Graham FisherBy Graham Fisher

Updated November 2026

A lawn mower won’t start after storage usually comes down to one of a handful of problems: stale fuel, a weak or dead battery, a fouled spark plug, or a clogged carburetor. This guide walks you through checking each of these in a logical order, so you can find the cause without replacing parts you don’t need. Most of the time, a mower that sat all winter or through the off-season can be brought back to life with a few hours of work and some basic supplies.

Before You Start: Safety and Tools

Work on the mower with the engine off, the spark plug wire disconnected, and the key removed if it has one. For battery-powered models, remove the battery or switch it off before touching the blade. Keep the mower on a flat surface, and never run it indoors or in an enclosed garage.

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

Gather these supplies before you begin:

  • Fresh gasoline (ideally with ethanol content of 10% or less) or a fuel stabilizer
  • A socket set and a spark plug socket (typically 5/8-inch or 13/16-inch)
  • A wire brush and a feeler gauge for checking the spark plug gap
  • A multimeter for testing battery voltage
  • Clean rags and a small container for used fuel
  • Carburetor cleaner, if you plan to clean the carburetor

Step 1: Check the Fuel and Replace It If It’s Stale

Stale fuel is the most common reason a mower won’t start after storage. Gasoline begins to degrade within about 30 days, especially ethanol-blended fuel, which absorbs moisture and separates. Over a season, this leaves a varnish-like residue that clogs the carburetor jets and the fuel line.

To check the fuel, look at the color and smell. Fresh gas is clear to light amber and smells sharp. Stale fuel looks darker, may have a sour or varnish smell, and sometimes shows water or sediment at the bottom of the tank.

How to Drain and Refill

  1. Place a container under the carburetor drain screw (if equipped) or disconnect the fuel line at the carburetor and drain the fuel into the container.
  2. Dispose of the old fuel at a local hazardous waste facility. Don’t pour it on the ground or into a drain.
  3. Add fresh gasoline with a fuel stabilizer, following the stabilizer’s mixing ratio on the label.
  4. Prime the engine by pressing the primer bulb several times, if your model has one.

Step 2: Inspect the Spark Plug

A fouled or worn spark plug can prevent starting even when fuel is fresh. Carbon buildup, oil residue, and moisture all reduce spark quality. Remove the spark plug and inspect the tip.

A healthy plug has a light tan or gray color on the electrode. Black, sooty deposits suggest a rich fuel mixture or a dirty air filter. Wet or oily deposits can mean oil is entering the combustion chamber. White or blistered electrodes suggest overheating or incorrect heat range.

Cleaning or Replacing the Plug

  • If the plug is lightly fouled, clean the electrode with a wire brush and let it dry.
  • Check the gap with a feeler gauge. Most small-engine plugs are set to about 0.030 inches (0.76 mm), but confirm the value in your owner’s manual.
  • If the porcelain is cracked or the electrode is badly worn, replace the plug with the exact model specified by the manufacturer.

When reinstalling, hand-tighten first, then use the socket to snug it down. Overtightening can damage the threads in the cylinder head.

Step 3: Test and Charge the Battery

If your mower has an electric start, a weak battery is a likely culprit. Batteries lose charge over winter, and lead-acid batteries can sulfate and fail permanently if left discharged.

Use a multimeter to check voltage. A fully charged 12-volt battery reads about 12.6 volts or higher. Readings near 12.0 volts suggest a partial discharge, and anything below about 11.8 volts usually means the battery needs a slow charge or replacement. Consult a battery professional if you’re unsure how to handle a deeply discharged or swollen battery.

Also inspect the battery terminals. Corrosion appears as a white or green crust. Clean it with a mixture of baking soda and water, rinse with a little clean water, and dry thoroughly before reconnecting. Tighten the cable clamps so they don’t rock.

Step 4: Check the Air Filter

A dirty or oil-soaked air filter restricts airflow and can flood the engine with fuel, making it hard to start. Foam filters should be washed in warm soapy water, squeezed dry, and lightly coated with engine oil. Paper filters should be replaced if they’re clogged or damp.

Step 5: Clean the Carburetor

If fuel is fresh and the plug is good, the carburetor is the next suspect. Varnish from old fuel can block the tiny jets and passages. Symptoms include hard starting, rough idling, and the engine dying shortly after starting.

Basic Carburetor Cleaning

  1. Remove the air filter housing and disconnect the throttle and choke linkages.
  2. Remove the carburetor bowl and gasket. Note the orientation of the float.
  3. Spray carburetor cleaner through the jets and passages. Use compressed air to clear them.
  4. Reassemble with new gaskets if the old ones are hard or torn.

