Lawn Mower Surging | Fix Engine Hunting Problems
Your mower engine speeds up and slows down repeatedly. This annoying symptom is called surging or hunting. The engine can’t maintain steady RPM and fights with itself. Here’s how to diagnose and fix it.
Table of Contents
What Causes Surging
Surging happens when the engine’s fuel and air supply fluctuates or when the governor overreacts to load changes.
The cycle:
- Engine runs lean (not enough fuel)
- RPM drops
- Governor opens throttle to compensate
- Engine gets more fuel and speeds up
- Governor closes throttle
- Engine runs lean again
- Repeat
[NEED REAL IMAGE: Engine governor linkage]
Three main culprits:
- Carburetor issues (most common)
- Governor problems
- Air leaks at intake
Quick Fixes
Try these before tearing anything apart.
1. Replace the Air Filter
A dirty air filter restricts airflow and causes surging. This is the easiest and cheapest fix.
- Remove old filter
- Inspect for dirt and debris
- Install new filter
- Test run the engine
Cost: $5-10 for most mowers
2. Use Fresh Fuel
Old gas causes all kinds of problems including surging.
- Drain old fuel
- Add fresh gas (less than 30 days old)
- Add fuel stabilizer
- Run engine for 5 minutes
3. Check the Gas Cap Vent
A clogged gas cap vent creates vacuum in the tank and starves the engine.
- Remove cap while engine surges
- If surging stops, the cap is blocked
- Clean or replace the cap
4. Clean Spark Plug
A fouled plug causes inconsistent ignition.
- Remove and inspect plug
- Clean carbon deposits
- Check gap (typically 0.030”)
- Replace if worn or damaged
Carburetor Causes
The carburetor is responsible for most surging problems.
Clogged Jets
Partially blocked jets restrict fuel flow inconsistently.
Main jet: Controls fuel at higher RPM. Partial blockage causes surging under load.
Idle jet: Controls fuel at low RPM. Blockage causes surging at idle.
Fix: Remove and clean the carburetor thoroughly. See our How to Clean a Carburetor guide.
[NEED REAL IMAGE: Carburetor jets close-up]
Fuel Delivery Issues
Inconsistent fuel supply causes hunting.
Check:
- Fuel filter - replace if discolored
- Fuel lines - look for cracks or kinks
- Fuel tank - check for debris
- Fuel pump (riding mowers) - verify output
Float Level
If the float is set wrong, fuel level in the bowl fluctuates.
Too low: Not enough fuel available Too high: Engine runs rich
Adjustment: Float should be parallel with carburetor body when inverted. Bend float tang slightly to adjust.
Worn Carburetor Parts
Internal wear causes fuel metering problems.
Worn needle valve: Inconsistent fuel shutoff Worn throttle shaft: Air leak at shaft Cracked diaphragm (if equipped): Fuel pump failure
If the carburetor is worn, replacement is often easier than rebuilding.
Governor Problems
The governor maintains engine speed under varying loads. Problems here cause surging.
How the Governor Works
Mechanical governor: Uses spinning weights to sense engine speed. Connected to throttle via linkage.
Air vane governor: A fin senses airflow from the flywheel. More common on smaller engines.
Governor Linkage Issues
Binding linkage:
- Pivot points stuck or rusty
- Springs stretched or broken
- Linkage bent
Check by:
- Moving throttle lever by hand
- Should move freely without resistance
- Springs should return lever to position
Fix: Lubricate pivot points. Replace bent or broken parts.
[NEED REAL IMAGE: Governor spring and linkage]
Governor Adjustment
If linkage is fine, adjustment may be needed. This varies by engine.
General procedure:
- Loosen governor arm clamp bolt
- Move throttle to high speed position
- Rotate governor shaft fully in same direction
- Tighten clamp bolt
- Test run
Warning: Incorrect adjustment can cause overspeeding. Consult your engine manual for specific procedure.
Governor Spring
A stretched or broken spring causes erratic operation.
Test: Compare to new spring length. Replace if stretched more than 10%.
