Graham FisherBy Graham Fisher

Updated November 2026

A lawn mower that sat for a year and won’t start is almost always dealing with the same handful of problems: stale fuel that has turned to varnish, a battery that has discharged and sulfated, a carburetor jet clogged with residue, or a spark plug that has fouled. This guide walks you through checking each of those in a logical order so you can find the fault without replacing parts you don’t need. Most mowers that have been stored badly can be revived with a few hours of work and some basic supplies.

Why a Mower Fails After Sitting for a Year

Gasoline is the first suspect, and for good reason. Modern ethanol-blended fuel begins to degrade within about 30 days, and after a year the lighter aromatic compounds have evaporated while the remaining gum and varnish coat the carburetor, fuel lines, and fuel filter. Ethanol also attracts moisture, which can separate and sit at the bottom of the tank. A carburetor that looks clean on the outside can still have jets blocked by this residue.

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Next in line are the battery and the spark plug. Lead-acid batteries left discharged for months develop sulfation, a hard crystalline buildup on the plates that reduces capacity permanently in many cases. Lithium-ion packs on cordless mowers also lose charge slowly and can drop below their safe voltage range if left empty. Spark plugs left in a cylinder can accumulate carbon and oil, and the electrode can corrode or the gap can close up.

Finally, mechanical and air-intake issues show up after long storage. Dried oil can leave the engine hard to turn over, the air filter may have collected debris and moisture, and rodents sometimes nest in air boxes or chew wiring harnesses. Work through the checks below in order, since the easiest fixes come first.

Before You Start: Safety and Tools

Disconnect the spark plug wire before doing any work near the blade or engine, and remove the battery or disconnect it on electric-start models. Drain or handle old fuel outdoors, away from heat sources, and wear eye protection when cleaning with solvent. Dispose of old gasoline at a local hazardous waste facility rather than pouring it onto the ground.

Gather these supplies before you begin:

  • Fresh gasoline (or fuel-stabilized gas) and a clean container
  • Fuel stabilizer and a can of carburetor cleaner
  • Socket set, screwdrivers, and a spark plug socket
  • Replacement spark plug matching the manufacturer’s part number
  • Fine wire brush or sandpaper for electrodes
  • Feeler gauge for setting the spark plug gap
  • Clean rags and a small wire or pipe cleaner for jets
  • Multimeter for checking battery voltage

Step 1: Check and Replace the Fuel

Start by looking at the fuel. If the mower has been sitting with gas in the tank, assume it has gone bad. Drain it into an approved container. Gas that smells sour, varnishy, or sharp rather than like gasoline should be discarded. Refill with fresh fuel mixed with stabilizer if the mower will be stored again later.

Check the Fuel Lines and Filter

Inspect the fuel line for cracks, hardening, or softness. Replace any line that feels brittle. If there is an inline fuel filter, replace it, since residue tends to collect there first. Make sure fuel is flowing to the carburetor by pulling the line free and checking for a steady flow when you prime the bulb or pull the starter.

Step 2: Inspect the Battery

On battery-powered mowers, measure the voltage with a multimeter. Many manufacturers specify a fully charged reading for their packs, so check the model’s documentation for the exact figure. If the voltage is very low, put the battery on its recommended charger and let it run through a full cycle. A battery that will not hold a charge, swells, or shows physical damage should be replaced rather than charged repeatedly. Never attempt to charge a visibly damaged lithium pack.

Step 3: Test and Clean the Spark Plug

Remove the spark plug and check its condition. A dark, oily, or heavily carboned tip suggests a fuel or oil problem. A white, blistered tip can indicate overheating or a lean mixture. Clean the electrode gently with a wire brush, then check the gap against the specification in your owner’s manual using a feeler gauge. Reset the gap if needed. If the plug is worn, cracked, or badly corroded, replace it with the correct part. Plugs are inexpensive relative to the time spent troubleshooting, so replacing a doubtful one is often the faster route.

Step 4: Clean or Rebuild the Carburetor

The carburetor is the most common culprit after a long storage period. Remove the air filter cover, and then the carburetor, following the manufacturer’s sequence. Spray carburetor cleaner into the bowl, jets, and passages. Use a fine wire, never a drill bit, to clear the small jets, and blow everything dry with compressed air. Inspect the float and needle valve for stickiness or damage. If the mower still runs poorly after cleaning, a carburetor rebuild kit matched to the engine model usually restores proper operation.

Tip: Take photos as you disassemble the carburetor. Small linkages and springs are easy to misplace, and the picture makes reassembly much simpler.

Step 5: Check the Air Filter and Air Intake

Remove the air filter and inspect it. A paper filter that is damp, oil-soaked, or clogged should be replaced. A foam filter can usually be washed in mild detergent, squeezed dry, and lightly re-oiled if the manufacturer calls for it. Look inside the air box for nesting material, moisture, or droppings, and clean it thoroughly.

