Briggs And Stratton Backfiring Through The Carburetor And Will Not Start

When you buy via links on our site, we may earn an affiliate commission at no cost to you. Learn more.

When you try turning on your Briggs and Stratton engine, but it just gives a  loud bang and splutters before dying, it is called backfiring and can be disturbing.

When your Briggs and Stratton backfires through the carburetor, it might be an indication that there’s an issue with the mower that needs urgent attention.

In this article, I share easy-to-follow steps on what to do when your Briggs and Stratton engine backfires through the carburetor.

Tools Needed For The Job

To troubleshoot your mower for backfiring, you will need the following tools and materials.

  • Gloves
  • Safety glasses
  • Carburetor cleaner
  • Screwdriver
  • Pliers
  • Wrench
  • Test tube
  • Wire brush
  • Socket set
  • Compressed air

Reasons Why Your Briggs And Stratton Might Backfire And Not Start

There are quite a few reasons why your Briggs and Stratton lawn mower engine might backfire through the carburetor and not start, and they include;

  • Ethanol in fuel
  • Damaged spark plug
  • Broken flywheel key
  • Carburetor adjustment set too lean
  • Wrong engine timing
  • Dirt in the carburetor

Briggs and Stratton Backfiring Through The Carburetor And Will Not Start: Causes And Fixes

  1. Ethanol In Fuel

Using fuel with an ethanol level higher than the recommended level might cause your mower to backfire or not start. Fuel high in alcohol is unhealthy for your mower’s engine, and you should avoid using them.


Carry out an alcohol test on your fuel to determine the level of ethanol it contains.

How To Run An Alcohol Test  In Simple Steps

  • Get a test tube or a small olive bottle about six to seven inches.
  • Make a line on the bottle, about two inches above the bottom.
  • Fill water into the bottle up to the line.
  • Fill the rest of the bottle with gas.
  • Cover the bottle, shake it, leave it and let it stand.

The Result: If there is ethanol in the gas, it will mix up with the water. Check the line to see if the water level has increased. If the water level increases, the gas contains ethanol and should not be used.


Drain the bad fuel from the tank and replace it with the recommended type.

  1. Damaged Spark Plug

When the spark plug is old or damaged, it creates a weak spark which may fail to ignite the fuel vapor when the exhaust pipe opens up. As a result, the spark will find its way to the exhaust and ignite unburned fuel, causing a backfire.


Remove the spark plug and check its condition using a spark plug tester.


  • Clean the spark plug using a wire brush
  • Replace the spark plug if it’s damaged
  1. Broken Flywheel Key 

The flywheel key connects the flywheel to the crankshaft.  When the key breaks, it causes a disconnection between the flywheel and the crankshaft, and this can cause a backfire when you try to start your engine and prevent it from starting. 

Usually, the flywheel key breaks when your mower runs over hard objects, like a rock.


Disassemble your mower to check the flywheel. The flywheel key should align with the keyhole on the crankshaft. If it isn’t, it might indicate damage.

How To Check If The Flywheel Key Is Damaged 

  • Disconnect the spark plug
  • Disconnect the flywheel brake spring
  • Remove the flywheel nut
  • Take off the starter recoil cut to reveal the flywheel key
  • Check if the flywheel key is correctly aligned with the crankshaft


Replace the damaged flywheel key 

  1. Carburetor Adjustment Set Too Lean

When the carburetor adjustment is set too lean, there will be an imbalanced mixture of air and gas. 

When there is little gas and too much air released into the exhaust valve, it burns more slowly, and there will be some residual air and fuel after the ignition, causing a backfire.


Check your carburetor’s high-speed mixture adjustments.


Clean and adjust your carburetor correctly

How to Adjust Carburetor High-Speed Mixture

The high-speed mixture is the mixture of air and fuel the carburetor provides when the mower is in motion. You can adjust your Briggs and Stratton high-speed mix by following these steps:

  • Locate the high-speed adjustment screw.
  • Remove the filter covering.
  • Adjust the high-speed screw.
  • Set the throttle speed.
  • Attach the casing. 
  • Start your mower to test it.
  1.  Wrong Ignition Timing

If the ignition timing of the mower is wrong, it will cause the sparks to start either too early or too late, which could cause a backfire. 

If your mower’s timing is wrong, you could experience other symptoms like overheating, knocking, and increased fuel usage.


Check your engine tuning and see if you set the timing correctly.


Adjust the timing of your mower, following the instructions on your user manual, or have a professional do it.

  1. Dirt In The Carburetor

A dirty carburetor can cause your Briggs and Stratton engine to backfire. When dirt clogs the carburetor, it messes with the air-fuel balance, causing a backfire.


Engine not starting, poor engine performance, engine running lean or rich, overheating, and backfiring are signs that your carburetor might be dirty.


Disassemble and clean your carburetor

How To Clean Your Carburetor In Simple Steps

  • Using a screwdriver, remove the air filter to access the carburetor.
  • Using a socket set or the nut driver, remove the carburetor. 
  • Disassemble the carburetor.
  • Using a carburetor cleaner, clean the carburetor and the carburetor parts.
  • Dry the carburetor parts with compressed air or allow them to air dry.
  • Reassemble the carburetor.
  • Mount the carburetor back on the mower
  • Test the mower

General Maintenance To Prevent Backfire In Your Briggs And Stratton Mower Engine

  • Keep your carburetor clean. 
  • Check your carburetor adjustment from time to time
  • Avoid using fuel with a high level of ethanol
  • Perform a routine check on your mower’s engine
  • Avoid running your mower over rocks and hard stones.


As discussed in this article, your Briggs and Stratton engine backfiring could be caused by many different reasons like a broken flywheel key, wrong carburetor adjustment, a damaged spark plug, and a dirty carburetor. 

Troubleshoot and inspect your engine to find the possible causes, and follow the steps provided in this article to fix the problem. However, if the problem persists after following all the steps, you might need to consult a professional repairer.