A lawn mower that refuses to start is a common homeowner frustration. You’re ready to tackle your overgrown yard, but your trusty mower just sputters or remains stubbornly silent. Before you resign yourself to hiring a lawn service, know that many starting problems are easily fixable with a bit of troubleshooting. This comprehensive guide will walk you through the most common culprits and how to resolve them, getting you back to mowing in no time.
Understanding the Basics of a Lawn Mower Engine
Before diving into troubleshooting, it’s important to understand the basic components that make your lawn mower engine run. A typical gasoline-powered lawn mower relies on a four-stroke engine cycle: intake, compression, combustion, and exhaust. Fuel and air mix in the carburetor (or fuel injector in newer models), the piston compresses the mixture, the spark plug ignites it, and the resulting explosion drives the piston down, turning the crankshaft and ultimately the blade. If any of these steps are interrupted, the engine won’t start.
Fuel System Overview
The fuel system is crucial. It consists of the gas tank, fuel lines, fuel filter (sometimes), and the carburetor. The carburetor’s job is to mix air and fuel in the correct ratio for combustion. Problems in any of these components can prevent fuel from reaching the engine.
Ignition System Fundamentals
The ignition system creates the spark needed to ignite the fuel-air mixture. This system includes the spark plug, ignition coil, and magneto. A weak or nonexistent spark will prevent the engine from starting.
Compression Issues: A Brief Look
While less common than fuel or ignition problems, low compression can also prevent starting. Compression refers to the piston’s ability to seal the cylinder and compress the fuel-air mixture.
Troubleshooting Steps When Your Lawn Mower Won’t Start
Now, let’s get into the practical steps you can take to diagnose and fix your starting problem. We’ll proceed in a logical order, starting with the easiest and most common issues.
1. The Obvious Checks: Fuel and Safety Features
Always start with the basics. It might seem obvious, but overlooking these simple things is surprisingly common.
Check the Fuel Level
Make sure there’s gas in the tank! This is the most frequent reason mowers don’t start. Don’t just rely on the gauge; actually look inside the tank. If the fuel is old (more than a month), it can degrade and become gummy, preventing proper combustion. Old fuel is a common culprit. Drain the old fuel and replace it with fresh gasoline. Consider adding a fuel stabilizer to extend the life of your fuel.
Inspect the Spark Plug Wire
Ensure the spark plug wire is securely connected to the spark plug. A loose connection will prevent the spark from reaching the cylinder. Push it firmly onto the spark plug terminal.
Engage the Safety Features
Lawn mowers have several safety features designed to prevent accidental starts. Make sure the bail lever (the handle you hold down while mowing) is fully engaged. Check the blade brake lever if your mower has one. Some mowers also have a safety key or button that must be properly inserted or activated. Double-check all safety features.
2. Addressing Fuel-Related Problems
If the obvious checks don’t solve the problem, it’s time to delve deeper into the fuel system.
Check the Fuel Valve or Shut-Off
Many mowers have a fuel shut-off valve, usually located near the fuel tank. Make sure this valve is in the “on” or “open” position.
Examine the Air Filter
A dirty air filter restricts airflow to the carburetor, leading to a rich fuel mixture (too much fuel, not enough air) that can prevent starting. Remove the air filter and inspect it. If it’s dirty, clean it with compressed air or replace it. A clean air filter is essential for proper engine performance. A blocked air filter restricts airflow.
Inspect and Clean the Spark Plug
The spark plug is crucial for igniting the fuel-air mixture. Remove the spark plug and inspect it. Look for signs of fouling, such as carbon buildup, oil, or gasoline. Clean the spark plug with a wire brush or spark plug cleaner. Check the spark plug gap using a spark plug gap tool. The correct gap specification is usually found in your owner’s manual. If the spark plug is heavily fouled or damaged, replace it. A faulty spark plug will not provide the necessary spark to ignite the fuel. A clean, properly gapped spark plug is critical.
Test for Spark
To test for spark, reattach the spark plug wire to the spark plug. Ground the spark plug against the engine block (touch the metal part of the spark plug to a metal part of the engine). Pull the starter cord. You should see a blue spark jumping across the spark plug gap. If there is no spark, the ignition coil may be faulty.