Air Leaks
Air entering the intake outside the carburetor leans out the mixture and causes surging.
Common Leak Locations
- Carburetor to intake gasket
- Intake manifold to engine
- Carburetor mounting bolts loose
- Cracked intake manifold
- Throttle shaft wear
How to Find Leaks
Spray test:
- Start engine
- Spray carburetor cleaner around intake areas
- RPM change indicates leak at that location
- Work systematically around gaskets and joints
Fixing Air Leaks
Gasket replacement:
- Remove carburetor
- Scrape old gasket material
- Install new gasket
- Tighten bolts evenly
Throttle shaft wear: If the shaft wobbles in the carburetor body, air leaks through the gap. Replacement carburetor needed.
[NEED REAL IMAGE: Intake manifold gasket location]
Brand-Specific Issues
Honda Surging
Honda engines rarely surge when properly maintained.
Common causes:
- Dirty air filter
- Auto-choke malfunction
- Carburetor needs cleaning
Honda-specific: Check the thermowax auto-choke actuator. Failure here affects fuel mixture.
Briggs & Stratton Surging
Very common complaint on Briggs engines.
Common causes:
- Carburetor gum from old fuel
- Governor spring stretched
- Air filter dirty
Briggs-specific: Many use plastic carburetors that don’t clean as well as metal. Consider replacement if cleaning doesn’t help.
Craftsman Surging
Since most Craftsman mowers use Briggs engines, same solutions apply.
Additional checks:
- Fuel cap vent
- Fuel line condition
- Self-propelled models - check for binding in drive system affecting engine load
Toro Surging
Toro Recyclers with Briggs or Kohler engines.
Common causes:
- Standard carburetor issues
- Governor linkage binding
- Fuel cap vent
Toro-specific: Some models have primer-based starting. Cracked primer bulb causes air leaks and surging.
John Deere Surging
John Deere riding mowers often use Briggs Intek engines.
Common causes:
- Dirty air filter (foam element clogs fast)
- Fuel delivery issues
- Governor adjustment
Riding mower specific: Check fuel pump operation if fuel delivery suspected.
Kohler Surging
Kohler engines have slightly different carburetor designs.
Common causes:
- Same as others - fuel and air issues
- Solenoid-equipped carbs may have failing solenoid
Kohler-specific: Idle solenoid on carburetor can fail, causing erratic idle. Check for 12V at solenoid with key on.
When Surging Indicates Bigger Problems
Surging usually points to fuel or air issues. But sometimes it indicates:
Low Compression
Worn rings or valves cause inconsistent combustion.
Test: Compression gauge should show 60-90 PSI. Low compression = engine wear.
Ignition Problems
Failing coil or inconsistent spark.
Test: Watch for spark while engine surges. Inconsistent spark indicates ignition issue.
Internal Governor Damage
Worn governor gear or weights inside engine.
Symptom: Governor adjustment doesn’t help, linkage moves erratically.
Fix: Engine disassembly required. Often not cost-effective on older mowers.
Systematic Diagnosis
Work through this checklist in order:
- ☐ Fresh fuel in tank
- ☐ Clean air filter
- ☐ Gas cap vent clear
- ☐ Spark plug clean and gapped
- ☐ Fuel filter replaced
- ☐ Fuel lines clear
- ☐ Carburetor cleaned
- ☐ Intake gasket sealed
- ☐ Governor linkage free
- ☐ Governor spring intact
- ☐ Compression test (if above fails)
Most surging problems resolve by step 7.
Related Guides
- How to Clean a Carburetor
- Lawn Mower Won’t Start
- Craftsman M220 Problems
- Toro Recycler Repair Guide
- Honda GCV160/GCV190 Problems
Summary
Surging is annoying but usually easy to fix. Start with the basics - fresh fuel, clean air filter and clear fuel cap vent. If those don’t work, clean the carburetor. Most surging problems stop right there. Governor issues and air leaks are less common but straightforward to diagnose. Work through the checklist methodically and you’ll find the cause.