How to Fix Backfiring or Afterfiring Briggs & Stratton Engines

If you’re hearing a loud pop or bang from your engine, you might be dealing with either a backfire or an afterfire. While they sound similar, they’re caused by different things—and the good news is, they’re usually easy to fix.

What’s the Difference?

• Backfire: This happens while the engine is running and is caused by combustion happening outside the engine’s combustion chamber.

• Afterfire: This occurs after the engine is shut off—you'll hear that loud “bang” a few seconds after turning off your mower.

These aren’t usually signs of serious engine damage, but they can indicate your engine needs a little attention. Let’s walk through the common causes and fixes.

Common Causes & Fixes

1. Sudden Throttle Changes

Problem: Slamming the throttle closed too quickly can cause fuel to build up and ignite outside the engine.
Fix: Always reduce throttle slowly before shutting off the engine.

2. Shutting Down at High Speed

Problem: Turning the engine off while it’s still running fast can lead to afterfire.
Fix: Let the engine idle for 15–30 seconds before shutting it off.

3. Wrong or Old Fuel

Problem: Low-quality or stale gasoline can cause incomplete combustion.
Fix: Use fresh, clean fuel and the correct fuel type for your engine. If in doubt, check your engine manual.

4. Carburetor Issues

Problem: A misadjusted carburetor can make the fuel/air mixture too rich or too lean.
Fix: You may need to adjust or clean the carburetor. Refer to your mower engine’s manual or have a small engine technician take a look.

5. Spark Plug Problems

Problem: A worn or dirty spark plug can misfire.
Fix: Inspect and replace the spark plug if it looks fouled or damaged.

Quick Tips to Prevent Engine Backfire

• Reduce throttle gradually before shutting down.

• Avoid shutting down at full speed.

• Keep fuel fresh and clean.

• Stay up to date on engine maintenance (especially spark plug and carburetor cleaning).