Why Does A Carburetor Only Work With The Choke On?

Published: 20.6.23

Updated: 2.8.24

If you've ever wondered, "Why does a carburetor only work with the choke on?" you're not alone.

It's a question that puzzles many, and the answer lies in the intricate mechanics of a carburetor and the function of the choke.

Here's the short answer:

If a carburetor only functions when the choke is on, even after the engine has adequately warmed up, it signals an imbalance in the engine's fuel-air mixture. This imbalance is typically due to factors like improper adjustment of the mixture system, a dirty carburetor, or problems with components such as the gasket or air intake. Regular maintenance and prompt troubleshooting can help restore optimal engine performance.

Read on for a more detailed answer...

Understanding the Basics: What is a Carburetor?

A carburetor is a mechanical device that blends air and fuel for an internal combustion engine.

It's essentially the lung of your engine, responsible for delivering the right mixture of air and fuel to keep it running smoothly. Without it, your engine wouldn't be able to function.

The basic principle of a carburetor is relatively simple, but its operation involves several key components working together:

1. Throttle Valve: This is a pivoting flap that controls the amount of air flowing into the engine. When you press the accelerator, it opens wider to allow more air in.

2. Venturi: This is a narrow section in the carburetor's air passage. As air flows through this constriction, it speeds up and creates a pressure drop.

3. Float Chamber: This component maintains a constant level of fuel, ready to be drawn into the air stream. It works like a toilet tank, with a float that rises and falls with the fuel level, controlling the fuel inlet.

4. Jets: These are precisely calibrated holes that meter the amount of fuel entering the air stream. The main jet supplies fuel at higher speeds, while smaller jets handle low-speed operation.

5. Choke: This is a valve that restricts the air intake, enriching the fuel mixture for cold starts.

Here's how these components work together:

1. Air enters the carburetor through the air filter.
2. As it passes through the venturi, its speed increases and pressure drops.
3. This pressure drop draws fuel from the float chamber through the jets.
4. The fuel mixes with the air, creating a fine mist.
5. This air-fuel mixture then travels through the intake manifold to the engine cylinders.

The carburetor constantly adjusts this mixture based on engine speed and load, ensuring optimal performance across different operating conditions.

While modern vehicles have largely replaced carburetors with fuel injection systems, carburetors are still commonly found in small engines and older vehicles.

Understanding how a carburetor works is crucial for diagnosing issues like the one mentioned in the article title, where the engine only runs with the choke on.

This typically indicates a problem with the fuel-air mixture, which we'll explore further in subsequent sections.

The Function of the Choke

When the engine is cold, it needs slightly more fuel in the fuel/air mixture to start, which is what the choke does. 

The choke increases the amount of fuel in the mixture, creating a richer mixture, allowing the engine to function at its best until it warms up enough to run on its normal operating fuel mixture. Now, you may be wondering...

Why a Carburetor Only Works When Choked

When the choke is turned off, the engine gets too little fuel and too much air for it to run and it stalls.

Here are some possible causes:

Engine Fuel-Air Mixture System Out of Adjustment

If the fuel-air mixture isn't precisely balanced, the engine may only run when the choke is on. It's a delicate dance, with no room for missteps.

Related: Why Does My Motorcycle Backfire?

Dirty or Gummed Up Carburetor

Old gasoline can gum up a carburetor, creating a sticky residue that restricts fuel flow.

This can cause your engine to stall unless the choke is engaged.

A thorough cleaning can often resolve this issue.

Here's a step-by-step guide on how to clean your carburetor:

1. Remove the Carburetor

   • Turn off the fuel valve and disconnect the fuel line.
   • Remove any air filters or covers blocking access.
   • Disconnect throttle and choke linkages.
   • Unscrew the mounting bolts and carefully remove the carburetor.

2. Disassemble the Carburetor

   • Take photos or make notes to aid reassembly.
   • Remove the float bowl by unscrewing the retaining bolt.
   • Carefully remove the float and needle valve.

3. Clean the Components

   • Use a carburetor cleaner spray to clean all parts.
   • Pay special attention to the jets, which are prone to clogging.
   • Use compressed air to blow out all passages.

4. Inspect and Clean Jets

   • Remove the main jet and pilot jet.
   • Hold them up to the light to check for blockages.
   • Use a thin wire to clear any obstructions in the jets.

5. Clean the Emulsion Tube

   • This tube has several small holes that can get clogged.
   • Ensure all holes are clear of debris.

6. Check the Float Valve

   • Inspect the rubber tip for wear or damage.
   • Ensure the metal tip moves freely.

7. Reassemble and Reinstall

   • Put all components back together in reverse order.
   • Replace any damaged gaskets or O-rings.
   • Reinstall the carburetor on the engine.

