Graham FisherBy Graham Fisher

Updated November 2026

A lawn mower that won’t start after winter storage usually has one of a handful of problems: old gasoline that has gone stale, a battery drained by months of sitting, a fouled spark plug, a clogged carburetor, or a dirty air filter. This guide walks you through the checks in the order that tends to solve the problem fastest, so you can work from the simplest fixes to the more involved ones. Most owners can get a lawn mower that won’t start after winter storage running again in under an hour with basic hand tools.

Start With the Safety Checks

Before you touch anything, make sure the mower cannot move or start unexpectedly. Disconnect the spark plug wire, or for an electric mower, remove the battery or unplug the charger. Engage the parking brake on a riding mower and make sure the blade is not spinning if you need to tip the machine. Work on a flat, level surface and wear eye protection when you are checking fuel and cleaning parts.

Shopping for one? See our guide: Lawn Mower Won't Start After Running Out of Gas: 3 Best Starting Aids Compared.

If you are unsure about any step that involves the fuel system, the electrical system or a blade removal, consult a professional small-engine repair shop. Gasoline vapors and battery acid are both hazardous, and a mistake can cause injury.

Check the Fuel First

Stale gasoline is the most common reason a mower refuses to start after winter. Gasoline begins to break down within about 30 days, and by spring the lighter fractions have evaporated while varnish-like residue has formed. That residue can clog the carburetor jets, the fuel filter and the fuel line.

Look at the fuel in the tank. Fresh gas is clear to light amber. Fuel that is dark, smells sour or varnish-like, or shows a layer of sediment should be drained. Drain it into an approved container and take it to a hazardous waste collection site. Do not pour it down a drain or onto the ground.

After draining, add fresh gasoline with a stabilizer or refill with fresh, ethanol-free fuel if you can find it. Many small engines are damaged by ethanol blends over time, so check your owner’s manual for the fuel grade and ethanol limit it specifies.

Checking the Fuel Line and Filter

  • Inspect the fuel line for cracks, stiffness or soft spots.
  • Look for a small inline fuel filter and replace it if it looks dark or restricted.
  • Confirm the fuel shutoff valve, if equipped, is fully open.
  • Make sure the primer bulb, if present, feels firm and springs back after squeezing.

Inspect and Charge the Battery

Electric and battery-assisted mowers, along with riding mowers that use a 12-volt starting battery, often fail to start after winter because the battery discharged while sitting. Lead-acid batteries self-discharge over time, and a battery left in a cold garage can lose much of its charge. Lithium-ion batteries lose less charge at rest but can still arrive in storage with a low state of charge that prevents them from starting the mower.

For a lead-acid battery, check the voltage with a multimeter. A fully charged 12-volt battery reads about 12.6 volts or higher at rest. Below roughly 12.2 volts, charge it before testing again. Clean any corrosion from the terminals with a baking soda and water paste, rinse, dry, and reattach the cables tightly. Then charge the battery with a smart charger matched to its chemistry.

For battery-powered mowers, follow the manufacturer’s charging instructions. Many owners report that a battery that was stored fully discharged for a long time will not accept a charge until it has sat on the charger for a while, so give it time before deciding it is dead.

Replace or Clean the Spark Plug

A spark plug that has been exposed to moisture, old fuel or oil over winter can be fouled with carbon or wet. Remove the plug with a spark plug socket, inspect the tip, and compare it to the manufacturer’s recommended plug in the owner’s manual.

Plug Condition What It Usually Means Action
Black, sooty tip Rich fuel mixture or dirty air filter Clean or replace the plug, check the air filter
Wet, oily tip Oil or fuel in the cylinder Dry the plug, check for an overfilled tank or oil leak
White or blistered insulator Lean mixture or overheating Replace the plug and check the carburetor adjustment
Dry, clean tip with a light gap Plug is probably fine Check the gap and spark wire connection

Set the electrode gap to the value listed in your manual, usually around 0.030 inch for many small engines, and make sure the spark plug boot seats firmly on the plug. A new plug is inexpensive and removes one variable from the troubleshooting process.

Clean the Air Filter

Air filters trap dust and debris over the mowing season and can collect moisture during storage. A clogged filter starves the engine of air, leading to a rich mixture that can flood the cylinder and make starting difficult.

  1. Remove the air filter cover, usually held by a wing nut or a few clips.
  2. Take out the filter and tap it gently to remove loose debris.
  3. For a paper filter, replace it if it is dirty or damp. Paper filters should not be washed.
  4. For a foam filter, wash it in warm, soapy water, squeeze it dry, and lightly oil it if the manual calls for it.
  5. Reinstall the filter and cover, making sure it seals properly against the housing.

Clean the Carburetor

If the fuel was stale or left in the carburetor over winter, varnish may block the small jets and passages that deliver fuel to the engine. This is one of the most frequent causes of a lawn mower that won’t start after winter storage, and it often shows up as a mower that starts briefly and then dies.

