When your lawn mower refuses to start, it can be incredibly frustrating, especially when the grass is getting tall. This guide will show you exactly how to fix a lawn mower that won’t start by walking you through the most common causes. A non-starting mower can be frustrating, but the issue usually lies within a few common and fixable systems.
You don’t need to be a mechanic to solve most of these problems. With some basic tools and a methodical approach, you can often get your mower running again in under an hour. We’ll start with the simplest checks and move to more complex issues.
Always remember to disconnect the spark plug wire before working on your mower for safety. Let’s get started.
How To Fix A Lawn Mower That Won’t Start
This comprehensive guide is structured to help you diagnose the problem step-by-step. Follow the checklist below in order, as it’s designed to solve the easiest and most frequent issues first.
Many starting failures are caused by just three things: bad fuel, a dirty air filter, or a disconnected spark plug. By checking these areas first, you save yourself alot of time and effort.
Initial Safety And Preparation Steps
Before you touch anything, ensure the mower is on a flat, stable surface. Make sure the engine is cool to the touch if it was recently used.
Gather a few basic tools: a socket set or wrenches, a screwdriver, a wire brush, and a container for old fuel. Having these ready will make the process smoother.
Disconnect The Spark Plug
This is the most important safety step. Locate the spark plug wire—it’s a thick rubber boot connected to the top of the engine. Firmly pull it off the plug’s terminal. This prevents the engine from accidentally starting while you’re working.
Check Your Mower’s Manual
If you have the owner’s manual, keep it nearby. It has model-specific diagrams and instructions that can be invaluable, especially for locating parts like the fuel shut-off valve or carburetor drain.
The First Checks: Fuel, Air, And Spark
These three elements are essential for any small engine to run. If one is missing or compromised, your mower won’t start. We’ll examine each one systematically.
Is There Fresh Fuel In The Tank?
Old gasoline is the number one cause of starting problems. Gasoline begins to degrade in as little as 30 days, forming gums and varnishes that clog the fuel system.
- Check the fuel level first. It seems obvious, but it’s often overlooked.
- Smell the fuel. If it smells sour or like varnish, it’s bad.
- If the fuel is old or contaminated, you must drain the entire system. This includes the tank, fuel line, and carburetor.
Always use fresh, clean gasoline with the correct octane rating, and consider adding a fuel stabilizer if you store your mower for the season.
Is The Air Filter Clean?
A clogged air filter starves the engine of oxygen, making it impossible to start. Locate the air filter cover, usually a plastic box on the side of the engine.
- Remove the cover by loosening the screw or unclipping the fasteners.
- Take out the filter. If it’s a paper filter, hold it up to the light. If no light passes through, it needs replacing.
- If it’s a foam filter, wash it gently in warm, soapy water, rinse, and let it dry completely before adding a few drops of clean engine oil and reinstalling.
Is The Spark Plug Functioning?
The spark plug provides the ignition for the fuel-air mixture. A fouled or damaged plug won’t create a spark.
- Using a spark plug socket, carefully remove the plug from the engine.
- Inspect the electrode (the small metal L-shaped piece at the tip). It should be a light tan or gray color. If it’s black and sooty or has a white, blistered appearance, it needs replacement.
- Check the gap between the electrode and the center tip with a feeler gauge, adjusting it to the specification in your manual (typically .030 inches).
- If the plug is cracked or the electrode is badly worn, simply replace it with a new one of the correct type. They are inexpensive.
Diagnosing Common Mechanical Issues
If fuel, air, and spark are all good, the problem may be mechanical. These issues require a bit more investigation but are still very fixable.
Checking The Carburetor And Fuel Line
The carburetor mixes fuel and air. Old fuel leaves behind sticky residues that clog its tiny jets and passages.
- First, check the fuel line (the hose from the tank to the carburetor) for cracks or blockages.
- Locate the carburetor bowl underneath the carburetor. There is often a small drain screw or bolt. Place a container underneath and open the screw to drain any old fuel or debris.
- For persistent problems, the carburetor may need to be removed and cleaned with carburetor cleaner. This involves disassembling small parts, so take pictures as you go to remember how to reassemble it.
Examining The Recoil Starter Assembly
If the pull cord is hard to pull or doesn’t retract, the starter mechanism may be jammed or broken.
Inspect the cord for fraying. Listen for grinding noises when you pull it, which could indicate a problem with the pulley or spring inside the starter housing. Sometimes the issue is simply that the rope has come off it’s guide.
Testing The Flywheel Key
The flywheel key is a small metal piece that connects the flywheel to the crankshaft. If you hit a rock or solid object, the key can shear, throwing the engine’s timing off.
This will prevent the mower from starting, even if everything else is perfect. Replacing a sheared key requires removing the flywheel nut, which needs a special tool and significant force. This may be a job for a professional if you’re not comfortable.
