Fix Surging Lawn Mower

VROOM-vroom-VROOM-vroom. That’s the sound of a surging engine and it’s driving you crazy, right? Nine times out of ten it’s a gunked-up carburetor. The other time it’s an air leak or bad gas. Either way, we can fix it without spending much.

Table of Contents

• What Causes Surging
• Quick Diagnosis
• Air Filter and Intake
• Carburetor Cleaning
• Air Leak Detection
• Governor Problems

What Causes Surging

The Surge Cycle

1. Engine runs lean (not enough fuel)
2. RPM drops
3. Governor opens throttle
4. Engine gets fuel burst
5. RPM spikes
6. Governor closes throttle
7. Cycle repeats

Common Causes

Cause	Frequency	Fix Difficulty
Dirty air filter	Very common	Easy
Clogged carburetor	Very common	Medium
Air leak at carb	Common	Easy-Medium
Governor linkage	Less common	Medium
Fuel delivery issue	Less common	Easy-Medium

[NEED IMAGE: Lawn mower surging causes diagram]

Quick Diagnosis

5-Minute Checks

1. Air filter - Remove and inspect
2. Fuel freshness - Old gas causes problems
3. Visible linkage - Anything disconnected?
4. Gasket condition - Carb mounted tight?
5. Fuel flow - Fuel reaching carb?

Choke Test

Run mower and move choke slightly toward closed:

• Surging stops = Running lean (carburetor issue)
• Surging continues = Likely governor or air leak
• Engine bogs = Not a lean condition

Air Filter and Intake

Air Filter Replacement

A dirty air filter restricts airflow causing lean surging.

Symptoms:

• Surging at idle
• Loss of power
• Black smoke (if very dirty)

Fix:

1. Remove air filter cover
2. Remove filter element
3. Inspect - replace if dirty
4. Clean foam pre-filter (if equipped)
5. Reinstall

Air filter cost: $8-15

[NEED IMAGE: Lawn mower air filter inspection]

Intake Manifold

Check the plastic or metal tube between carburetor and engine.

Problems:

• Cracks allow air leaks
• Loose mounting bolts
• Damaged gaskets

Carburetor Cleaning

Why Carburetors Clog

• Old fuel leaves varnish
• Ethanol in gas attracts water
• Debris enters through fuel
• Small passages clog easily

Jets That Cause Surging

Main jet - Affects high speed operation Pilot/idle jet - Affects idle and low speed Emulsion tube - Mixes fuel and air

Partial blockage in any of these causes surging.

Cleaning Procedure

Tools needed:

• Carburetor cleaner spray
• Thin wire or jet cleaning kit
• Screwdrivers
• New gaskets (recommended)

Steps:

1. Remove air filter housing
2. Disconnect fuel line (have rag ready)
3. Disconnect throttle/choke linkage
4. Remove carburetor (2 bolts typically)
5. Remove float bowl (1 bolt)
6. Remove jets (if accessible)
7. Spray all passages with carb cleaner
8. Clear jets with wire or compressed air
9. Inspect needle and seat
10. Reassemble with new gaskets
11. Reinstall and test

[NEED IMAGE: Carburetor cleaning steps for surging fix]

When to Replace vs Clean

Clean if:

• First time surging
• Carb looks serviceable
• Gaskets available

Replace if:

• Multiple cleaning attempts failed
• Visible damage
• Parts worn

Carburetor cost: $25-50 aftermarket

Air Leak Detection

Common Leak Points

1. Carburetor mounting gasket
2. Intake manifold gasket
3. Crankshaft seals
4. Carburetor body cracks

Detection Methods

Spray test:

1. Start engine
2. Spray carb cleaner around gaskets
3. RPM change indicates leak location
4. Be careful - flammable!

Visual inspection:

1. Look for damaged gaskets
2. Check bolt tightness
3. Inspect for cracks

Fixing Air Leaks

Gasket replacement:

1. Remove carburetor
2. Scrape old gasket material
3. Install new gasket
4. Torque bolts evenly

Gasket cost: $5-15 for set

Governor Problems

How the Governor Works

The governor maintains constant engine speed:

1. Engine speeds up → governor closes throttle
2. Engine slows down → governor opens throttle
3. Continuous balance keeps speed steady

Governor Surging Causes

• Linkage loose - Governor arm slipped
• Spring wrong - Wrong tension or position
• Linkage binding - Restricted movement
• Internal wear - Governor gear/weights worn

[NEED IMAGE: Governor linkage adjustment]

Governor Adjustment

Static adjustment (engine off):

1. Loosen governor arm clamp bolt
2. Move throttle to wide open
3. Rotate governor shaft fully
4. Tighten clamp bolt
5. Test operation

See Toro Carburetor Linkage Diagram for detailed governor information.

Governor Spring

Wrong spring symptoms:

• Constant surging
• Wrong engine speed
• Speed varies with load

Check:

• Spring in correct hole
• Spring not stretched
• Correct spring for engine

Other Causes

Fuel Delivery Issues

Fuel filter clogged:

• Inconsistent fuel supply
• Surging under load
• Replace filter ($5-10)

Fuel line kinked/cracked:

• Restricted flow
• Air entering line
• Replace line

Tank vent blocked:

• Vacuum forms in tank
• Fuel stops flowing
• Clean or replace cap

Ignition Problems

Weak spark:

• Can cause surging (less common)
• Replace spark plug first
• Check ignition coil gap

Repair Summary

Fix Order

1. Replace air filter
2. Use fresh fuel
3. Clean carburetor
4. Check for air leaks
5. Adjust governor (if needed)
6. Replace carburetor (last resort)

Cost Summary

Fix	DIY Cost
Air filter	$8-15
Fuel filter	$5-10
Carb cleaning	$10-20
Carb gaskets	$5-15
Carburetor	$25-50
Governor spring	$5-10

Related Guides

• How to Clean a Carburetor
• Toro Carburetor Linkage Diagram
• Lawn Mower Won’t Start
• Briggs & Stratton Engine Problems

Summary

Lawn mower surging is almost always a carburetor or air delivery problem. Start with the air filter - it’s cheap and easy. If surging continues, clean the carburetor thoroughly. Check for air leaks at mounting gaskets. Governor issues are less common but can cause identical symptoms. Most surging fixes cost under $50 in parts.