When your mower refuses to start, a systematic check of fuel, spark, and air often reveals the simple culprit. This guide will show you exactly how to fix a lawn mower that will not start, walking you through the most common problems in a logical order. You don’t need to be a mechanic to solve most of these issues. With a few basic tools and some patience, you can get your mower running again and save a costly trip to the repair shop.
We will start with the easiest, most common fixes and work our way to more complex diagnostics. Always remember to disconnect the spark plug wire before working on your mower to prevent any accidental starts. Safety first is the best policy for any repair job.
How To Fix A Lawn Mower That Will Not Start
The key to sucessful troubleshooting is to follow a process. Don’t just start taking things apart randomly. The classic internal combustion engine needs three things to run: fuel, spark, and compression. We’ll focus on the first two, as compression issues are less common and often require professional help. By checking each system methodically, you’ll find the problem.
Start With The Simple Safety And Operational Checks
Before you dive into the engine, eliminate these basic oversights. They are the most frequent reasons a mower won’t start and are the easiest to fix.
- Check the Fuel Valve: On some mowers, there is a fuel shut-off valve. Make sure it is in the “On” position.
- Engage the Blades Correctly: Many mowers have a safety feature that prevents starting if the blade engagement lever is not in the proper position. Ensure it’s disengaged.
- Check the Parking Brake: Similarly, the parking brake may need to be fully engaged for the engine to turn over.
- Prime the Engine: If your mower has a primer bulb, press it 3-5 times to draw fuel into the carburetor. Don’t over-prime, as this can flood the engine.
- Use the Choke: For a cold engine, set the choke to the “Closed” or “Start” position. Remember to open it once the engine warms up.
Investigating Fuel System Problems
Old or contaminated fuel is the number one cause of lawn mower failure. Gasoline begins to degrade in as little as 30 days, forming gums and varnishes that clog the delicate passages in your carburetor.
Step 1: Check The Fuel Level And Quality
It sounds obvious, but always check that there is actually fuel in the tank. Next, check its condition. If the fuel is over a month old, it’s suspect. Drain the old fuel from the tank and the carburetor bowl into an approved container. Fill the tank with fresh, clean gasoline. For added protection, use a fuel stabilizer with every fill-up, especially if you store your mower for the season.
Step 2: Inspect The Fuel Line And Filter
Trace the fuel line from the tank to the carburetor. Look for cracks, brittleness, or leaks. Squeeze the primer bulb if you have one; it should feel firm and fill with fuel. If the fuel line is damaged, replace it. Many mowers also have an inline fuel filter. Hold it up to the light; if it looks dark or clogged, replace it with a new one. These are inexpensive parts.
Step 3: Clean or Replace The Carburetor
If fresh fuel doesn’t help, the carburetor is likely clogged. You can often clean it without a full replacement.
- Disconnect the spark plug wire and drain the fuel.
- Remove the air cleaner assembly to access the carburetor.
- Locate the carburetor bowl (usually held on by a single bolt) and carefully remove it.
- Spray the inside of the bowl, the float, and the main jet (the small brass piece with a hole in it) thoroughly with carburetor cleaner.
- Use a thin wire or a dedicated jet cleaning tool to clear the tiny passage in the jet. Reassemble everything and try starting the mower.
Checking For A Spark Ignition Issue
If the fuel system checks out, the next step is to see if the engine is getting spark. A faulty spark plug or a disconnected ignition coil are common culprits.
How To Test For Spark
- Remove the spark plug wire boot by pulling it straight off the plug.
- Use a spark plug socket to remove the spark plug itself.
- Reconnect the spark plug wire to the plug.
- With insulated pliers, hold the metal body of the spark plug against a clean, unpainted part of the engine cylinder head.
- Have a helper pull the starter cord or turn the key. You should see a bright blue spark jump across the plug’s electrode gap. A weak orange spark or no spark at all indicates a problem.
Replacing The Spark Plug
Even if you see a spark, a worn plug can cause starting problems. Remove it and check the condition. The electrode should not be excessively worn, fouled with black carbon, or coated in white residue. The gap should match your mower’s specification (usually found in the manual). It’s often easiest to simply install a new spark plug of the correct type and gap. This is a cheap and effective fix.
Inspecting The Ignition Coil
If there’s no spark with a new plug, the ignition coil (also called the magneto) may be faulty. This component generates the electrical current for the spark. Check that the coil’s wiring is intact and that it is properly gapped from the flywheel. A small business card’s thickness is a typical gap. Replacing an ignition coil is more advanced but manageable with a service manual.
Addressing Airflow Restrictions
An engine needs air to mix with fuel for combustion. A clogged air filter starves the engine of air, causing it to not start or run poorly.
Locate the air filter cover, usually held by clips or a screw. Remove the cover and take out the air filter. If it’s a paper filter, hold it up to the light. If you can’t see light through it, it needs to be replaced. If it’s a foam filter, wash it gently in warm, soapy water, rinse, let it dry completely, and then apply a few drops of clean engine oil before reinstalling. Never run the engine without the air filter, as this can cause serious damage.
When The Engine Cranks But Won’t Start
If the starter rope pulls easily or the engine turns over with the key but doesn’t fire, you have different set of issues to consider.
- Valve Clearance: On some engines, especially overhead valve models, tight valve clearances can prevent starting. This adjustment requires specific tools and knowledge.
- Flywheel Key: If the mower hit a solid object, the shear key on the flywheel may have broken. This throws the ignition timing off. Inspecting this requires removing the flywheel nut, which often needs a special tool and an impact wrench.
- Low Compression: Worn piston rings or a leaking valve can cause low compression. You can test this with a compression gauge. If compression is very low, a major engine repair is likely needed.
Preventative Maintenance To Avoid Future Problems
The best way to fix a lawn mower that won’t start is to prevent the problem in the first place. A little routine care goes a long way.
- Always use fresh fuel and add a stabilizer if storing for more than 30 days.
- At the end of the mowing season, either run the carburetor dry or use a fuel stabilizer in a full tank.
- Change the air filter annually or more often in dusty conditions.
- Replace the spark plug every season or after 25 hours of use.
- Check the oil level regularly and change it according to the manufacturer’s schedule.
- Keep the underside of the mower deck clean of caked-on grass to prevent rust and improve airflow.
Frequently Asked Questions
Why does my lawn mower have no spark?
The most common reasons for no spark are a faulty spark plug, a disconnected or damaged spark plug wire, or a failed ignition coil. Always start by checking and replacing the spark plug first, as it’s the simplest fix.
What do I do if my mower engine is flooded?
An engine floods when too much fuel enters the cylinder. To fix it, move the throttle or choke to the “Run” or “Open” position. Then, with the spark plug wire disconnected, pull the starter cord several times to clear excess fuel from the cylinder. Reconnect the wire and try starting normally.
How can I tell if the carburetor is bad on my lawn mower?
Signs of a bad carburetor include the engine not starting, running roughly, stalling, or leaking fuel. Often, a thorough cleaning resolves the issue. If cleaning doesn’t work, or if parts are visibly damaged, a replacement carburetor may be necessary.
Where is the air filter on a lawn mower?
The air filter is typically housed in a black plastic box near the top of the engine, often on the side. It’s cover is usually secured with a single screw, a wing nut, or metal clips. Consult your mower’s manual for the exact location.
Can old gas cause a lawn mower not to start?
Yes, old gasoline is the leading cause of starting problems. Gasoline degrades quickly, losing volatility and forming gums that clog the carburetor. Always drain old fuel or use a stabilizer if you plan to store your mower with fuel in the tank.