When your lawn mower refuses to start, a systematic approach will help you find the root cause. Learning how to start a lawn mower that won’t start is often a matter of checking a few key systems in the right order. You don’t need to be a mechanic to solve most common problems. This guide will walk you through the process from simple fixes to more involved checks, ensuring you can get back to your yard work quickly.
How To Start A Lawn Mower That Won’t Start
The frustration of a silent engine is familiar to every lawn owner. Before you consider costly repairs or replacement, follow this structured troubleshooting path. Most non-starting issues stem from just a handful of culprits: old fuel, a dirty air filter, a faulty spark plug, or a clogged carburetor. We’ll start with the easiest, no-tools solutions and progress to more detailed steps.
Immediate No-Tool Checks And Safety
Always begin with safety and the most obvious potential oversights. It’s easy to miss a simple step, especially at the start of the mowing season.
- Check the Fuel Level: It sounds obvious, but it’s a common mistake. Ensure there is fresh gasoline in the tank.
- Inspect the Blades: Ensure the mower is on a level surface and that nothing is obstructing the blade, like a thick branch.
- Verify the Controls: For a push mower, the handlebar must often be engaged. For a riding mower, ensure it’s in neutral or park, the brake is set, and the blade engagement lever is off.
- Prime the Engine: If your mower has a primer bulb, press it 3-5 times to draw fuel into the carburetor.
- Use the Choke: If the engine is cold, set the choke to the “closed” or “on” position. If it’s warm, leave it open.
Step One: Addressing The Fuel System
Bad fuel is the number one reason lawn mowers fail to start. Gasoline begins to degrade in as little as 30 days, forming varnish and gum that clogs the delicate passages in your carburetor.
Draining Old Gasoline
If the fuel is over a month old, or if you didn’t add a stabilizer before storage, you need to drain it. Use a siphon pump or a turkey baster dedicated to this task to remove the old fuel from the tank. Dispose of it properly at a local hazardous waste facility.
Adding Fresh Fuel And Stabilizer
Fill the tank with fresh, clean gasoline. For small engines, it’s best to use ethanol-free fuel if available, as ethanol attracts moisture. Always add a fuel stabilizer if you plan to store the mower for more than a few weeks. This simple step can prevent most starting issues next season.
Checking The Fuel Line And Filter
Trace the fuel line from the tank to the carburetor. Look for cracks, brittleness, or kinks. Some mowers have an inline fuel filter. If it looks dark or dirty, replace it. A clogged filter starves the engine of fuel.
Step Two: Inspecting The Ignition System
If fresh fuel doesn’t help, the problem may be a lack of spark. The spark plug is a critical and inexpensive component that wears out over time.
Removing And Reading The Spark Plug
First, disconnect the spark plug wire for safety. Use a spark plug socket to carefully remove the plug. Examine its condition:
- Normal Wear: The electrode will be slightly worn but relatively clean.
- Fouled with Fuel: Wet, black deposits indicate a flooded engine or a rich fuel mixture.
- Carbon Fouled: Dry, sooty black deposits suggest too much fuel or a dirty air filter.
- Oil Fouled: Shiny, wet black deposits mean oil is leaking into the combustion chamber.
Cleaning Or Replacing The Spark Plug
For light carbon deposits, you can clean the electrode with a wire brush. Check the gap with a feeler gauge and adjust it to the specification in your manual (typically .030 inches). However, replacement is often best. Spark plugs are cheap, and a new one ensures a strong spark. Before installing, apply a small amount of anti-seize compound to the threads to prevent future seizing.
Checking For Spark
Reconnect the plug wire to the plug. With insulated pliers, hold the plug’s metal body against the engine block, ensuring the electrode is not touching. Pull the starter cord or turn the key. You should see a bright blue spark jump across the gap. No spark indicates a faulty plug, a disconnected or broken wire, or a deeper ignition coil problem.
Step Three: Evaluating Air Flow And The Carburetor
Your engine needs the correct mix of air and fuel to run. A restriction in air flow or a clogged carburetor disrupts this balance.
Cleaning Or Replacing The Air Filter
Locate the air filter cover, usually held by a screw or a snap latch. Remove the filter. If it’s a paper filter, hold it up to the light. If you can’t see light through it, it needs replacement. If it’s a foam filter, wash it in warm, soapy water, rinse thoroughly, and let it dry completely. Before reinstalling a foam filter, lightly coat it with clean engine oil and squeeze out the excess. A dirty air filter is a very common cause of poor starting and performance.
Basic Carburetor Cleaning
The carburetor mixes air and fuel. If it’s clogged with old fuel residue, fuel cannot reach the engine. Many mowers have a single bolt or nut on the bottom of the carburetor bowl. Place a small container underneath and loosen this bolt to drain any old fuel or debris from the bowl. Sometimes, this simple drain is enough to clear a blockage.