Do not poke wire into the jets, because it can enlarge the opening and change the fuel mixture permanently.

Step 6: Check the Fuel Line and Fuel Filter

Rubber fuel lines become brittle and can crack over time, letting air in or allowing fuel to leak. Squeeze the line to check for hardness or cracks. Many mowers also have an inline fuel filter, which should be replaced if it looks dark or restricted.

Step 7: Check the Oil Level and Condition

Low or contaminated oil can trigger a low-oil safety shutoff on many modern mowers, which prevents the engine from starting. Check the dipstick, add oil to the full mark if needed, and change the oil if it looks milky (possible water contamination) or very dark.

Step 8: Verify the Safety Switches

Many mowers have safety features that block starting unless certain conditions are met. Check the operator presence bar, the blade engagement lever, and the seat switch on riding models. Make sure the throttle and choke are in the correct starting positions. A stuck safety switch is a common cause of a mower that cranks but never fires.

Common Mistakes

  • Skipping the fuel check. Many owners jump straight to the spark plug, but stale fuel causes more no-start problems than any other single factor.
  • Using old gas with an empty stabilizer. Stabilizer doesn’t reverse damage in fuel that has already degraded.
  • Forcing the throttle. Pumping the throttle or holding the choke on too long can flood the engine.
  • Cranking for extended periods. Repeated cranking drains the battery and can overheat the starter. Wait a few minutes between attempts.
  • Ignoring the air filter. A quick check takes less than a minute and rules out a surprisingly common cause.

Tips for Preventing This Next Season

  • Add fuel stabilizer to the tank before storage, then run the engine for a few minutes so the treated fuel reaches the carburetor.
  • Or run the tank completely dry by letting the engine stop on its own after the fuel is used up.
  • Change the oil and replace the spark plug at the end of each season.
  • Store battery-powered mowers with the battery charged, and keep the battery somewhere above freezing.
  • Store the mower in a dry location, and cover it to limit dust and moisture.

If you’re dealing with a particular brand, you may also find helpful troubleshooting on the site, such as Lawn Mower Won't Start After Winter: 7 Fixes That Work for general cold-storage issues, or Lawn Mower Won't Start After Sitting: 8 Causes and Fixes for a broader look at mowers that sat unused for a long time. If the problem started after you worked on the engine, Lawn Mower Won't Start After Replacing Carburetor: 6 Parts Compared covers related checks.

  • Fuel stabilizer. Keeps gasoline from breaking down during storage and makes next season’s startup far more reliable. View on Amazon
  • Digital multimeter. Lets you check battery voltage accurately before deciding whether to charge or replace. View on Amazon
  • Spark plug socket and gap tool. Makes plug removal and gap checks quick and prevents thread damage. View on Amazon
  • Battery terminal cleaning brush. Removes corrosion from posts and clamps for a solid electrical connection. View on Amazon

FAQ

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

Gasoline can start to degrade in as little as 30 days, especially ethanol-blended fuel. Using a fuel stabilizer extends its life considerably, but the safest approach is to use fresh fuel before storage or drain the tank.

Can I use starting fluid to get my mower running?

Starting fluid can help diagnose a fuel delivery problem, but it’s a temporary fix. If the engine starts briefly and dies, the real issue is usually the fuel system or carburetor, and you should address that instead.

Why does my mower crank but not start?

This usually means the battery has enough power to turn the engine, but the engine isn’t getting fuel, spark, or both. Check fuel freshness, the spark plug, and the air filter first.

Should I drain the gas before storing my mower?

Either drain the tank completely or add a stabilizer and run the engine until the treated fuel reaches the carburetor. Both approaches prevent varnish buildup over the off-season.

How do I know if my spark plug is bad?

Remove and inspect it for cracks, heavy deposits, or electrode wear. If it looks damaged or the gap is out of spec, replace it with the model listed in your owner’s manual.

Can a dead battery cause a mower not to start after storage?

Yes, on electric-start models. Batteries self-discharge during storage, and a deeply discharged lead-acid battery can be damaged. Test the voltage and recharge it before assuming a bigger problem.

When should I take the mower to a repair shop?

If you’ve checked the fuel, plug, battery, filter, and carburetor and the engine still won’t start, or if you see fuel leaks, cracked components, or unusual smoke, consult a qualified small-engine technician.

Conclusion

A lawn mower that won’t start after storage is usually fixable with a systematic approach. Start with fuel and the spark plug, then move to the battery, air filter, and carburetor. Taking the time to check each system in order will help you avoid unnecessary parts replacement. Store the mower properly next season, and you’ll spend far less time troubleshooting when spring arrives.

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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.