Lawnmower backfires when trying to start, the causes and fixes.


Lawnmowers are important machines for many homeowners to maintain their gardens. They use small engines to power the blades that trim the grass. Occasionally, you may encounter problems like backfiring when you are trying to start it. As soon as you pull the starter cord or press that start button, the mower exerts a loud noise, and ultimately shutting the engine off. It may feel like a severe issue, but do not worry. This blog post will help you identify the possible causes and how to fix them.

Lawnmower backfires when trying to start, the causes and fixes:

  • Step 1. You should use fuel that has no blends of ethanol.
  • Step 2. Check the carburetor to make necessary adjustments.
  • Step 3. Check the engine temperature and pressure.
  • Step 4. Repair and replace damaged spark plugs.
  • Step 5. Repair the flywheel key if it is broken.
  • Step 6. Check the engine timing and lubricate necessary components.
  • Step 7. Carburetor not working correctly.

Backfires can be a headache, given that they are so loud and uncomfortable. Trying to operate a backfiring engine may cause damage to the inlet/exhaust valves or the piston head in cylinders. It may also cause harm to the cutting blades. The bottom line of a backfiring engine is a fault with two crucial engine systems, i.e., the ignition and fuel delivery system. Whenever you have problems regarding sputters, backfires, or engine stalls, these systems and their associated components need to be inspected for faults.  

Troubleshooting steps for a lawnmower engine backfiring:

Lawnmower engines are small engines, typically four-stroke, that use controlled explosions in the combustion chamber to drive the blade and wheels. Backfiring is an irritating problem that you can quickly fix yourself. If you are not confident in resolving the issue, ask a professional for help.

● Step 1. Check the fuel quality:

Backfiring may occur if the gas that you use is bad. If you use gas with a high amount of ethanol, then your engine may backfire. A fuel that contains more blends of alcohol is also less suited for small engines. It is better to use fuel with soft blends of alcohol to keep your lawnmower’s engine safe and correctly working.

Fix: Remove the bad gas from the tank and replace it with a new better-suited type.

● Step 2. Adjust the carburetor:

The carburetor is an essential part of every engine that prepares and provides the engine’s air-fuel charge. Carburetors are quite susceptible to developing clogs and restrictions often owing to old or bad fuel quality. Adjusting your carburetor from time to time is important for every engine if you wish to keep it running smoothly. If the carburetor is not adjusted correctly, you may witness backfires and stalls.

If the carburetor adjustment is set too lean, it may create issues. Usually, the backfires occur through the carburetor when starting. Backfiring happens if the carburetor is not adjusted well. It appears when something is wrong with the valves or rocker arm area.

Fix: Use a carburetor cleaner, or disassemble and clean it. After it is clean, adjust it correctly.

● Step 3. Keep the temperature and pressure normal:

Moreover, backfiring also depends on engine temperatures. If the engine temperature is too high, it can cause backfiring. Check the temperature of your engine. If it is just idling, it should have an average operating temperature and not feel like an electric heater. A cause for the high engine temperature can be a low fuel pressure. This results in a wrong fuel-air ratio with too much air in the combustion chamber. The engine is unable to create more power to ignite fuel. Low fuel pressure also affects the fuel pumps and old fuel filters.

Fix: Check the fuel pump if your lawnmower has one. Also, a partially clogged fuel filter or fuel lines can be the cause of the problem. Check, clean, or replace them.

● Step 4. Replace weak or damaged spark plugs:

Spark plugs are another essential part of the lawnmower’s engine. If the spark plug is damaged or worn, it creates a weak spark. As a result, the fuel may not ignite correctly in the cylinder and might ignite when it reaches the hot exhaust muffler. As a result, a loud backfire is produced.

Fix: Remove the spark plugs and check the condition. Use a spark plug tester if you are not sure. You can try cleaning the spark plug with a metal brush.  Replace the spark plug immediately if it is damaged or worn.

