Why Does Your Lawn Mower Engine Speed Up and Slow Down?

Lawn mower on green lawn

When you finally mow the lawn after weeks of not using the mower, you don’t want to hear a roaring engine that dies within seconds. Regardless of the quality and brand of your mower, it is common to notice such changes. That being said, do you know why your lawnmower engine speeds up and slows down frequently?

A damaged air filter in the engine is the most common cause of an erratic engine. The presence of the combustion engine on lawnmowers means you have to deal with many small parts, including air filters, carburetors, spark plugs, and many others. Your machine can behave irresponsibly if any of these parts is malfunctioning.

However, you do not have to worry about this as it is one of the issues you can handle on your own without having to contact a professional. This blog post aims to discuss some common causes of an erratic mower engine, besides suggesting some quick solutions. Stay tuned to find out how to get rid of a surging mower engine.


What Causes Your Lawn Mower Engine to Speed Up and Slow Down?

Many things can affect the performance of your mower’s engine in this regard. Before we dive into the issues, it is pertinent to mention what having a mower engine that speeds up and slows down frequently means.

No matter how old or new your mower is, you want it to perform as it should. There are no issues with the engine if it only experiences a surge for some seconds during the startup. After some time of inactivity, it is common for machines to behave in this way.

However, when it starts doing these power fluctuations while you use it continuously, it needs to be fixed. This problem can lead to the complete failure of the machine if you do not pay timely attention to its resolution. This phenomenon is also known as hunting and surging and occurs when an engine experiences frequent surges and staggers.

Because of this, we always insist on a timely solution to the engine’s speeding fluctuations. Let’s now go back to the causes of this up-and-down surge of the engine.

Without knowing the cause of the problem, it is impossible to solve it. That is why you should pay attention to the following sections regarding what can cause your mower engine to speed up and slow down frequently.


Inefficient Air Filter

Mowing is a rigorous activity that involves a lot of dust and tiny particles of grass. Despite the quality of your device, you will probably have to maintain it from time to time. The air filter is one of the essential elements of the combustion engine present in many mowers. It maintains the airflow within the device to ensure its proper functioning.

Once dust and other unwelcomed visitors affect its capability to supply enough air, the engine responds negatively. As a result, you will notice a slowdown in the speed of the machine.

On the other hand, when the dust or any other cause of blockage gets removed due to excessive force exerted by the engine, you will notice a surge in the speed.

However, the air filter may get out of order as you burden the device with excessive use. That is why we always suggest our readers diagnose the inferior air filters whenever they face any speed issues.

The best thing about the air filter is that it is quite affordable you can easily be replaced. But before you buy one, you should see if the old one can handle further use. Never waste your resources on unnecessary things.


Issues With the Spark Plug

The spark plug is another part that deteriorates over time. Its gradual decrease in terms of performance varies from device to device. However, you can suspect its inefficiency if you cannot start the device properly after some period of inactivity.

You can look for the following things when it comes to problems with the spark plug, which kills your engine’s speed:

  • Corrosion within the plug
  • Wire connectivity



The main reason behind spark plug failure is the corrosion of the metal. At first, you need to remove the plug from its socket to analyze its condition. If there is minimal corrosion, you can quickly get away with it by rubbing it with an anti-corrosion brush. However, you should consider replacing the plug if the entire surface is corroded.


Wire Connectivity

The plug’s wire is the most crucial part as it establishes the connection between different parts of the engine. The plug cannot get the required current if the connected wire gets out of order or is not connected properly.

You can check this part within a matter of seconds. All you have to do is ensure the proper connectivity between the plug and the rest of the engine. You can also replace the wire if it gets damaged. Follow the company’s provided manual to see how it works for your model.


Problems With the Fuel Usage

You must consider many factors when choosing the type of fuel you use for your engine. The most crucial aspect in avoiding any fuel-related problems is to only use the type of fuel type suggested by the manufacturer.

Many companies use different mixing formulas for the production of fuels. In other words, you will get different percentages of butane, xylene, benzene, and other substances when you buy energy from different companies.

In this regard, you can check the user manual provided by the company to avoid any possible errors with the fuel usage.

However, you may still encounter problems if you use the type of fuel recommended by the manufacturer. Although, the problem arises for entirely different reasons. If you leave the residual fuel within the machine’s container for an extended period, it starts messing up the rest of the components.

Consequently, you are most likely to encounter clogging in several of the tiny components of the engine. As a result, the engine will exert maximum force to give the desired output, resulting in the unwanted speed up of the device.

You may also mix water with the fuel, which can trigger an unusual response from the engine. A heavy rainstorm or condensation on hot summer days can result in this sort of mix.

The first thing you need to do in this regard is replace the existing fuel with a fresh gallon of gasoline. But, before you add the newer fuel, do not forget to thoroughly clean the gas tank and other valves associated with the tank and pipe.


How to Fix Sluggish Lawn Mower Engine?

Apply the following things to get rid of the problem:

  • Clean or replace the spark plug
  • Change the fuel
  • Replace the damaged air filter with a new one
  • Get rid of corroded elements in the engine


How to Fix a Surging Engine?

Besides following the steps mentioned above, you can follow these guidelines to get rid of a surging engine.


Carburetor Malfunction

The malfunctioning carburetor of the mower’s engine is the primary cause of surging power issues. Dirt and consistent exposure to unwanted trivial substances make it inefficient and reduce its functionality.

It is totally fine if you find them inefficient. Almost any lawn owner can treat these minor elements by following two to three simple steps. However, we advise you to pay a visit to a professional if you are not good at assembling and disassembling things.

For DIY enthusiasts, the carburetor is not something that you will find on the exterior of the engine. Instead, you need to disassemble many parts to reach it. Before getting the desired results, remove the following pieces:

  • Air filter
  • Fuel container (Some models)
  • Breather pipe
  • Keeper ring

Once you are done disassembling those, you can quickly remove the carburetor from the engine to get things done.

The carburetor should be cleaned with a device that can reach every corner. However, if its condition is too bad for reuse, you must avoid reinstalling it in the engine. Instead, buy a new one at a nearby parts store by giving information such as the model and type of engine.

If you are satisfied with the cleaning, you can reassemble everything to see if it works. However, if cleaning or replacing the engine’s carburetor does not work for you, your only remaining option is to check the quality of the fuel and other such details.



Weekend warriors who mow the lawn after weeks are irritated by the frequent surges and hunting of their mowers’ engines. Fortunately, multiple things can help you in this regard. Follow the detailed guidelines mentioned above to fix the problem permanently.

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