Why Your Engine Surges Up and Down
Engine surging—also called “hunting”—happens when the engine repeatedly revs up and down instead of holding a steady speed. This is almost always caused by an inconsistent air-fuel mixture or a governor that can’t stabilize RPM.
1. Carburetor Running Lean
The most common cause of surging is a lean carburetor. When the main jet is partially clogged, the engine receives just enough fuel to rev up, then starves and drops RPM, repeating the cycle.
Internal link: Check the Symptoms Index for related fuel issues.
External reference: EPA Gasoline Basics
2. Dirty or Partially Blocked Fuel Filter
A restricted fuel filter causes inconsistent fuel flow. The engine receives enough fuel to surge upward, then starves and drops back down.
3. Vacuum Leak at Intake or Carburetor
A cracked intake boot, loose carburetor mounting bolts, or damaged gasket can pull in extra air, causing unstable RPM.
4. Governor Linkage Out of Adjustment
The governor controls engine speed. If the linkage is bent, loose, or sticking, the engine will constantly hunt for the correct RPM.
Internal link: Visit the FAQ for common troubleshooting questions.
5. Old or Stale Fuel
Fuel older than 30 days loses volatility, causing inconsistent combustion. Drain the tank and refill with fresh gasoline.
6. Clogged Pilot Jet
The pilot jet controls low-speed fuel delivery. If it’s clogged, the engine will surge at idle or low throttle.
Internal link: The Start Here page covers basic maintenance steps.
When to Seek Professional Help
If cleaning the carburetor, replacing the filter, and checking for vacuum leaks doesn’t fix the surging, a technician can perform ultrasonic carburetor cleaning or governor recalibration.