Basic Cleaning Steps

  • Remove the air filter housing and disconnect the fuel line from the carburetor.
  • Remove the carburetor bowl and the float, then inspect the needle valve and seat for debris.
  • Spray carburetor cleaner through the jets and passages, following the product label and working in a ventilated area.
  • Use compressed air to clear passages, but avoid pushing debris deeper into the jets.
  • Reassemble with new gaskets if the old ones are hard or torn.

If the carburetor is heavily varnished or the jets cannot be cleared, a carburetor rebuild kit or replacement carburetor may be the more reliable choice. Many owners find that a complete carburetor replacement costs less time and frustration than repeated cleaning attempts.

Check the Starting System

If the engine turns over slowly or not at all, the starting system may be the issue. On a pull-start mower, check that the recoil rope is not frayed and that the starter pulley and spring are engaged. On an electric start mower, inspect the starter solenoid, wiring and key switch for loose connections or corrosion. A failing starter motor often produces a clicking sound when the key is turned.

Engine compression also matters. Pull the spark plug and look into the cylinder while cranking the engine. If the piston does not rise and fall freely, or if the recoil feels unusually hard, the engine may have a stuck valve or a seized component, which calls for a professional diagnosis.

Common Mistakes

  • Assuming the engine is bad too early. Most no-start conditions after winter come from fuel, battery or air filter problems, not engine failure.
  • Ignoring the fuel shutoff and choke. Set the choke correctly for a cold start, and make sure the fuel valve is open.
  • Flooding the engine. Pulling the starter cord repeatedly with the choke fully closed can flood the cylinder. Remove the spark plug, dry it, and crank the engine with the throttle wide open to clear excess fuel before trying again.
  • Using stale gas to test. If you are unsure about the fuel, drain it and refill before troubleshooting further.
  • Skipping the battery on electric start models. A weak battery can look like a starter problem.

Tips for Next Winter

Preventing a no-start condition is easier than fixing one. Before storing the mower, run the engine until it stops to empty the carburetor, or add a fuel stabilizer to a full tank and run the engine for a few minutes so treated fuel reaches the carburetor. Change the oil, clean or replace the air filter, and remove or replace the spark plug if it is worn. Store the mower in a dry place, and keep the battery on a maintenance charger if it is a battery-powered or electric start model.

Stabilized fuel and a small set of hand tools cover most of the work described above. A fuel stabilizer helps protect gasoline during storage, a multimeter lets you check battery voltage accurately, a smart battery maintainer keeps a stored battery topped up, and a basic spark plug socket with a gap tool makes plug service quicker.

  • Fuel stabilizer, because it slows the breakdown of gasoline during storage. View on Amazon
  • Digital multimeter, because it gives an accurate battery voltage reading before you decide whether to charge or replace. View on Amazon
  • Smart battery maintainer, because it keeps a stored battery at a safe charge level over winter. View on Amazon
  • Spark plug socket with gap tool, because it makes removing and gapping a plug faster and less likely to damage the threads. View on Amazon

FAQ

How long can gasoline sit in a mower before it goes bad?

Gasoline can start to degrade within about 30 days, especially in warm conditions or if it contains ethanol. Fuel stabilizer extends that window, but it is still wise to drain or treat fuel before long storage periods.

Can I use the mower if the engine starts but runs rough?

A rough-running engine usually points to a fuel, air or spark problem. Check the fuel, air filter and spark plug first. If the roughness persists after those checks, the carburetor likely needs cleaning or adjustment.

Should I drain the gas before winter storage?

Either drain the tank and run the carburetor dry, or add a stabilizer to a full tank and run the engine briefly so the treated fuel reaches the carburetor. Both methods help prevent varnish buildup.

How do I know if the battery is the problem?

If the starter motor turns very slowly, the lights are dim, or you hear a clicking sound, the battery is a likely cause. Test the voltage with a multimeter and charge it before deciding it needs replacement.

Can I jump-start a riding mower battery?

Yes, using a second 12-volt battery or a jump starter designed for vehicles, connecting positive to positive and negative to a good ground. Follow the manufacturer’s instructions, and disconnect the cables in the correct order afterward.

Why does my mower start and then die?

This often indicates a clogged carburetor jet, a fuel supply problem or a vent that is blocked in the gas cap. Check the fuel cap vent, then clean or rebuild the carburetor if the problem continues.

When should I take the mower to a repair shop?

Consult a professional small-engine technician if you find internal engine damage, a stuck or bent valve, a faulty starter that you are not comfortable replacing, or any issue involving the fuel system that you cannot resolve safely yourself.

Conclusion

A lawn mower that won’t start after winter storage is usually fixed by addressing fuel, battery, spark plug, air filter and carburetor in that order. Work through the checks methodically, and you will likely find the cause without replacing major parts. Store the mower properly next season to make spring startup quick and uneventful.

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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.