Graham FisherBy Graham Fisher

Updated November 2026

A lawn mower that won’t start and makes a popping sound usually has a fuel, spark or air problem. In most cases you can narrow it down in under 20 minutes by checking the fuel, air filter, spark plug and carburetor in that order. This guide walks through each check so you can find the cause and get the engine running again.

What the Popping Sound Tells You

The sound itself is a useful clue. A single pop from the muffler or carburetor usually means the engine is getting fuel but not firing cleanly, often because the mixture is too lean or the timing is off. Repeated popping while cranking typically points to a weak spark or a fuel delivery problem. Backfiring through the carburetor often means a clogged air path or a stuck valve. Note when the pop happens, whether it’s during cranking, right after starting, or when you give it throttle, because that narrows the list.

Shopping for one? See our guide: Lawn Mower Won't Start Without Starter Fluid: 5 Best Options Compared. If you want more background, see How to Start a Troy Bilt Lawn Edger: A Step-by-Step Guide for Beginners.

Check the Fuel and Air First

Most popping starts come from fuel that is old, contaminated or not reaching the engine. Gasoline that sits longer than about 30 days can break down and leave varnish in the carburetor. Ethanol blends also absorb moisture over time. Start with these checks:

  1. Drain or replace the fuel if it’s more than a month old, and use fresh gasoline with the octane your owner’s manual specifies.
  2. Confirm the fuel shutoff valve is open and the fuel line is not kinked or cracked.
  3. Remove the air filter and inspect it. A paper filter that is dirty or oil-soaked will starve the engine of air and cause popping. Replace it if it’s clogged, and tap out loose debris from a foam filter or wash it according to the manual.

Inspect the Spark Plug

A fouled or worn spark plug can produce a weak spark that misfires and pops. Remove the plug with a spark plug socket and look at the tip. A sooty black coating points to a rich mixture, often from a dirty air filter. An oily wet tip suggests oil in the combustion chamber. White or blistered electrodes mean the engine is running too hot or the plug is the wrong heat range. Clean the tip with a wire brush, check the gap against the specification in your manual, and replace the plug if the electrodes are worn. Reinstall it snugly, then test the spark by grounding the plug against the engine while cranking.

Clean the Carburetor

If fuel and spark check out, the carburetor is the next suspect. Varnish and debris can block the idle jet or main jet, letting fuel and air mix incorrectly. Popping through the carburetor, especially when the engine tries to start or idle, is a common symptom. Carefully remove the carburetor, spray the jets and passages with carburetor cleaner, and blow them out with compressed air. Do not push wire through the jets, because that can enlarge the opening. If the bowl is cracked or the float sticks, replace the carburetor or its rebuild kit.

  • Spark plug socket set: The right socket keeps the plug from cracking when you remove it. View on Amazon
  • Carburetor cleaner spray: Dissolves varnish in jets and passages without damaging gaskets when used as directed. View on Amazon
  • Feeler gauge set: Lets you check and set the spark plug gap accurately. View on Amazon

Common mistakes

  • Adding fresh fuel without draining the old fuel from the tank and carburetor.
  • Replacing the spark plug without checking the gap, which can cause a weak spark on a new plug.
  • Running the engine with the air filter removed, which can change the mixture and make popping worse.
  • Using a wire or pin to clear jets, which can permanently enlarge them.
  • Ignoring a loose or cracked spark plug wire boot, which can cause intermittent misfires.

FAQ

Can bad gas cause a lawn mower to pop?

Yes. Stale or water-contaminated gasoline can cause misfires and popping, especially in carburetors. Drain the tank and carburetor, then refill with fresh fuel.

Is popping from the muffler dangerous?

A single pop is often a normal sign of a lean or rich mixture, but repeated loud backfiring can damage the muffler or engine. If the problem continues after basic checks, consult a small engine repair professional.

Should I replace the spark plug first?

Not always. Check the fuel and air filter first, since they are quicker and cheaper to inspect. If those are fine, inspect or replace the spark plug next.

Conclusion

Most popping, non-starting mowers come down to old fuel, a dirty air filter, a fouled spark plug or a clogged carburetor. Work through those four in order and you will likely find the problem. If the engine still won’t start, a small engine technician can run more detailed tests.

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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.