Lawn Mower Is Surging

If your lawn mower is surging, you know how frustrating it can be. That erratic revving up and slowing down makes it impossible to get a clean cut and is a clear sign something needs your attention. As a gardener, I see this problem all the time, especially during peak season when our equipment is under incredibly high demand. The good news is, it’s usually a simple fix you can handle yourself with some basic tools and a bit of patience.

Let’s walk through the common causes and solutions. A surging engine typically points to a fuel or air delivery issue. The engine is essentially starving for a consistent mix, causing it to hunt for the right speed. We’ll start with the easiest checks and work our way to the more involved ones. You’ll have your mower running smoothly again in no time.

This specific problem, where the engine speed repeatedly increases and decreases on its own, is our main focus. Understanding why it happens is the first step to fixing it. Think of your mower’s engine like your own body—it needs a steady, clean supply of air and fuel to work properly. A surge is a symptom of a disruption in that supply.

Primary Culprits: The Usual Suspects

Most surges stem from a handful of common issues. These are the first places you should look before assuming the worst. They are often quick and inexpensive to address.

Old or Contaminated Fuel: Gasoline degrades over time and can form gummy residues that clog the system.
A Dirty or Clogged Air Filter: A restricted air flow creates a fuel-rich mixture, confusing the engine.
A Faulty or Clogged Carburetor: This is the heart of the fuel system and where most surge problems originate.
A Sticking or Damaged Governor: This component’s job is to maintain engine speed; if it fails, surging is a direct result.
Vacuum Leaks: Cracks in fuel lines or gaskets allow unwanted air into the system.

Step-by-Step Diagnosis and Fix

Always start with safety. Disconnect the spark plug wire to prevent any accidental starts. Work in a well-ventilated area and have some clean rags on hand. Let’s go through the fixes in order.

1. Check the Fuel System First

Bad gas is the number one cause of small engine problems. If your fuel is more than 30 days old, it’s suspect. Drain all the old fuel from the tank and the carburetor bowl. Use a fuel stabilizer with fresh gasoline to prevent this issue in the future. Also, inspect the fuel cap; a clogged vent in the cap can create a vacuum in the tank and restrict fuel flow.

Next, look at the fuel line. Follow it from the tank to the carburetor. Check for any cracks, brittleness, or kinks that might be restricting flow. A simple visual inspection can often reveal a problem here.

2. Inspect and Clean or Replace the Air Filter

A dirty air filter is a very common and easy fix. Locate the air filter housing—it’s usually a plastic box near the carburetor. Remove the cover and take out the filter. If it’s a paper filter, hold it up to the light. If you can’t see light through it, it needs replacing. If it’s a foam filter, wash it gently in warm, soapy water, rinse, let it dry completely, and then apply a few drops of clean engine oil before reinstalling.

Never run your mower without its air filter. This can allow debris into the engine and cause far more serious damage than a surge.

3. The Carburetor: A Deep Clean

If fresh fuel and a clean air filter don’t solve the surge, the carburetor is likely the issue. Gummy residues from old fuel clog the tiny jets and passages inside. You have two main options: a cleaning or a replacement.

For a cleaning, you’ll need to remove the carburetor. Take a picture or make notes of how the linkages connect before you disconnect anything. Here’s a basic process:

Disconnect the fuel line and any linkage springs.
Remove the bolts holding the carburetor to the engine.
Carefully disassemble the carburetor, paying close attention to the small parts.
Soak the metal parts in a carburetor cleaner solution for the time specified on the can.
Use a soft wire or a can of compressed air to clear all the jets and passages.
Reassemble with a new carburetor gasket kit, which is cheap and ensures a good seal.

Sometimes, especially if the carburetor is very old or damaged, it’s more cost-effective to buy a replacement carburetor. They are often surprisingly affordable and come pre-tuned, saving you a lot of hassle.

4. Understanding and Adjusting the Governor

The governor is a clever mechanical device that automatically opens and closes the throttle to maintain a set engine speed under changing loads (like going from thin to thick grass). If the linkage is sticking or the spring is damaged, it can’t do its job, leading to surging.

First, locate the governor. It’s usually connected to the throttle lever on the carburetor by a thin metal arm and a spring. With the engine off, move the linkage by hand. It should move freely and snap back to its position. If it’s sticky, clean it with a penetrating oil. If the spring is stretched or broken, replace it.

Warning: Adjusting the governor screw itself is a precise procedure. If the internal governor gear is damaged, it often requires professional repair. Consult your mower’s manual before making any adjustments to the governor screw, as an incorrect setting can cause the engine to run at dangerously high RPMs.

5. Hunting for Vacuum Leaks

A vacuum leak introduces unmetered air into the intake system, leaning out the fuel mixture. Common leak points include the carburetor mounting gasket, the intake manifold gasket, and cracked fuel lines or primer bulb.

With the engine running at a low idle (if possible), carefully spray a small amount of carburetor cleaner or starting fluid around the gasket areas. If the engine speed changes (revs up or smooths out momentarily), you’ve found your leak. The gasket or part in that area will need to be replaced.

Prevention is Better Than Cure

To avoid the problem of a surging mower next season, a few end-of-season habits make all the difference. This proactive care saves you time and money, and ensures your mower is ready when you need it.

Always Use Fuel Stabilizer: Add stabilizer to your gasoline can every time you fill it, or directly into the mower tank at the end of the season. Run the engine for a few minutes to get the treated fuel into the carburetor.
Perform End-of-Season Maintenance: Before storage, change the oil, clean or replace the air filter, and sharpen the blade. A clean machine starts easier and runs better.
Store It Properly: Either run the carburetor dry or use the stabilizer method. Store your mower in a dry, covered place.

When to Call a Professional

If you’ve gone through all these steps and your lawn mower is still surging, the issue might be more internal. It could point to worn engine components, such as valve problems or a failing flywheel key, which affect ignition timing. At this stage, the cost and complexity of the repair may warrant taking it to a small engine repair shop. They have the specialized tools and expertise to diagnose and fix these less common issues efficiently.

FAQ Section

Why does my lawn mower engine surge at idle?
Surging at idle is almost always caused by a clogged idle jet or circuit in the carburetor, or a vacuum leak. A thorough carburetor cleaning is the typical fix.

What causes a lawn mower to surge under load?
If the surging happens primarily when you’re actually cutting grass, it often points to a governor problem. The governor can’t respond correctly to the increased demand, causing the engine speed to fluctuate.

Can bad spark plug cause surging?
While a bad spark plug more commonly causes misfiring or hard starting, a weak or intermittent spark can contribute to rough running that might resemble a surge. It’s a cheap and easy part to replace as part of your troubleshooting.

Is it safe to use a mower that is surging?
It’s not recommended. The inconsistent speed puts extra stress on engine components and leads to a poor quality cut. It’s best to diagnose and fix the issue before continuing to use the mower.

How do you fix a surging engine?
As outlined above, the fix follows a process: 1) Replace with fresh, stabilized fuel. 2) Clean/replace the air filter. 3) Clean or replace the carburetor. 4) Check governor linkage and springs. 5) Inspect for vacuum leaks. This sequence solves the vast majority of surge problems.

Dealing with a surging lawn mower can be annyoing, but it’s rarely a death sentence for the machine. By methodically checking the fuel, air, and governing systems, you can almost always identify and solve the problem yourself. Remember, regular maintenance, especially proper fuel management, is the key to preventing this issue from happening in the first place. With your mower running smoothly again, you can get back to enjoying a well-kept lawn without the frustrating soundtrack of an engine that can’t make up it’s mind.