Briggs And Stratton Backfire Through The Exhaust

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Does your Briggs and Stratton engine backfire through the exhaust? 

Do you hear a loud popping sound when trying to start or shut down your lawn mower engine? Not only can this be annoying but it can also cause harm to the engine over time.

In this article, I share some of the common reasons your Briggs and Stratton engine backfires, troubleshooting tips to help you diagnose and fix the problem, plus simple steps you can take to avoid engine backfires.

Why Does My Briggs And Stratton Backfires Through The Exhaust?


Backfire occurs when unused fuel enters the engine before the valves are closed or when fuel escapes into the exhaust system. It can be a result of one of the following:

  • Carbon buildup
  • Low fuel quality 
  • Carburetor adjustment set too lean
  • Engine overheating
  • Lowering engine speed too fast
  • Worn out spark plug
  • Wrong engine timing

Briggs and Stratton Backfires Through Exhaust: Possible Causes, Diagnosis, and Fixes

1.  Carbon buildup

Carbon buildup is the primary cause of engine backfires. Carbon buildup and deposits are by-products of combustion in an engine. It is usually caused by incomplete combustion of fuel.

It appears in the form of black soot which hardens on internal engine components disrupting airflow and as a result, areas, where carbon has been deposited, will experience overheating. 

Overheating occurs due to carbon’s ability to hold heat. This can lead to engine backfires and a reduction in engine efficiency.

Diagnosis: Try starting the lawn mower. If you notice black smoke(soot) coming out of the exhaust, that is an indication that there is carbon buildup in the engine.


  • Use fuel additives like SeaFoam in your fuel
  • Spray carb cleaner into the engine to decarbonize it

2. Low Fuel Quality 

Another reason your engine backfires through the exhaust could be low fuel quality. Fuels with high alcohol content are more combustible, and this could cause premature combustion outside the combustion chamber causing your engine to pop and backfire.

Fix: Change fuel to one with lower ethanol content.

3. Carburetor Adjustment Set Too Lean

Your carburetor adjustment set too lean could also be the reason your engine backfires. The carburetor controls the fuel-air ratio that is sent into the combustion chamber for combustion.

If the carburetor is set to allow only a small amount of fuel into the engine, it could cause your engine to backfire.

Diagnosis: Check the temperature of your engine’s exhaust. If it is too hot or glowing red, the carburetor might be set too lean.

You can also use an Air Fuel Ratio (AFR) gauge to check the air-fuel ratio of the carburetor.

Fix: Adjust the carburetor setting to allow more fuel into the engine.

How To Adjust The Carburetor Settings On A Briggs And Stratton Engine

  • Turn off the engine and let it cool down 
  • Remove the air filter housing and air filter 
  • Locate the adjustment screw on the side of the carburetor
  • Rotate the screw clockwise with a screwdriver until you feel a resistance
  • Turn the screw in the opposite direction for about 1½ turns to fine-tune it.
  • Reassemble the air filter 

4. Engine Overheating

Your Briggs and Stratton engine popping/backfiring could also be a result of overheating.

The combustion process won’t effectively use the mixture inside the combustion chamber if your engine is operating at unusually high temperatures.

This results in low fuel efficiency and will cause fuel residue will remain in the exhaust. When these residues come in contact with heated engine components like the exhaust muffler, it causes an ignition outside the chamber, which can cause the engine to backfire.

Some of the causes of engine overheating include:

  • Blocked air vents
  • Dirty/Clogged air filter 
  • Clogged cooling fins
  • Insufficient engine oil

Diagnosis: Some symptoms of overheating include:

  • smoke coming out of the engine
  • loud popping sounds
  • engine shutting down suddenly are signs that your engine is overheating.


  • Check the air vents, air filters, and cooling fins of your lawn mower. Clean or replace any dirty or clogged
  • Check the quantity of engine oil in the oil tank and top up if it is too little

5. Lowering Engine Speed Too Fast

Decelerating your engine too quickly can also cause your engine to backfire because it makes the engine timing go out of sync with the throttle and affects the fuel mix ratio.

Reducing the speed of your engine too fast can affect the fuel-air ratio and cause your engine to backfire due to the sudden reduction in fuel supply.

Fix: Gradually reduce the speed of your engine. Do not allow the throttle to bounce back to the rest position immediately.

6. Worn out spark plug

This is another reason your engine may backfire through the exhaust. A worn-out spark plug can cause a lawnmower engine to backfire. 

If the spark plug is damaged or worn, it will not produce a good spark. Because of this, the fuel will not ignite inside the combustion chamber but somewhere inside the hot exhaust muffler. This results in an engine backfire.

Diagnosis: Take out the spark plug and inspect the ends for signs of carbon build-up. If it looks dark or burnt, the spark plug is worn out and should be replaced.

Fix: Replace the damaged spark plug with a new one

7. Wrong Engine Timing

If you have checked all the above reasons and the issue persists, wrong motor timing might be the problem. The timing of the engine process is important for the smooth running of the engine. 

If the timing is not right, the fuel might ignite outside the combustion chamber while the intake valves are still open resulting in engine backfires.

Diagnosis: If your engine is running rough, backfiring, and making bumpy sounds, the timing might be off.

Fix: A small engine technician should be contacted to troubleshoot and fix the engine.

How To Avoid Briggs & Stratton Engine Popping And Backfiring 

Here are some precautionary measures that can help avoid the problem of an engine backfiring on your lawn mower:

1.  Turn off the engine slowly after use


Turning off the engine of your lawn mower too fast when it is still very hot can result in a loud popping sound and an engine backfire. You should turn it off slowly to avoid this.

Wait for about 5 minutes after use to allow the engine to cool for some time before shutting down your engine.

2. Adjust the carburetor correctly


A wrongly adjusted carburetor can cause engine backfires. Adjust the carburetor to allow a reasonable amount of fuel into the exhaust chamber.

This will help to avoid engine dehydration and overheating which can cause your lawnmower to backfire.

3. Avoid engine overheating

One of the causes of Briggs and Stratton engine popping is overheating. Avoid overworking your engine as this could lead to engine overheating.

Also, ensure that the quantity of fuel in the engine is sufficient to run it. Low oil levels result in overheating.


Your Briggs and Stratton engine might backfire as a result of carbon buildup, low-quality fuel, or a worn-out spark plug.

To determine the cause and solve the problem, carefully examine and troubleshoot the essential parts of your lawn mower’s engine using the troubleshooting tips in this article. Then, apply the recommended fixes.