Riding Lawn Mower Sputtering And Backfiring

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Is your riding mower sputtering and backfiring? Is it running irregularly? Do you hear a loud bang when you try to start your mower’s engine? This is undoubtedly a sign that something is wrong internally.  

In this article, I explain the common reasons your lawn mower sputters and backfires, either before starting your mower, while the mower is running, or when it is shutting down.

I also share troubleshooting tips to help you diagnose and fix the problem to get your mower running smoothly again.

Reasons Your Riding Mower Sputters And Backfires

Some common reasons your riding mower sputters and backfires include:

  • Clogged/dirty air filter
  • Faulty spark plug
  • Water in the gas tank
  • Clogged fuel filter/fuel lines
  • Carburetor problems
  • Clogged mower deck

Tools You Need For This Job

Some tools you may need when troubleshooting include:

  1. Work gloves
  2. Screwdriver
  3. Socket wrench
  4. Pliers
  5. Empty can
  6. Clean rag
  7. Water hose
  8. Scraper
  9. Carburetor cleaner
  10. Siphon

Riding Mower Sputters And Backfires: Possible Causes And Fixes

1. Clogged/Dirty Air Filter

The primary reason your riding mower sputters and backfires is a dirty air filter. The engine may be starving of air brought about by a clogged or dirty air filter.

Air entering the mower’s engine passes through the air filter for it to be rid of debris and harmful particles. If the filter is clogged, air intake is reduced and this could cause your riding mower to sputter and backfire.

Diagnosis:

Open the rectangular air filter housing located at the side of the engine. 

Visually inspect the air filter. If it is darkened, dusty, or stained with oil, it may be clogged.

Fix:

Paper air filters are best replaced. Foam air filters can be washed and reused but I would recommend replacing them for best engine performance.

2. Faulty Spark Plug

A faulty spark plug can cause your riding mower to sputter and backfire. If the engine isn’t receiving sufficient spark, it could react by sputtering and backfiring.

This could result from carbon buildup on the spark plug, burnt porcelain, or spark plug wear as a result of frequent heating, cooling, and reheating.

Diagnosis:

Remove the spark plug and examine its ends for cracks, carbon buildup, or burnt ends. These symptoms are signs of a faulty spark plug.

Fix:

Use a wire brush to remove carbon buildup and oil stains.

Replace the spark plug if it is broken, cracked, or burnt.

3. Water In The Gas Tank

The presence of water in the gas tank will cause water to enter the engine and mix with the fuel and air mixture. This is another reason your riding mower may sputter and backfire.

Water impedes the correct ignition of the gasoline because it will not ignite when it gets into the cylinder. As a result, the engine will sputter and backfire whenever the water tries to ignite.

Water in the tank could be caused by condensation, a loose fuel cap, or it could have been present in the fuel cans and was transferred into your mower. 

Diagnosis:

Remove all fuel from the tank. If water is mixed with fuel, it could be the cause of the problem.

Fix:

Empty the gas tank and carburetor bowl.

Refill the tank with new fuel and start the engine.

4. Clogged Fuel Filter/Fuel Lines

Insufficient fuel reaching the carburetor can cause the engine to sputter and backfire as a result of the fuel mixture being too lean.

The fuel gets to the carburetor through the fuel filter and fuel lines. These parts can get clogged with debris from the fuel tank and reduce fuel flow, causing the engine to run on a lean fuel mixture.

Diagnosis:

Inspect the amount of fuel flowing through each fuel line and fuel filter. If fuel isn’t flowing freely, it is clogged.

Fix:

Replacing the clogged air filter is the only way to fix it. 

You can replace the fuel lines or spray carb cleaner through them to remove residual debris.

5. Carburetor Problems

The carburetor is in charge of combining the proper amounts of gasoline and air needed to keep the engine running smoothly. The mower sputters and backfires if this mixture is off. 

Jets, springs, diaphragms, needle valves, and floats are internal components of the carburetor that may be faulty and will need to be cleaned or replaced. 

The mower’s performance is affected by the idle and mix of the carburetor, which is adjustable by a few screws on the outside of the device. The carburetor may be set too lean and this could cause the engine to backfire and sputter.

Diagnosis

Open up the engine and take out the carburetor with a screwdriver. Then examine all jets, bowls, diaphragm, valves, and floats for damage or cracks.

Fix:

  • First, use carb cleaner to clean the entire carburetor focusing on the jets and bowls
  • Replace any damaged parts 
  • If the carburetor is too damaged, rebuild it with a carburetor repair kit or replace the carburetor.

Without prior experience, the carburetor can be challenging to clean or repair due to its complexity. Do not hesitate to contact an experienced technician if you do not have mechanical experience.

6. Clogged Mower Deck

As you use your mower to work, the deck can get clogged with grass. Check the underside of the unit for caked-on grass. Excessive grass under the deck will make the mower sputter. 

This is because the caked grass will prevent the belt, pulleys, and blades from moving freely, causing the mower to sputter as it runs.

Diagnosis:

Check the deck of your mower for stuck grass or debris.

Fix:

  • Use a water hose to blast grass from the deck
  • Remove the spark plug using a wrench to prevent the mower from turning on while working underneath. 
  • Scrape the grass from hard-to-reach places using a small scraper tool
  • Start the engine to uncover more grass, then repeat the cleaning process 

Lawn Mower Sputters But Runs

Your lawn mower sputtering while running indicates fuel delivery problems. If your lawn mower sputters while running, it could be a result of:

  • Water in the fuel tank
  • Clogged fuel filter 
  • Blocked fuel lines 
  • Carburetor problems
  • Clogged mower deck

These conditions prevent an adequate flow of fuel to the engine which can lead to insufficient fuel needed for proper functioning.

