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What Causes a Lawn Mower to Run Rich?

Lawnmowers are motor-driven machines that chop, trim or mow grass patches, fields, lawns and gardens. The engine spins a revolving blade or blades to cut the grass. 

Lawnmower engines are typically gasoline-powered with a spark plug. As they are used by humans to help maintain their lawns, they have become an integral part of many people’s lifestyles. 

Over time the lawn mower gets old, the components corrode, some gets dirty or clogged. And so many other problems. One such problem occurs when it runs rich.

In this article, we’ll be discussing the reason behind a lawn mower engine to run rich and some of the possible solutions to the issues.

Reasons why a lawn mower runs rich

When the engine is running rich in fuel and air, it will draw little air than fuel needed. This will cause the engine to run hotter. The engine is receiving too much fuel and too little air, which can be caused by a stuck fuel injector

When that happens, it creates extra smoke and creates a significant increase in noise levels coming from the engine. Another reason might be dirty, clogged with debris or carbon deposits from a malfunctioning spark plug. It also occurs for a restricted air filter.

Restricted air filter

Air filters are very important for engines, especially for gasoline engines. The function of an air filter when it comes to gasoline engines is to prevent dirt and debris from entering the engine.

Restricted air filter of a lawn mower in a person's hand
Restricted air filter

This prevents dirt from clogging up the engine. Air filter prevents problems such as overheating, poor running, and even serious damage. A restricted air filter can cause a lawn mower to run rich, consumes more fuel than air, which will lead to engine damage.

A dirty air filter causes the mower to not let out enough air, which will lead to several problems. It can cause problems such as stalling, running poorly, and excessive wear on components. 

Sooty spark plugs

Spark plugs are small piece of equipment in the engine. Spark plugs are used to provide the necessary spark for ignition and ignite the fuel-and-air mixture in a combustion engine. 

Faulty spark plug of a lawn mower
Faulty spark plug

They convert the electrical energy into heat, which then ignites the fuel and mixes it with air. The spark plug contains small metal tips called electrodes that create a high voltage spark across an air-fuel mixture that creates enough heat to ignite it. 

One of the reasons why spark plugs may cause a lawn mower to run rich is that they can get fouled up with dry, black soot from the engine which causes the electrodes to foul up with carbon. 

With carbon on its electrodes can cause incomplete combustion and lead to high emissions, which might be a reason the engine is running rich.

Stuck float or needle in carburetor

A carburetor is a device that regulates the air/fuel mixture ratio in an internal combustion engine. A float needle is a needle attached to the float of a carburetor. 

It is used to control the amount of fuel entering the engine. The controls fuel flow by moving up and down in response to pressure on the float. Thus it regulates the supply of oil. 

Stuck float needle in the carburetor causes a lawn mower to run rich. This will cause a mix of too much fuel and not enough fuel going into the engine, which can cause it to run lean or rich. 

Change old float in the carburetor of a lawn mower
Change old float in the carburetor

When a lawn mower is running rich, the engine will struggle to get enough fuel. Sometimes the engine will stall, sometimes it won’t idle smoothly or backfire. In both cases the engine won’t run efficiently and could potentially damage the motor or cause other issues.

Frequently Asked Questions

What causes a lawn mower to rev up and down?

The engine of a lawn mower is designed so that it can only run when it has enough airflow to operate which is why the blades don’t turn at all when the engine is not running. 

When the engine is blocked, you may hear the motor revving up repeatedly. There are a few reasons for this. It could be due to debris near the air filter that cleans the air to feed into the engine and drive it. 

Once these are blocked, you will hear a hunt or surge in airflow that can cause your lawn mower’s engine to slow down or even stop altogether. It also causes the engine to surge.

Why is my lawn mower not running smoothly?

If your lawn mower starts vibrating or running erratically when you turn it on and smokes out of the exhaust when you start it up, then what you need to replace some of the components.

If your mower seems to idle poorly, this could be due to a bad spark plug, while other times it could be because there is not enough oil being supplied or the air filter needs replacing.

While replacing these parts might seem like a costly and time-consuming process, a good tune-up can make your mower run smoother.

How do I know if my lawn mower engine is bad?

The best way to know if your engine is bad is by listening to it. If you hear a knocking sound, your engine may be worn out and needs replacement. 

If the exhaust is emitting excessive black smoke, the engine may be damaged and needs repair or replacement. A vibrating engine can sometimes indicate that the parts are worn out and need fixing and replacing. 

A poor-running lawn mower could also cause this problem if you’re running on high-quality fuel or using poor quality fuel such as gas from an old gas can.


If a lawn mower is running rich, it will draw too much air and not enough fuel for combustion. This leads to a loss of power. 

A lawn mower running too rich will draw little air than what’s needed for combustion, which can cause a number of problems that lead to the engine running rich. 

Most of the time, it is caused by an incorrect mixture of fuel and air. This lack of proper mixing leads to a build-up of unburnt fuel and carbon in the engine. It might also need a costly repair.

Here is why a lawn mower runs rich. We’re also discussing the fix. These articles will be useful for repairing your lawn mower but if you can’t fix it yourself, ask for professional help or contact your dealer.

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