Electrical And Safety Switch Problems
Modern mowers have several safety switches that prevent the engine from starting under unsafe conditions. A faulty switch can mimic other problems.
Understanding Safety Interlock Switches
Common safety switches include the blade engagement lever switch, the seat switch (on riding mowers), and the brake/clutch switch. If any of these fail or are out of adjustment, the circuit to the spark plug is broken.
Consult your manual to locate these switches. You can sometimes bypass them temporarily for testing purposes, but they should be repaired or replaced for safe operation.
Inspecting The Battery (For Electric Start Models)
If your mower has an electric start that just clicks or does nothing, the battery is the first suspect.
- Check the battery terminals for corrosion (a white or blue crusty substance). Clean them with a wire brush and a baking soda/water solution.
- Ensure the battery cables are tight and secure.
- Test the battery voltage with a multimeter. A fully charged 12-volt battery should read about 12.6 volts. If it’s below 12 volts, it needs to be recharged or replaced.
Step By Step Troubleshooting Flowchart
Follow this logical sequence to find the root cause of your mower’s starting issue.
- Safety First: Disconnect spark plug wire. Ensure mower is on level ground.
- Check Fuel: Is the gas fresh and is the tank at least half full? If no, drain and refill.
- Check Air: Remove and inspect the air filter. Clean or replace if dirty.
- Check Spark: Remove the spark plug, check its condition and gap. Replace if faulty.
- Check Carburetor: Drain the carburetor bowl. Look for clogged fuel lines.
- Check Mechanical: Does the pull cord operate smoothly? Does the blade turn freely?
- Check Electrical: (Electric start) Test battery and connections. Check all safety switches.
By following these steps, you will identify the problem in vast majority of cases.
Preventative Maintenance To Avoid Future Problems
The best way to fix a starting problem is to prevent it from happening in the first place. A little routine maintenance goes a long way.
End-Of-Season Storage Procedure
Proper storage is crucial. Never just park a mower with fuel in the carburetor for the winter.
- Add a fuel stabilizer to a full tank of fresh gas and run the engine for 5 minutes to circulate it through the system.
- Alternatively, you can run the engine until it stops to empty the carburetor, then drain any remaining fuel from the tank.
- Change the oil and clean or replace the air filter.
- Remove the spark plug, add a teaspoon of oil into the cylinder, and slowly pull the starter cord to coat the cylinder walls. Reinstall the plug.
Regular In-Season Maintenance Schedule
During the mowing season, get into these good habits:
- Check the air filter every 25 hours of use.
- Change the engine oil according to your manual’s schedule (usually every 50 hours or seasonally).
- Keep the underside of the mower deck clean of grass clippings.
- Always use fresh fuel and consider adding a fuel stabilizer with every fill-up, especially if you mow infrequently.
When To Call A Professional
While most issues are DIY-friendly, some situations warrant a trip to the small engine repair shop.
If you’ve checked all the common issues and the mower still won’t start, the problem could be internal, like low engine compression, a damaged valve, or a broken connecting rod. These repairs require specialized tools and expertise.
If you are not comfortable with tasks like removing the carburetor for a full cleaning or testing engine compression, it’s wise to hire a pro. They can often diagnose and fix the problem quickly, saving you time and potential frustration.
Frequently Asked Questions
Why Does My Lawn Mower Have No Spark?
A lack of spark is usually caused by a fouled or damaged spark plug. It can also be due to a disconnected or damaged spark plug wire, a faulty ignition coil, or a tripped safety switch preventing the circuit from completing. Always check the plug first.
What Would Cause A Lawn Mower To Not Start After Winter?
This is almost always caused by old, degraded gasoline clogging the carburetor jets and fuel passages. The fuel evaporates over time, leaving a sticky residue. Draining the old fuel and cleaning the carburetor is the standard fix for a mower that won’t start after storage.
How Do I Know If My Lawn Mower Carburetor Is Bad?
Signs of a bad or dirty carburetor include the engine not starting, starting but then dying immediately, running roughly, or leaking fuel from the bowl. If you’ve ruled out fuel, spark, and air, the carburetor is the likely culprit, especially if the mower was stored with untreated fuel.
Can Bad Gas Cause A Lawn Mower Not To Start?
Yes, bad gasoline is the single most common cause of starting failure. Gasoline oxidizes and forms gums that block the small ports in the carburetor. If your mower has been sitting for more than a month with the same fuel, draining the system and using fresh gas is your first step.
Why Won’t My Lawn Mower Start When I Pull The Cord?
If you pull the cord and nothing happens—no sound or resistance—the recoil starter assembly may be broken or disconnected. If the cord pulls but the engine doesn’t turn over, the engine could be seized, or the flywheel key might be sheared. First, ensure the blade engagement lever is disengaged, as this is a common oversight.