● Step 5. Check the flywheel key:

Flywheels are located after the engine’s crankshaft and are connected to the transmission. Sometimes, when you run your mower over a large rock, it can damage or even break the flywheel key. This can result in the mower backfiring when you try to start it. The key connects the flywheel to the crankshaft.

When the key is damaged, the connection is lost, and the lawnmower doesn’t propel forwards and may backfire.

Fix: Repair the flywheel by replacing its key.

As a preventative step, always remove those large rocks from your garden whenever you perform the mowing operation. These obstructions are not suitable for the mower. If the flywheel key is broken, then your mower might stutter and backfire when you try to start it. It’s not usable until the flywheel key is repaired or replaced.

● Step 6. Check the lawnmower timing:

If your lawnmower is not tuned correctly, it can result in the engine not able to work correctly. Tuning involves the timing of the engine, and if not set correctly, it can ignite a spark plug at the wrong time. This may result in a backfire. Over time the timing between the engine and valves can change, and they need proper tuning. Damaged fuel filters may also cause backfiring.

Fix: Not all lawnmower engines can be tuned-up. Setting the timing on the engine depends greatly from the engine to the engine. Check the user manual for more information. The typical interval for a lawnmower engine is around 50 hours or once a season.  

● Step 7. The carburetor is not working correctly:

If you notice that your carburetor is not working correctly, it can also result in a backfire. It can be the result of dirt in the fuel or bad gas.

Fix: Use a carburetor cleaner or disassemble the carburetor and clean it. Replace it if it is not possible to clean.

Other possible causes of backfiring

Some other potential causes of a backfire are:

  1. Regular maintenance: You should examine and maintain the engine, carburetor, and fuel line regularly.
  2. Rust: Try to remove the rust and metal deposits in the gas tank and other parts of the engine. Rust in the gas tank can find its way to the engine and increase wear and tear.
  3. Damaged engine parts: If you find out that some crucial parts of your lawnmower’s engine are damaged, such as crankshaft or pistons, you need to replace them. If you are not comfortable with that type of fixes, ask a professional for help.

Some Precautionary Measure

To increase the life span of your lawnmower, there are some measures you should take into consideration:

  1. Turn the engine off slowly: You should turn off the engine slowly. Let it idle first so it can cool down a little bit. Too fast can cause a backfire while shutting it off. 
  2. Adjust the carburetor: You need to make sure that the carburetor is adjusted correctly before starting the engine. A wrongly adjusted carburetor can cause backfires.
  3. Hot engine: If the lawnmower backfires, you need to be careful as the engine might overheat, and you may get burned by it. Check with a professional if you do not know how to fix it.

Related Questions:

1. Is backfire harmful for a lawnmower engine?

Yes, a backfire can be harmful to your lawnmower. It affects not only the engine but also the exhaust. Backfiring can damage the engine as they are rapid bursts of fire in the wrong place. The ignition of the gas-air mixture should only happen in the combustion chamber specifically designed for it.

2. What is a lawnmower backfire?

Backfire is a condition described as a loud bang or explosion. It may occur while starting, running, or shutting down the engine. Backfire occurs if the combustion process extends to the outside of the combustion chamber. It can be harmful to the engine.

3. What causes the backfire in a lawnmower engine?

Backfiring is caused by combustion or explosion from unburnt fuel in the exhaust. There are several causes why a backfire can occur, like clogged pipelines, faulty spark plugs.

4. Can a spark plug cause a lawnmower to backfire?

Yes, a spark plug can cause a lawnmower to backfire. If the spark plug is damaged or worn, it will not produce a good spark. As a result, the fuel might not ignite inside the cylinder but somewhere inside the hot exhaust muffler. This results in a loud backfire.

Final Remarks

Backfiring is a common problem in a lawnmower but, depending on the cause, is not difficult to fix. There can be different times when a backfire occurs, like starting, get your feet from the gas, or shutting down the engine. The common causes and fixes of backfiring have been highlighted in the blog post. Proper maintenance of your lawnmower will help extend the lifetime and decrease the chance of a backfire.

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