Small Engine Runs Rough
In This Guide
An engine that stumbles, surges, or idles unevenly is annoying and usually easy to fix. When a small engine runs rough, it is almost always struggling to get a clean, steady mix of fuel and air. Here are the common causes in order of likelihood, plus the fix for each.
Surging Up and Down
If the engine speeds up and slows down on its own, that hunting is the textbook sign of a lean condition from a partly clogged carburetor. The idle passage is restricted, so the engine cannot hold a steady speed.
Cleaning the carburetor almost always cures surging. Work through the carburetor cleaning steps.
Stumbling and Misfiring
- Fouled spark plug. A worn or sooty plug misfires under load. Replace it and set the gap.
- Dirty air filter. Chokes the engine and richens the mix. Clean or replace it.
- Stale fuel. Burns unevenly. Drain and refill with fresh gas.
For Two Stroke Engines
On two stroke machines like many trimmers and older blowers, rough running is often the wrong fuel mix. Too little oil or stale premix causes stumbling and smoke.
If Cleaning Does Not Help
When fresh fuel, a clean filter, a new plug, and a clean carburetor still leave it rough, the cause runs deeper. A vacuum leak, a governor fault, or low compression can all cause uneven running, and those are worth a technician's eye.
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.