You’re out mowing your lawn and all of a sudden your lawnmower starts revving up and down for no apparent reason. Due to the hunting and surging, it’s tough to keep the mower moving in a straight line, and you may end up with an uneven cut. And now you may wonder why does my lawn mower rev up and down.

Actually, there are a few reasons this may be happening. It doesn’t just include lawnmower problems but also missteps made by users. Although it might be a bother when your lawnmower is misbehaving, hopefully, this guide will help you figure out the causes and how to solve them.

So, let’s get started.

Why Does My Lawn Mower Rev Up And Down : Main Reasons

surging lawn mower engine

If your lawn mower engine is bouncing up and down, it’s called “hunting and surging.” It means that the engine can’t maintain a consistent speed and instead fluctuates between higher and lower speeds. This can be caused by several things:

NO #1. A Dirty Air Filter

A lawn mower relies on a small engine to power its blades, and like any engine, it needs a steady supply of air to run properly. The air filter helps maintain the mower’s engine clean by trapping dirt and debris before it can enter the combustion chamber.

However, over time the air filter can become clogged with dirt, restricting the flow of air and causing the engine to run rough.

One way to tell if a dirty air filter is causing your lawn mower to rev up and down is to remove the lawn mower’s sputter and hold it up to the light. If you can’t see light passing through the filter, it’s time for a new one. A clean air filter ensures your lawn mower runs smoothly all season long.

NO #2. Overfilling the Tank

Overfilling the gas tank on your lawn mower can cause a number of problems, the most serious of which is engine damage. When you overfill the tank, gas will spill out and come into contact with the engine.

This can lead to a build-up of harmful deposits that can eventually lead the engine to seize up. In addition, excessive fuel can also cause the carburetor to become flooded, leading to starting problems.

To avoid these problems, always make sure to check the level of gas in the tank before starting riding the lawn mower. And when filling up, stop when the nozzle clicks off automatically. This fuel stabilizer makes sure you don’t accidentally overfill the fuel tank.

NO #3. Spark Plug is Damaged or Dirty

A lawn mower engine typically contains a spark plug that helps to ignite the gasoline and start the machine. With time, the spark plug can become damaged or dirty, which can make it rev rapidly. In some cases, the spark plug may need to be replaced entirely.

Other times, simply cleaning the plug with a wire brush or other abrasive tool can be enough to restore its functionality. In either case, it is important to identify and address the problem as soon as possible, as a damaged or dirty spark plug can ultimately lead to engine failure.

NO #4. The Carburetor is Dirty

Lawn mowers, like any engine, have a carburetor (except electric mower) that helps deliver the right mixture of fuel and air to the engine. After a while, the carburetor cleaner can become dirty, causing it to run erratically.

One tell-tale sign that the carburetor needs to be cleaned is if the engine speeds up and slows down on its own. This is because the carburetor is not delivering the proper mix of gasoline, hence the engine’s “hunger” for more liquid fuel.

As a result, the lawn mower will rev up in an attempt to get more fresh gas, but then slow down as the carburetor becomes overloaded. Cleaning the carburetor is a relatively easy fix with carb cleaner that can be done with a few simple tools.

NO #5. Using the Wrong Gasoline

One of the most common reasons why your mower may rev high and low is that you’re using the wrong type of gasoline. Eventually, the engine in your lawn mower will begin to build up deposits from the different types of fuel you use. These deposits can make your mower’s engine run less efficiently, which can lead to revving.

To avoid this, be sure to use the high-quality fresh fuel recommended by the manufacturer of the lawn mower. In most cases, this will be high-octane gasoline.

Additional Reasons for Lawn Mower Rev Ups And Downs

engine speed

There are also a few user-related reasons that may cause your lawnmower to hunt and surge. These include:

1. Incorrect Blade Setting

One potential reason why lawn mowers rev up and down because of user actions is incorrect the blade settings. If the blades are positioned too low, they can catch on the ground and force the engine to rev up. Similarly, when they are set too high, they can create resistance against the ground, causing the fan to spin fast.

In either case, it’s important to adjust the blade setting so that it’s level with the ground. This will help to ensure that the engine runs smoothly and doesn’t rev up unnecessarily.

2. Hitting Something While Mowing

When you hit anything during mowing, it makes your engine rev up/down. This is because the blade is no longer able to rotate freely. Engines have to work harder to turn the blade, which causes the revs to increase.

Once you clear the obstruction, it should resume moving normally. If the revs do not return to normal, or if the engine stalls completely, it may be a sign that the blade is damaged and needs to be replaced.

3. Worn or Damaged Blades

Another reason is blade wear and damage. When blades are rusted or corroded, they are less able to cut grass effectively. Consequently, the lawn mower has to do more work to mow the lawn, resulting in the engine revving upward and downward. In most cases, worn-out blades can be replaced.

4. Clogged Mower Deck

When the deck is full of grass, it can restrict the movement of the blades, preventing them from rotating at a consistent speed. Therefore, the vehicle works more intensively to turn the blades, leading to an increase in revs.

To fix this problem, simply clear out the deck and make sure that there is nothing blocking the blades from moving freely.

You Can Also Check: Can You Put a Larger Deck on Your Lawnmower?

5. Going Too Fast

Lawn mowers are designed to operate at a certain speed, and going too fast will result in the engine going up and down. The throttle shaft can become worn, resulting in a loss of speed control. This is usually due to the fact that the blades are unable to rotate at the proper speed, causing them to stall.

In addition, operating the riding mower too quickly will also put unnecessary strain on the engine, which can lead to problems in the long run. If you find the mower is constantly accelerating and decelerating, it’s best to slow down and take your time.

Frequently Asked Questions

There are a few frequently asked questions that may help you troubleshoot your lawn mower issue.

1. Why Does My Lawn Mower Sound Like It Is Surging?

Small engines, like lawn mowers, can surge when there is an issue with the air and gas mixture. When the engine reaches full speed, it is unable to maintain that speed because of the lack of mixture. This makes it stall or repeat the process.

2. Can Spark Plugs Cause Surging?

Spark plugs can cause your car to surge if they are dirty or worn. This is because the spark plugs aren’t igniting the fuel in your engine at the proper rate, which can result in an uneven speed. If your mower is surging, it indicates that you need to clean or replace your spark plugs.

3. Why is My Lawn Mower Not Running Smoothly?

When your lawnmower doesn’t run smoothly, it could be due to air intake or fuel system problems. If your engine starts but isn’t running smoothly, it is probably one of these systems that needs attention. You should also check the spark plugs and wires to ensure they are in good condition.


As you can see, there are a few different reasons why your lawn mower might be revving up and down. Be sure to check these things before taking your lawn mower to a mechanic. These include an air filter, fuel filter, and spark plug. With a little troubleshooting, you should be able to fix the problem yourself.

Also, as a user, you should know that going too fast, hitting a hard object while mowing, or having a clogged deck can also cause this problem. So be sure to go slowly and be careful as you mow your lawn. Hope our content solved all your question about why does my lawn mower rev up and down.