This imbalance of air and fuel in your engine can cause your mower to sputter while it is running.

Fix:

Troubleshoot the entire fuel system including the fuel lines, fuel filter, fuel tank, and carburetor as directed above.

Check for blockage in any of these parts and clean or replace any clogged component if found.

Also, check the mower deck to ensure that the moving parts aren’t clogged with grass. If it is, use a water hose and scraper to unclog it.

Lawn Mower Sputters When Blades Are Engaged

Your lawn mower sputtering only when the blades are engaged indicates that there is a problem with the motor. This could be a result of:

  • Damaged drive belts
  • Worn out pulleys
  • Faulty safety switch

1. Damaged drive belts

The belt of your mower can become slack or glazed and cause it to sputter as a result. 

When the belt is glazed or damaged, it may start to slip and cause the mower to sputter when you engage the blades.

Lower your mower’s deck to the lowest setting and remove the belt guard to check for this problem. You can open it up by consulting your mower’s handbook.

Fix:

Replace your belt if it is glazed, worn out, damaged, or poorly routed. If your belt exhibits symptoms of wear, you’ll need to replace it.

If re-routing the belt is the problem, your owner’s manual will give you directions to reroute it. 

2. Worn out pulleys

The pulleys of your lawn mower are normally engaged by the drive belt and are in charge of rotating the spindles. 

As a result, if a particular pulley becomes stuck and does not spin properly, your mower is likely to splutter once the blades are engaged.

To diagnose, disconnect the drive belt from the pulley and engage the blades. 

Now, spin the idler pulleys by hand to see if they revolve or generate a low-pitched sound. If this is not the case, you will need to repair the damaged pulleys. 

Fix:

Pulley replacement is a difficult task and it should be performed by a lawn mower professional with technical experience.

3. Faulty safety switch

Motor safety switches are designed to assist in preventing serious accidents. When the switch is triggered, the motor receives an immediate signal to stop.

If you suspect the safety switch is to blame, open your mower and check to see if the switch is correctly plugged in. If it is, check for damage. If it appears to be worn out, replacing it may resolve the problem.

Fix:

  • Unbolt the seat and slide it forward to find the safety switch under your riding mower seat.
  • Remove the seat from its mount by unscrewing the locking tab.
  • Once you’ve accessed the safety switch, physically inspect it for damage.
  • However, switch damage is not always evident and may require testing with a multimeter.
  • If the outcome is anything other than a ‘zero ohms’ reading when the switch is turned off, your safety switch is damaged and must be replaced.

What Causes A Lawn Mower To Backfire When Starting?

A lawn mower can backfire when starting as a result of a damaged flywheel or sheared flywheel key.

Many mower components protect other, more expensive components. To safeguard the more expensive components, like the crankshaft, these less expensive components, like the flywheel, sacrifice themselves. 

Your mower’s flywheel breaks and won’t engage when you restart it if you run over a big rock while mowing or run into an obstacle with the blades. When you try to start your engine, it will sputter or backfire.

Fix:

A mower with a damaged flywheel cannot be used until the flywheel is fixed since it will not start normally. 

Take the mower to a professional for repair before attempting to use it again.

What Causes A Lawn Mower To Backfire When You Turn It Off?

Your lawn mower may backfire when you turn it off as a result of shutting it down too fast. 

When you reduce your engine speed too quickly, fuel continues to be pushed through it since the engine isn’t expecting it to stop so abruptly.

However, because the ignition is now turned off, no spark is produced by the spark plug, and the fuel and air mixture is not ignited in the combustion chamber. 

As an alternative, the fuel finds its way into the exhaust system. The muffler inside the exhaust is extremely hot, and when unburned fuel vapor contacts it, it ignites, resulting in the loud backfiring.

Fix:

You must gradually reduce the engine speed and allow the lawn mower to idle for at least 30 seconds before turning it off to avoid a backfire. This will guarantee that extra fuel doesn’t enter the exhaust system.

Riding Lawn Mower Backfires While Running

If your mower starts okay but then backfires after a while, the carburetor may be set too lean or too rich.

The carburetor ensures that the air-fuel mixture is just right for combustion to occur. Backfiring can occur when there is too much air and not enough fuel, or too much fuel and not enough air.

Fix:

Adjust the carburetor screw to the right setting to allow the right quantity of fuel and air into the engine.

Why Does My Lawn Mower Backfire And Not Start?

Your lawn mower backfiring and not starting could be a result of wrong engine timing. This is usually caused by a damaged flywheel or flywheel key

The flywheel of your mower can get damaged over time due to frequent use and wear. Partially disassemble the engine and check the state of the flywheel and key. 

Fix:

You will need to replace the flywheel and flywheel key to fix the problem if this is the case. Take the mower to a professional technician to get this done.

Conclusion

Your riding mower sputtering and backfiring could be a result of a clogged/dirty air filter, faulty spark plug, water in the gas tank, clogged fuel filter/fuel line, carburetor problems, clogged mower deck, or damaged flywheel.

Follow the troubleshooting guides in this article to diagnose the problem, then apply the recommended fixes to get your mower running smoothly again.