The Mystery of the Auto Choke: How Does It Work on Your Lawn Mower?

Imagine yourself on a sunny Saturday morning, ready to tackle your overgrown lawn. You pull the starter cord on your trusty lawnmower, but instead of the satisfying roar of the engine, you hear a sputtering cough and nothing more. Frustrated, you try again, and again, but the engine just won’t catch. What’s going on?

This scenario is a common frustration for many lawnmower owners, and the culprit might be the elusive “auto choke.” This seemingly mysterious component plays a crucial role in starting your lawnmower, but its function often remains a mystery. In this article, we’ll unravel the mysteries of the auto choke, explain how it works, and explore why it might be giving you trouble.

Understanding the Basics:

The auto choke is a device designed to enrich the fuel-air mixture in your lawnmower’s engine during cold starts. In simpler terms, it helps your engine get the extra fuel it needs to fire up when it’s cold. Think of it like a temporary “booster shot” for your lawnmower. Once the engine warms up, the choke automatically adjusts to allow the appropriate fuel-air mixture, ensuring optimal performance.

How Does the Auto Choke Work?

The auto choke operates through a variety of mechanisms, but the most common type relies on a bimetallic strip. This strip is made of two different metals that expand and contract at different rates when exposed to heat. Here’s how it works:

1. Cold Start:

  • When the engine is cold, the bimetallic strip is contracted.
  • This contracted state restricts the flow of air into the carburetor, creating a richer fuel-air mixture.
  • This rich mixture is essential for easy starting when the engine is cold.

2. Warm-Up:

  • As the engine starts and begins to warm up, the bimetallic strip heats up.
  • The different metals expand at different rates, causing the strip to bend.
  • This bending opens the air passageway, gradually allowing more air to enter the carburetor.
  • The fuel-air mixture becomes leaner as the engine warms up, allowing for efficient operation.

3. Fully Warmed Up:

  • Once the engine reaches operating temperature, the bimetallic strip is fully heated and has reached its maximum bend.
  • The air passageway is fully open, allowing for the optimal fuel-air mixture for optimal performance.

Common Auto Choke Problems and Troubleshooting

While the auto choke is a helpful mechanism, it can sometimes malfunction, leading to starting difficulties. Here are some common issues and how to troubleshoot them:

1. Sticking Choke

  • Problem: The bimetallic strip might become stuck in a partially open or closed position, preventing the choke from fully engaging or disengaging.
  • Solution: Try manually opening and closing the choke lever to see if it moves freely. If it’s stuck, you might need to clean it or replace it.

2. Worn or Damaged Bimetallic Strip

  • Problem: Over time, the bimetallic strip can wear down or become damaged, affecting its ability to bend and regulate the air flow.
  • Solution: Inspect the bimetallic strip for any signs of damage. If it’s worn, you’ll likely need to replace it.

3. Clogged Air Passageway

  • Problem: Debris or dirt can accumulate in the air passageway, restricting airflow and preventing the choke from functioning properly.
  • Solution: Clean the air passageway with compressed air or a small brush.

4. Incorrect Choke Adjustment

  • Problem: The choke adjustment screw may be set incorrectly, leading to a too-rich or too-lean fuel-air mixture.
  • Solution: Refer to your lawnmower’s manual for the correct choke adjustment setting. Adjust the screw as needed.

5. Other Engine Issues

  • Problem: Starting difficulties can also be caused by other engine issues, such as a clogged carburetor, spark plug problems, or a faulty fuel system.
  • Solution: If you’ve ruled out auto choke issues, it’s essential to check these other components to ensure proper engine function.

Alternatives to the Auto Choke

While the auto choke is a widely used system, some lawnmowers employ alternative starting mechanisms. These include:

1. Manual Choke:

  • How it works: Instead of an automatic system, manual chokes require the user to manually adjust a lever or knob to enrich the fuel-air mixture during cold starts.
  • Pros: Allows for more precise control over the choke.
  • Cons: Requires the user to manually adjust the choke, which can be inconvenient.

2. Electric Choke:

  • How it works: Electric chokes use a solenoid valve to control airflow, often triggered by a key switch or a button.
  • Pros: Automatic and typically more reliable than manual chokes.
  • Cons: Requires a battery and can be more expensive.


The auto choke is an important component that helps your lawnmower start smoothly, especially in cold conditions. While it’s generally a reliable system, it can sometimes malfunction, leading to starting difficulties. By understanding how the auto choke works and troubleshooting common problems, you can ensure a smooth start every time you need to mow your lawn.

Remember, if you’re unsure about diagnosing or repairing any auto choke issues, it’s always best to consult a qualified technician to avoid further damage to your lawnmower. With a little knowledge and care, you can keep your lawnmower running smoothly and your lawn looking its best.


What is an auto choke?

An auto choke is a device found on gasoline engines, including lawn mower engines, that helps the engine start and run smoothly when cold. It works by temporarily restricting airflow to the engine, enriching the fuel-air mixture. This allows the engine to run at a higher idle speed until it warms up, preventing it from stalling. The auto choke is often a spring-loaded diaphragm that is connected to a thermostat. As the engine warms up, the thermostat opens, releasing the diaphragm and allowing more air into the engine.

How does an auto choke work on a lawn mower?

The auto choke on a lawn mower typically consists of a lever or a butterfly valve that restricts airflow to the carburetor. When the engine is cold, the choke is engaged, restricting airflow and enriching the fuel mixture. This allows the engine to run at a higher idle speed, providing a richer fuel-air mixture for easier starting. As the engine warms up, the thermostat opens, reducing the choke’s restriction on airflow and allowing the engine to run at a normal idle speed. The warmer engine requires less fuel and therefore needs a leaner air-fuel mixture.

Why is an auto choke important?

An auto choke is important for a few reasons. First, it helps the engine start easily when cold. Cold engines need a richer fuel-air mixture to ignite and run smoothly. Second, the auto choke helps prevent the engine from stalling when it is cold. If the engine were to start with too much air and not enough fuel, it could stall. The auto choke ensures that there is enough fuel to keep the engine running smoothly until it warms up.

What happens if my auto choke is not working?

If your auto choke is not working, your lawn mower may have difficulty starting, especially when cold. It may also stall frequently, or run rough. In some cases, your lawn mower might even smoke excessively. If you notice any of these issues, it’s important to have your auto choke inspected and repaired by a qualified mechanic.

How can I tell if my auto choke is working?

The easiest way to tell if your auto choke is working is to observe the engine’s behavior when it is cold. If the engine starts easily and runs smoothly without stalling, your auto choke is likely working properly. However, if the engine struggles to start, stalls frequently, or runs rough, your auto choke may be malfunctioning.

How do I adjust my auto choke?

Adjusting the auto choke on your lawn mower requires some knowledge and experience. If you are not familiar with engine mechanics, it is best to leave this task to a professional mechanic. However, if you are comfortable working on engines, you can find instructions in your lawn mower’s manual or online. Typically, adjusting the auto choke involves turning a screw or adjusting a linkage.

What are some common problems with auto chokes?

Some common problems with auto chokes include sticking or binding, worn-out parts, and improper adjustment. If you suspect a problem with your auto choke, it’s best to have it inspected and repaired by a qualified mechanic. Attempting to repair the auto choke yourself could lead to further damage to the engine.

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