Dealing with a Clogged Carburetor
The carburetor is a complex component that meters fuel and air. Over time, it can become clogged with varnish from old fuel. This is a very common cause of starting problems.
To clean the carburetor, you may need to remove it from the engine. This process varies depending on the mower model, so consult your owner’s manual or watch a tutorial video. Once removed, disassemble the carburetor carefully, noting the location of each part. Use carburetor cleaner to clean all the jets and passages. Reassemble the carburetor and reinstall it on the engine. Cleaning a carburetor can seem daunting, but with patience and careful attention to detail, it’s a manageable task. A clogged carburetor is a frequent cause of starting issues.
Fuel Line Inspection
Check the fuel lines for cracks, leaks, or blockages. Replace any damaged fuel lines. A blocked fuel line will prevent fuel from reaching the carburetor.
3. Investigating Ignition System Issues
If the fuel system seems to be working properly, the problem may lie in the ignition system.
Check the Ignition Coil
The ignition coil generates the high voltage needed to create a spark. If the ignition coil is faulty, it won’t produce a spark. Testing an ignition coil usually requires a multimeter. Consult your owner’s manual or a qualified technician for instructions on how to test your specific ignition coil.
Inspect the Flywheel Key
The flywheel key is a small key that connects the flywheel to the crankshaft. If the flywheel key is sheared, the timing will be off, preventing the engine from starting. Inspecting the flywheel key requires removing the flywheel, which can be a more involved process.
4. Addressing Compression Problems (Less Common)
While less frequent than fuel or ignition problems, low compression can prevent starting.
Signs of Low Compression
If the engine feels easier to pull over than usual, it may indicate low compression. Other symptoms include a lack of power and excessive oil consumption.
Causes of Low Compression
Low compression can be caused by worn piston rings, a damaged cylinder, or a leaking valve. These issues typically require professional repair.
Tips for Preventing Future Starting Problems
Preventing starting problems is often easier than fixing them. Here are some tips to keep your lawn mower running smoothly.
- Use fresh fuel: Don’t use gasoline that’s been sitting for more than a month.
- Add fuel stabilizer: Fuel stabilizer prevents fuel from degrading.
- Clean or replace the air filter regularly: A clean air filter ensures proper airflow.
- Change the spark plug annually: A new spark plug ensures a strong spark.
- Empty the fuel tank before storing the mower for the winter: This prevents fuel from gumming up the carburetor.
- Follow the manufacturer’s recommended maintenance schedule: Regular maintenance keeps your mower in top condition.
When to Seek Professional Help
While many starting problems can be fixed with DIY troubleshooting, some issues require professional attention. If you’ve tried the steps outlined above and your mower still won’t start, it’s time to take it to a qualified repair technician. Complex engine problems, such as low compression or a faulty ignition coil, are best left to the experts.
By following these troubleshooting steps and preventative maintenance tips, you can keep your lawn mower running smoothly for years to come. Remember to always consult your owner’s manual for specific instructions and safety precautions.
Why won’t my lawn mower start, even though it has gas?
Even if your lawn mower has gasoline in the tank, the fuel itself might be the problem. Gasoline can degrade over time, especially if it’s been sitting in the mower for several months. Old gas can become gummy and prevent the carburetor from functioning correctly, leading to starting issues. Additionally, water can condense inside the fuel tank, further diluting the gasoline and hindering its ability to ignite.
Another common issue is a clogged fuel filter. This small component prevents debris from entering the carburetor. If the fuel filter is blocked, it restricts the flow of gasoline, preventing the engine from receiving the necessary fuel for combustion. Check the fuel filter for any visible signs of dirt or blockage. Replacing the fuel filter is often a simple and inexpensive solution.
How do I check the spark plug on my lawn mower?
First, disconnect the spark plug wire. This is a crucial safety step to prevent accidental starting. Next, carefully remove the spark plug using a spark plug wrench. Once removed, visually inspect the spark plug for any signs of damage, such as cracks, chips, or excessive carbon buildup. The electrode should be clean and intact.
