. To deal with a flooded engine, first, turn off the ignition and wait for a few minutes to allow the excess fuel to evaporate. Then, open the throttle – if your lawn mower has one – or hold down the primer button to allow more air into the engine. With the throttle open or primer held down, try starting the engine again without engaging the choke. This process should help clear the excess fuel and allow the engine to start. If the engine still does not start after several attempts, consult your lawn mower’s user manual or contact a professional technician for further assistance.

Carburetor

A clogged carburettor is one of the most common issues that can prevent a lawnmower from starting. This can happen when fuel is left in the mower for an extended period, causing some of the ingredients to evaporate and leaving behind a sticky substance that clogs the carburettor. If this is the case, try cleaning the carburettor with a carburettor cleaner. If that doesn’t work, it may be necessary to replace the entire carburettor.

A dirty carburettor can also cause a lawnmower to start and then die or not start at all. The carburettor mixes fuel with air to create the right fuel/air mixture for combustion. If the carburettor is dirty, it won’t be able to draw air into the engine, causing the mower to malfunction. A dirty air filter is often the culprit, as leftover petrol can condense and solidify in the filter, preventing it from pulling in the air. It’s also possible that the carburettor isn’t attached properly and is leaking air after it draws it in.

If you haven’t cleaned the carburettor recently, it’s worth checking again to see if it needs cleaning. Many models have a primer, a manually operated fuel pump near the carburettor that can fill it with fresh fuel in a few pump strokes. This can be especially useful if the mower has been run empty or unused for an extended period. Be sure to check the primer’s functionality with a few pump strokes before attempting to start the mower.

Cables

Damaged or loose cables can lead to various problems, such as difficulty starting the engine or loss of power. To check the electrical cables, begin by turning off the mower and disconnecting the spark plug to ensure safety. Examine the cables for any signs of wear, fraying, or damage, paying close attention to the connections at the battery, solenoid, and starter motor. Make sure the connections are clean, tight, and free of corrosion. If you notice any damaged cables, replace them promptly to avoid further complications.

Check the Spark Plug

Experiencing difficulty starting your petrol lawnmower might indicate a problem with the spark plug, which is responsible for igniting the fuel in the engine. Over time, spark plugs can become dirty, corroded, or disconnected. To inspect the spark plug, first, remove the lead and then carefully unscrew the plug using a spark plug spanner/socket. Examine the end of the plug for any signs of wetness from petrol, which could point to a carburettor blockage. Check the fuel line by removing and blowing through it to ensure it’s clear. If the plug is dry and the fuel line is clear, the issue is likely with the carburettor, which should be cleaned by a professional if you’re inexperienced in handling it.

If the spark plug appears wet or has visible corrosion, burns, or sludge, it’s best to replace it rather than clean it. For cleaning, use a grease-cleaning solution or solvent and gently sand the contacts. Make sure the gap is set correctly, as specified in the user manual. Always have a spare spark plug handy in case of a sudden malfunction.

When inspecting the spark plug, pay attention to the electrode and insulator for any buildup. If present, spray brake cleaner on the plug, let it soak for a few minutes and then wipe it clean. Reinstall the spark plug by hand before using a socket wrench to tighten it. If the problem persists, consider replacing the spark plug.

The spark plug’s condition can also be assessed by unscrewing it from the motor and visually inspecting it. If the electrodes have a metallic or fawn colour without soot, the spark plug can typically be used for another season. Slight discolourations can be removed with a wire brush. However, if the spark plug is black or oily, there may be a defect in the system that needs urgent repair. When purchasing a new spark plug, make sure the type designation is correct, which can be found on the ceramic body. Check the manual to make sure the new spark plug is compatible.

Check the oil

Regularly checking the oil levels in your lawn mower is important to prevent serious damage to the engine. Most lawnmowers have a dipstick to check the oil. Locate the dipstick, remove it, and wipe it off with a clean cloth. Reinsert the dipstick into the engine and remove it again to get an accurate reading for the oil level. Top off the oil if necessary, and ensure you use the recommended oil for your lawn mower. Regularly checking your lawn mower’s oil levels can help increase its longevity, reduce the chance of engine failure, and keep your lawn mower running smoothly.

The correct oil type can be found in the owner’s manual. Using the wrong oil can reduce efficiency and risk permanent damage.

Depending on the age of the engine oil, it may be time for an oil change. For commercial 4-stroke lawn mowers with normal use, it is recommended to change the oil once a year, ideally in the spring. An oil change should occur every 25 operating hours for petrol mowers with heavy use. The first oil change should occur after 5 to 6 hours of operation for newly run-in lawn mowers. These short change intervals compared to a car are because lawn mower petrol engines usually do not have an oil filter, making the oil dirtier more quickly.

Remember that the oil level should be checked not just at the start of the season but at regular intervals to ensure the longevity of the mower’s engine.

Battery

If your lawn mower features an electric start, it relies on a battery to provide the power needed for the engine to start. A charged battery is essential for the smooth operation of your mower. A flat battery could be the culprit if you’re experiencing difficulties starting your lawn mower.

To troubleshoot a flat battery, follow these steps:

Check the battery connections: Make sure the connections between the battery and the mower are clean and secure. Corrosion or loose connections can impede the flow of electricity, preventing the mower from starting.

Test the battery voltage: Using a multimeter, measure the voltage of the battery. A fully charged battery should read around 12.6 volts or higher. If the voltage is significantly lower, the battery may be flat or faulty.

Charge the battery: If the voltage reading is low, connect the battery to a compatible charger and allow it to charge for the recommended amount of time. Consult your lawn mower’s user manual for specific charging instructions.

Test the battery again: After charging the battery, measure the voltage once more using the multimeter. If the voltage remains low despite charging, the battery may be damaged or have reached the end of its lifespan, and a replacement may be necessary.

Check the charging system: If the battery continually loses charge or becomes flat, there could be an issue with the lawn mower’s charging system. In this case, consult your user manual or contact a professional technician for assistance.

Starter Solenoid

The solenoid is an essential component in a lawn mower’s electrical system, responsible for transferring electrical current from the battery to the starter motor when you turn the ignition key. Essentially, it acts as a switch, enabling the engine to start when needed. To troubleshoot a solenoid on a lawn mower, begin by ensuring the battery is charged and properly connected. If the engine still fails to start, locate the solenoid (often found near the battery or starter motor) and check its connections for any signs of corrosion, wear, or damage. Clean and tighten the connections as necessary. If the issue persists, use a multimeter to test the solenoid’s functionality by measuring the voltage at the input and output terminals while turning the ignition key. If the voltage reading does not change when the key is turned, the solenoid may be faulty and need replacement.

Conclusion

The information in this article should cover 99% of the issues stopping your lawn mower from starting. With the help of this article, you will be able to diagnose and repair any lawn mower that won’t start. You’ll learn how to check for spark plugs, fuel lines, air filters, fuel tanks and other possible causes of a lawn mower not starting. Also included are tips on when to replace parts or when it’s time to call in a professional. We hope this article helps you get your lawn mower running like new again!

Please read through each of the troubleshooting steps and be sure to follow all safety precautions before starting any repairs.