Using Carburetor Cleaner
For more stubborn issues, you can use aerosol carburetor cleaner. Remove the air filter and spray a short burst directly into the carburetor’s intake while pulling the starter cord. This can help dissolve minor varnish deposits. For a thorough cleaning, the carburetor may need to be removed and disassembled, which is a more advanced repair.
Step Four: Advanced Mechanical Checks
If the fuel, spark, and air systems check out, the issue may be mechanical. These checks require a bit more mechanical confidence.
Checking The Flywheel Key
The flywheel key is a small metal piece that connects the flywheel to the crankshaft. If the mower blade hits a solid object, the shear key can break to protect the engine. A broken key throws off the engine’s timing, preventing it from starting. You’ll need to remove the starter cover and the flywheel nut to inspect it. A broken key will have a clean split. Replacing it is straightforward if you have the tools.
Examining The Valves (For Overhead Valve Engines)
On overhead valve (OHV) engines, tight or stuck valves can cause a no-start condition. This requires removing the valve cover and checking the clearance between the valve stem and rocker arm with a feeler gauge. Adjusting valves is precise work best left to a manual or a professional if you’re unfamiliar.
Assessing Engine Compression
Low compression means the engine cannot build the pressure needed for combustion. You can check this with a compression gauge. Screw it into the spark plug hole and pull the starter cord several times. A reading below 80 PSI often indicates worn piston rings, a scored cylinder, or a blown head gasket. Significant compression loss typically requires professional engine repair.
Step Five: Electrical System Troubleshooting (For Riding Mowers)
Riding mowers have more complex electrical systems. A dead battery or a faulty safety switch are frequent problems.
Testing And Charging The Battery
Use a multimeter to check the battery voltage. A fully charged 12-volt battery should read about 12.6 volts. If it’s below 12 volts, charge it. Clean any corrosion from the battery terminals and ensure the connections are tight. A weak battery cannot power the starter solenoid effectively.
Checking Safety Interlock Switches
Riding mowers have safety switches on the seat, brake, and blade engagement lever. If one fails, it breaks the starting circuit. Consult your manual for their locations. You can often test them by bypassing them temporarily with a jumper wire, but be extremely cautious and ensure the mower is in neutral with the blades off. Replacing a faulty switch is the proper fix.
Listening For The Starter Solenoid Click
When you turn the key, listen for a solid “click” from the starter solenoid (usually near the battery). A rapid clicking sound usually indicates a weak battery. A single click or no click could mean a bad solenoid, a faulty ignition switch, or a broken connection in the safety circuit.
Preventative Maintenance To Avoid Future Problems
The best way to solve a no-start issue is to prevent it from happening. A small amount of seasonal maintenance saves immense frustration.
- Use Fuel Stabilizer: Always add stabilizer to your fuel can or directly to the mower’s tank at the end of the season and run the engine for a few minutes to circulate it.
- Change the Oil Annually: Old, dirty oil increases engine wear and can make starting harder. Change it according to your manual’s schedule.
- Replace the Air Filter and Spark Plug Each Season: Make this part of your spring tune-up routine. It’s inexpensive and highly effective.
- Store Properly: At season’s end, either run the carburetor dry or add stabilizer. Store the mower in a clean, dry place.
- Keep it Clean: Regularly clean grass clippings and debris from the mower deck and engine area to prevent overheating and corrosion.
Frequently Asked Questions
What Are The Most Common Reasons A Lawn Mower Won’t Start?
The top three reasons are stale fuel, a dirty or clogged air filter, and a fouled spark plug. These account for the vast majority of starting failures and are the first things you should check.
Why Does My Lawn Mower Have No Spark?
A lack of spark is usually caused by a worn-out spark plug. Other causes include a damaged spark plug wire, a loose connection, or a failed ignition coil. Always test with a known-good spark plug first.
How Do You Start A Lawn Mower That Has Been Sitting All Winter?
First, drain all the old gasoline from the tank and carburetor bowl. Add fresh fuel with stabilizer. Check the oil level and condition. Replace the air filter and spark plug. Prime the engine and use the choke before attempting to start it.
Can Bad Gas Cause A Lawn Mower Not To Start?
Yes, bad gasoline is the single most common culprit. Gasoline degrades quickly, forming gums that clog the carburetor jets and fuel lines, preventing fuel from reaching the engine.
When Should I Take My Lawn Mower To A Professional?
If you’ve checked fuel, spark, and air flow, and the engine still won’t start, or if you discover significant mechanical issues like very low compression, broken internal components, or complex electrical faults on a riding mower, it’s time to consult a small engine repair shop.