To test if the spark plug is producing a spark, reattach the spark plug wire to the spark plug. Hold the threaded metal part of the spark plug against a metal part of the engine (grounding it). Then, pull the starter cord. If you see a bright blue spark jumping across the spark plug gap, the spark plug is likely functioning correctly. If there’s no spark, or the spark is weak and orange, the spark plug needs to be replaced.
What should I do if my lawn mower’s air filter is dirty?
A dirty air filter restricts the airflow to the engine, which can lead to starting problems and poor performance. Remove the air filter cover and inspect the filter itself. If the filter is visibly dirty, clogged with debris, or excessively oily, it needs to be cleaned or replaced. A clean air filter allows the engine to breathe properly, ensuring optimal combustion.
If your air filter is a foam type, you can wash it with warm soapy water, rinse it thoroughly, and allow it to dry completely before re-oiling it lightly with air filter oil. If it’s a paper filter, try gently tapping it to remove loose debris. If it’s still dirty after tapping, replace it with a new one. Never try to wash a paper air filter, as this can damage the filter and render it ineffective.
How can I clean a lawn mower carburetor?
Cleaning a carburetor involves removing it from the engine, which may require some mechanical knowledge. Before removing the carburetor, ensure the fuel tank is empty or the fuel line is clamped off to prevent fuel leakage. Once removed, carefully disassemble the carburetor, taking note of the location of all parts. Spray each part thoroughly with carburetor cleaner to remove any varnish or buildup.
Pay particular attention to cleaning the jets and passages within the carburetor. Use a small wire or carburetor cleaning tool to ensure these passages are clear. After cleaning, reassemble the carburetor, ensuring all parts are properly seated and tightened. Reinstall the carburetor onto the engine and reconnect the fuel line. A clean carburetor will allow the engine to receive the correct air-fuel mixture for starting and running smoothly.
What role does the lawn mower’s safety switch play in starting issues?
Lawn mowers are equipped with safety switches to prevent accidental starting and operation. These switches are typically located on the handle, under the seat (for riding mowers), or near the blade engagement lever. If these switches are not properly engaged or are malfunctioning, the lawn mower will not start. The safety switch is designed to prevent the engine from running if the operator is not in the correct position or if the mower is not being operated safely.
Inspect all safety switches to ensure they are functioning correctly. Check for any loose wires, corrosion, or damage to the switch itself. Sometimes, simply cleaning the contacts of the switch can resolve the issue. If a safety switch is faulty, it may need to be replaced to allow the lawn mower to start. Bypass should never be attempted due to safety issues, the switches protect against accidental operation.
What if my lawn mower engine is flooded?
A flooded engine means there’s too much fuel in the cylinder, preventing it from igniting. If you’ve been repeatedly trying to start the mower, especially with the choke engaged, you may have flooded the engine. A tell-tale sign of a flooded engine is the strong smell of gasoline. Do not continue trying to start it with the same settings, as this will only worsen the flooding.
To address a flooded engine, disengage the choke completely and pull the starter cord several times. This will help to clear excess fuel from the cylinder. In some cases, you may need to remove the spark plug and allow the cylinder to air out for a few minutes. After airing, reinstall the spark plug and try starting the mower with the choke disengaged. If the engine still doesn’t start, consult the owner’s manual for specific flooding instructions.
When should I take my lawn mower to a professional repair shop?
If you’ve tried all the basic troubleshooting steps, such as checking the fuel, spark plug, air filter, and safety switches, and your lawn mower still won’t start, it may be time to consult a professional. Some problems, such as internal engine damage, require specialized tools and expertise to diagnose and repair. Attempting to fix complex issues without the proper knowledge could potentially cause further damage.
Additionally, if you’re uncomfortable working on small engines or lack the necessary tools, it’s best to take your lawn mower to a qualified repair shop. A professional technician can accurately diagnose the problem, perform the necessary repairs, and ensure your lawn mower is running safely and efficiently. The cost of professional repair may be well worth it to avoid further damage or personal injury.