A non-starting lawn mower can quickly turn a simple yard task into a frustrating repair project. Most common starting issues are simple to diagnose and fix with basic tools and a methodical approach. Before attempting any inspection or repair, always prioritize safety by disconnecting the spark plug wire and securing it away from the terminal. This prevents the engine from accidentally starting while you are working. Understanding the basic requirements for an engine to run—fuel, spark, and air—will guide your troubleshooting process effectively.
Troubleshooting: Why Your Mower Won’t Start
The engine requires a precise mixture of fuel and air, ignited by a strong spark, to achieve combustion. If your mower fails to start, the problem almost always lies with one of these three elements.
Fuel System Check
Begin by checking the fuel, as stale gasoline is a frequent culprit. Fuel can degrade and form gummy deposits in the carburetor in as little as 30 days, especially if it contains ethanol. Ensure the fuel tank has fresh, clean gasoline. Also, check that the fuel line is not kinked or pinched, which would restrict flow to the engine.
Spark Plug Inspection
Next, inspect the spark plug, which ignites the air-fuel mixture. Remove the spark plug using a socket and examine the electrode tip for fouling, which appears as a dark, oily, or carbon-covered residue. A fouled plug cannot generate a strong enough spark to start the engine reliably. If the plug is dirty, you can attempt to clean light deposits with a wire brush and spray cleaner, but replacement is often the most effective solution.
Check the spark plug gap, which is the distance between the center and ground electrodes, using a gapping tool. The correct measurement is typically found in your owner’s manual, often around 0.030 inches. To test for spark, reconnect the wire, ground the plug’s metal body against the engine block, and pull the starter cord. A visible, bright blue spark confirms the ignition system is working, directing your focus to the air and fuel systems.
Air Filter Assessment
Finally, check the air filter, which ensures the engine receives clean air for combustion. A dirty or clogged air filter restricts airflow, causing the engine to run “rich” (too much fuel, not enough air), which prevents starting. If you have a paper filter, gently tap it on a hard surface to dislodge loose debris. If it appears heavily soiled or oil-soaked, it must be replaced. Foam filters can often be washed in warm, soapy water, dried completely, and then lightly re-oiled before reinstallation.
Fixing Common Performance Problems
If the engine starts but runs poorly—such as stalling, surging, or losing power—the issue is typically related to the fuel delivery system. The carburetor is a common trouble spot, as it precisely meters the air and fuel mixture before it enters the combustion chamber. Deposits from old fuel can clog the tiny passages and jets inside the carburetor, leading to erratic engine performance.
A quick fix for minor clogs is to use a specialized carburetor cleaner spray. This can be directed into the air intake while the engine is running or sprayed directly into the carburetor throat after removing the air filter. This solvent action can dissolve varnish and gum without requiring full disassembly.
If your mower has a fuel filter, check it for visible debris or discoloration, as a clogged filter will starve the engine of fuel, causing it to surge or stall under load. Also, ensure that the choke and throttle cables are moving freely and correctly engaging their mechanisms. A sticky choke that does not fully open after starting will cause the engine to run rich and eventually stall. Finally, check the engine oil level, as low oil can trigger a safety sensor on some mowers, causing the engine to shut down abruptly.
Addressing Blade and Deck Issues
Mechanical issues related to the cutting system can affect engine performance, often manifesting as excessive vibration or strain on the engine. Excessive vibration is frequently caused by a bent or unbalanced blade, which creates a wobble as it spins at high speed. Inspect the blade for signs of impact damage, such as deep nicks or bends, and replace it if the damage is significant.
Poor cutting quality, where the grass appears torn or shredded, indicates a dull blade edge. You can remove the blade and sharpen it using a file or grinder, ensuring you maintain the original cutting angle and that the blade remains balanced. An unbalanced blade will cause the same damaging vibration as a bent one.
A significant buildup of caked-on grass clippings underneath the deck can also strain the engine and reduce cutting efficiency. This heavy, damp debris creates drag and prevents the blade from creating the necessary lift for a clean cut. Use a putty knife or scraper to thoroughly clean the underside of the deck, which reduces engine load and improves performance.
Essential Preventative Maintenance
Routine maintenance is the most effective way to prevent future breakdowns and extend the life of your lawn mower. A regular oil change is a fundamental task, as engine oil breaks down and accumulates contaminants over time, leading to increased friction and wear. Consult your owner’s manual for the correct oil type, typically an SAE 30 or 10W-30, and change it at least once per season or after every 50 hours of operation.
At the end of the mowing season, proper winterizing is necessary to protect the fuel system from degradation. Untreated gasoline will break down and leave behind sticky residues that clog the carburetor jets. You should either run the fuel tank completely dry or add a quality fuel stabilizer to the remaining gasoline and run the engine briefly to circulate the treated fuel.
Other scheduled tasks include cleaning the engine cooling fins, which are metal ridges that dissipate heat from the engine block. Debris buildup on these fins can cause the engine to overheat, leading to premature wear. Lubricate any moving parts, such as wheel axles and height adjusters, with a light oil or grease to ensure smooth operation and prevent rust during storage.