Graham FisherBy Graham Fisher

Updated November 2026

A petrol lawn mower that won’t start after winter usually has one of three problems: stale fuel gummed up in the carburetor, a fouled or dead spark plug, or a clogged air filter. This guide walks you through checking each one in a sensible order, so you can get the engine running without taking anything apart unless you need to.

Start With the Fuel

Fuel is the most common culprit. Gasoline begins to break down within a few months, and after a winter sitting in the tank, it often leaves varnish-like deposits in the carburetor jets. Owners frequently report that the mower will sputter briefly or not fire at all, even though the tank looks full.

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

Drain the old fuel into a container approved for gasoline and take it to a local disposal site. Refill with fresh fuel. If you didn’t use a stabilizer last fall, expect this step to matter most. For a broader look at fuel-related no-start issues, see Lawn Mower Won't Start After Sitting: 8 Causes and Fixes.

Check the Spark Plug

Once the fuel is fresh, inspect the spark plug. Pull the spark plug wire, remove the plug with a socket wrench, and look at the electrode.

  • Black, sooty tip: the engine has been running rich. Clean or replace the plug.
  • Wet, oily tip: the engine may be flooded. Dry the plug and try again after waiting several minutes.
  • White or blistered insulator: the plug may be overheated or damaged. Replace it.

Set the gap to the value listed in your owner’s manual, usually somewhere in the range manufacturers specify for small engines. Reinstall the plug finger-tight, then snug it with the wrench.

Clean the Air Filter and Carburetor

A clogged air filter starves the engine of air, and a dirty carburetor can keep fuel from reaching the combustion chamber. Remove the air filter cover and inspect the element. A paper filter that is caked with dirt should be replaced. A foam filter can usually be washed in mild soap and water, then squeezed dry and lightly oiled if the manufacturer calls for it.

If fresh fuel and a good spark don’t help, the carburetor may need cleaning. This is a more involved job. You’ll need to remove the carburetor, soak the jets in carburetor cleaner, and clear any passages with compressed air. If you’re not comfortable with that, a small engine shop can do it. For model-specific guidance, check Lawn Mower Won't Start After Storage: 8 Causes and Fixes.

Starting Procedure and Final Checks

Before you pull the cord, confirm the basics. Set the choke to the start position, the throttle to fast, and make sure the blade-engagement or safety bar is in the correct position for your model. Pull the cord firmly several times. If the engine fires but dies immediately, the carburetor is the likely cause. If it doesn’t fire at all, recheck the spark plug connection and fuel flow.

Tools Worth Having on Hand

  1. Socket wrench and spark plug socket
  2. Feeler gauge for checking plug gap
  3. Fresh gasoline and a fuel container approved for storage
  4. Replacement air filter and spark plug matching the owner’s manual
  5. Carburetor cleaner and a small wire brush

Common Mistakes

  • Pulling the cord endlessly with a flooded engine. Wait several minutes, then try with the choke open.
  • Reusing old fuel because it “looks fine.” Varnish problems often start before the color changes.
  • Skipping the air filter check, which is quick and often overlooked.
  • Forgetting the safety bar or blade engagement lever, which can block starting on many models.

A fuel stabilizer added to the tank each fall prevents most of the varnish buildup that causes winter no-starts. View on Amazon

A spark plug socket with a rubber insert makes removing and reinstalling the plug far less frustrating. View on Amazon

FAQ

Can I just add fresh gas and hope it starts?

Sometimes, but only if the old fuel was recent and the carburetor is clean. Stale fuel left in the carburetor often needs to be drained and replaced along with a check of the jets.

How do I know if the spark plug is bad?

Remove it and check the electrode color and condition. A plug that is heavily fouled, cracked, or has a worn electrode should be replaced. A quick spark test with the plug grounded against the engine can also confirm whether ignition is working.

Should I keep a stabilizer in the tank all winter?

Yes. Add stabilizer to fresh fuel, run the engine for a minute to circulate it through the carburetor, then store the mower. This is the most reliable way to avoid the problem next spring.

Conclusion

Work through fuel, spark plug, and air filter first, since those fix most winter no-starts. If the engine still won’t run, the carburetor is the next step. Consult a small engine technician if the repair goes beyond basic cleaning.

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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.