What Causes a Small Engine to Backfire Through the Carburetor?

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Backfire is a condition described by a loud poof, explosion or bang while the engine is shutting down or running. 

Backfire typically occurs when the engine is decelerated rapidly. It may feel like a serious issue but you do not need to worry about it.

Is the small engine of your lawn mower making loud noises? Curious to know exactly what causes an engine to backfire through the carburetor? This article will help you identify the possible causes, problems and recommend effective ways to fix them.

Common Causes of Small Engine Backfire

  • Lowering engine speed too fast.
  • Muffler construction can induce backfire
  • Gasoline which contains higher blends of alcohol
  • Some carburetors can cause backfire due to the sensitivity of internal transitional passages.
  • Higher than normal engine temperatures.
  • Carburetor adjustment set is too lean.

Troubleshooting Steps for a Lawn mower Engine Backfiring


Backfiring is a task that you can solve yourself. If you can’t handle it yourself, you should seek the help of a professional. 

Here are some troubleshooting tips:

Step 1: Check the Fuel Quality

Backfiring can happen if you make use of bad gas. Using gas with a high amount of ethanol might make the engine backfire too.

A gas that contains more blends of alcohol is not ideal for small engines. It is better to use gas with soft blends of alcohol. This will make your engine work correctly.

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

Step 2: Adjust the Carburetor


The carburetor is an important part of your mower’s engine. It provides and prepares the engine’s air-fuel charge. They are quite liable to developing restrictions and clogs owing to old or bad fuel quality.

It is important to adjust the carburetor at timely intervals so that the engine can run smoothly. The engine may stall or backfire if it is not adjusted correctly. If the adjustment is set too lean, it might also cause problems. Usually, the backfires occur through the carburetor when starting.

Fix: Ensure that you use a carburetor cleaner. You can also disassemble and clean it. Adjust it correctly after cleaning it.

Step 3: Keep the Temperature and Pressure Normal

If the temperature is too high, it might result in backfiring. Make sure that you inspect the temperature of your engine. If it is just idling, it should have an average operating temperature.

Low fuel pressure is also a cause of high temperature. This results in a wrong fuel-air ratio with too much air in the combustion chamber, making it unable to create more power to ignite fuel. Low fuel pressure affects old fuel filters and fuel pumps.

Fix: Check the fuel pump if your mowing machine comes with one. In addition, partially clogged fuel lines or fuel filters can be the cause of this problem. Make sure that you inspect, clean or replace them when necessary.

Step 4: Replace Weak or Damaged Spark Plugs

Spark plugs are an important component of the mower’s engine. Worn or damaged spark plugs create a weak spark, causing the fuel in the cylinder not to ignite properly, it may ignite when it gets to the hot exhaust muffler. With this, there will be a loud backfire.

Fix: Remove your spark plugs and check the condition. You can clean them with a metal brush or replace them immediately if they are worn or damaged.

Step 5: Check the Flywheel Key


Flywheels are connected to the transmission and are positioned after the engine’s crankshaft. Running your lawn mower over a large rock can damage the flywheel key. This can make your mower backfire when you want to start it.

Fix: Repair the flywheel by replacing the key.

Step 6: Check the Lawnmower Timing

If the mower is not tuned properly, the engine will not be able to work correctly. This can also ignite a spark plug at the wrong time. Furthermore, damaged fuel filters can also cause backfiring.

Fix: Setting the timing of engines varies from engine to engine. You can read through the user manual for more information. The usual interval for the engine is around fifty hours or once a season.

Step 7: The Carburetor is Not Working Correctly

If you observe that your carburetor is not working perfectly, it might cause a backfire. This can be as a result of bad gas or dirt in the fuel.

Fix: You can use a carburetor cleaner or disassemble the carburetor and clean it. Replace it if it isn’t possible to clean.


Backfire is a usual problem of several lawnmowers, but it is not a difficult thing to fix. The common causes and fixes of backfiring have been discussed in the article. Proper care and maintenance of your mowing machine will help extend the lifetime and reduce the chances of a backfire.