Small Engine Backfiring: Causes and Fixes
In This Guide
A loud bang from a mower or generator is startling, but it is usually telling you something fixable. Small engine backfiring happens when fuel ignites at the wrong moment, either in the intake or the exhaust. This guide explains why it happens and how to stop the pops for good.
Backfire vs Afterfire
The timing of the bang points to the cause. A bang through the carburetor while running is a true backfire, usually from a lean mix or ignition timing. A bang from the exhaust as you shut down is an afterfire, from unburned fuel igniting in the muffler.
The Most Common Cause
Most backfire complaints are afterfire on shutdown, and the cause is simple. Killing the engine at full throttle dumps raw fuel into a hot exhaust, where it ignites with a bang.
Other Causes to Check
- Stale fuel. Burns unpredictably and pops. Refill with fresh gas.
- Dirty carburetor. A wrong mixture causes both backfire and afterfire. Clean the carburetor.
- Wrong spark plug or gap. Fit the correct plug and set the gap.
- Sheared flywheel key. A worn key throws off ignition timing and causes hard backfiring. This one needs a technician.
When Backfiring Means Trouble
Occasional afterfire on shutdown is harmless once you idle down first. Persistent backfiring through the carburetor, especially with hard starting or lost power, can point to a timing fault like a sheared flywheel key. See the mower backfiring guide for model specific help, or have it checked.
Rather Let a Pro Come to You?
Wildwood Small Engine Repair brings the shop to your door. Over 10,000 repairs since 2019, house calls from $75 and tune-ups from $38, backed by a 4.9 star rating. Call 888-594-5516 to book same week service.