Step 6: Check Engine Compression and Turn It Over by Hand

With the spark plug removed, turn the engine over by hand or with the recoil. You should feel steady resistance from compression. If the engine turns freely with no compression, the valves may be stuck or the piston rings may have seized to the cylinder wall. A stuck valve can sometimes be freed by carefully adding a small amount of penetrating oil through the spark plug hole and letting it soak before attempting to turn the engine again. If the engine locks up or turns with extreme difficulty, consult a small engine repair shop before forcing it.

Step 7: Check the Safety Switches and Electrical Connections

Many mowers will not start unless the operator presence control is engaged, the blade brake is in the correct position, or the safety key is installed. Test each safety switch. On electric-start models, inspect the wiring for rodent damage and loose or corroded connectors. Clean connectors with contact cleaner and reseat them firmly.

Step 8: Try a Careful Start-Up Sequence

Once the fuel is fresh, the carburetor is clean, the plug is set, and the air filter is in place, reconnect the spark plug wire and set the controls to the start position. Use the choke as the manufacturer directs, usually closed for a cold start and opened after the engine catches. Pull the starter in a steady motion rather than yanking it. If the engine fires and then dies, check the choke and throttle settings and make sure fuel is reaching the carburetor.

Common Mistakes

  • Adding fresh gas on top of stale fuel without draining the tank first. The old fuel will still clog the carburetor.
  • Using a drill bit or wire that is too thick to clean the carburetor jets, which enlarges them and ruins the calibration.
  • Repeatedly cranking an engine that will not fire. This floods the cylinder and can wash oil off the cylinder walls.
  • Charging a damaged lithium battery or one that has become very hot or swollen.
  • Ignoring the spark plug gap, which can cause misfires even when the plug looks clean.
  • Forgetting to disconnect the spark plug wire before working near the blade.

Tips for Storing a Mower So This Doesn’t Happen Again

  • Add fuel stabilizer to the tank and run the engine for several minutes so the treated fuel reaches the carburetor before storage.
  • Change the oil before storing the mower, since used oil contains acids and contaminants.
  • Store cordless batteries at a partial charge in a cool, dry place, and check them periodically.
  • Keep the mower in a dry location and cover it lightly to limit moisture and rodent access.
  • Remove the spark plug and add a teaspoon of clean oil into the cylinder before storing, then turn the engine over by hand to coat the walls.

A fuel stabilizer keeps gas from breaking down during storage and is the single most useful product for preventing this problem. View on Amazon

A carburetor cleaner spray dissolves gum and varnish from jets and passages without requiring full disassembly for light cleaning. View on Amazon

A digital multimeter lets you measure battery voltage accurately and check connections for continuity. View on Amazon

A spark plug gap tool with a feeler gauge makes it easy to set the correct electrode spacing on a replacement or cleaned plug. View on Amazon

FAQ

Can I still use gas that has been in a mower for a year?

Usually not. After a year, gasoline has lost its volatile components and often contains varnish and water. Drain it and refill with fresh fuel to avoid carburetor problems.

Will a lawn mower battery recover after sitting dead for a year?

Sometimes. A lead-acid battery that has sulfated may recover partially with a slow charge, but capacity is often reduced permanently. A lithium pack that has discharged deeply may not accept a charge. Check the voltage and consult the manufacturer’s guidance before charging.

Do I need to replace the spark plug?

Not always, but it is wise to inspect it. If the electrode is fouled, worn, or the gap is off, replacing it with the correct part is inexpensive and often solves the problem.

How do I know if the carburetor is the problem?

If the engine turns over with a healthy spark but won’t fire, or it sputters and dies, the carburetor is a likely cause, especially after long storage with untreated fuel.

Can I use starter fluid to get the engine going?

A small spray of starting fluid can help confirm whether fuel delivery or ignition is the issue, but it should not be used repeatedly as a fix, since it can damage the engine over time.

What if the engine turns over by hand with no resistance?

That suggests a mechanical fault such as a stuck valve or seized piston. Stop attempting to force it and have a small engine technician inspect it.

Should I take it to a professional?

If you find internal damage, signs of seized components, or electrical faults you are not comfortable diagnosing, a qualified small engine shop can inspect it safely. Many mower problems are routine for them.

How often should I add stabilizer to stored fuel?

Add it whenever you store the mower with fuel in the tank, following the product’s directions. For mowers used seasonally, treating the fuel at the end of each season prevents most storage problems.

Conclusion

A lawn mower that sat for a year and won’t start usually comes back to life once you address stale fuel, the battery, the spark plug, and the carburetor in that order. Work methodically, keep your tools clean, and avoid repeatedly cranking a flooded engine. Proper storage next season will prevent most of these problems from returning.

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Graham Fisher

About Graham Fisher

Graham Fisher is part of our editorial team and reviews and edits our lawn mowers guides, checking product details against manufacturer specifications and owner feedback.