8. Final Adjustments

   • Reset the idle mixture screw to its original position.
   • Start the engine and make final adjustments as needed.

Remember, if you're not comfortable performing this maintenance yourself, it's best to consult a professional mechanic.

Regular carburetor cleaning can significantly improve engine performance and prevent issues like the one described where the engine only runs with the choke on.

By following these steps, you can effectively clean your carburetor and restore proper fuel flow, potentially resolving the issue of the engine only running with the choke on.

Regular maintenance of your carburetor can prevent such problems and keep your engine running smoothly.

Issues with Carburetor Gasket

The carburetor gasket prevents excess air from entering the engine.

If this gasket is damaged, it can result in an engine that only runs with the choke on.

Clogged Transition Circuit or Main Jet

The transition circuit or main jet could be clogged with debris. This can cause the engine to only run when the choke is closed.

Related: Should You Run With The Choke Open Or Closed?

Expert Tips for Carburetor Tuning

Properly tuning a carburetor can significantly improve engine performance, fuel efficiency, and overall reliability.

Here are some advanced tips and tricks for optimal carburetor tuning:

1. Understand Jetting

• Main Jet: Controls fuel mixture at high RPMs. If the engine sputters at full throttle, go up a size. If it runs rough or fouls plugs, go down.
• Pilot Jet: Affects low-speed performance. If the engine hesitates on acceleration, try a larger pilot jet.
• Jet Needle: Fine-tunes mid-range performance. Raising the clip position richens the mixture, lowering it leans it out.

2. Master the Mixture Screws

• Start with both idle mixture screws turned in fully, then back out 1.5 turns.
• Fine-tune by adjusting 1/4 turn at a time, aiming for the highest stable idle RPM.
• If you can't achieve a smooth idle, your jets may be incorrectly sized.

3. Synchronize Multiple Carburetors

• Use a vacuum gauge set to ensure all carburetors are drawing the same amount of air.
• Adjust the linkages until all cylinders are firing evenly at idle.

4. Optimize Float Level

• Set the float level with the carburetor inverted. An incorrect float level can cause flooding or fuel starvation.
• Generally, the float should be parallel to the gasket surface when inverted.

5. Consider Altitude and Temperature

• Higher altitudes require leaner jetting due to thinner air.
• Colder temperatures typically need richer mixtures, while warmer temps need leaner mixtures.

6. Use an Air/Fuel Ratio Meter

• For precise tuning, an A/F ratio meter can help you achieve the ideal mixture (around 14.7:1 for gasoline at idle and cruise, richer for full throttle).

7. Pay Attention to Throttle Plate Alignment

• Ensure the throttle plate closes completely and opens smoothly. Misalignment can cause idle issues and poor throttle response.

8. Experiment with Emulsion Tube Selection

• Different emulsion tubes can significantly affect mid-range performance. Experiment with various designs for your specific application.

9. Consider a Velocity Stack

• A properly sized velocity stack can improve airflow and throttle response, especially in performance applications.

10. Fine-tune with Spark Plug Reading

• After tuning, check spark plug color. A light tan color indicates a good mixture. White is too lean, dark or sooty is too rich.

11. Use the Right Tools

• Invest in a good quality vacuum gauge, tachometer, and carburetor synchronizer for multi-carb setups.
• A wide-band oxygen sensor can provide real-time air/fuel ratio data for precise tuning.

Remember, carburetor tuning is as much an art as it is a science. It requires patience, attention to detail, and a willingness to make small, incremental adjustments.

Always make one change at a time and thoroughly test before moving on to the next adjustment.

By mastering these advanced tuning techniques, you can ensure your carburetor performs optimally across all operating conditions, potentially resolving issues like the engine only running with the choke on and improving overall engine performance and efficiency.

Troubleshooting Carburetor Problems

Before starting, it's crucial to correctly diagnose your issue. First, try to start your engine.

If it only runs with the choke on or stalls when you try to turn the choke off, consider these potential issues:

Engine Fuel-Air Mixture System Out of Adjustment

Check the adjustment of the fuel-air mixture system. Incorrect adjustment is a common mistake.

You can usually resolve this issue by adjusting the carburetor's mixture screws.

Turn the screws in or out until you achieve a stable idle with the choke off.

Related: Does Leaving The Choke Closed Increase Fuel Consumption?

Dirty or Gummed Up Carburetor

If the engine isn't run regularly, gasoline in your carburetor can gum up over time.

Clean the carburetor using a carb cleaner spray.

Remove, disassemble it and spray the cleaner on all components, ensuring to remove any visible debris.

Always follow the cleaner manufacturer's instructions to avoid common mistakes like neglecting to clean small orifices or jets.

Related: A Step-by-Step Guide: How to Clean a Carburetor on Your Motorcycle

Issues with Carburetor Gasket

Inspect the carburetor gasket for any signs of wear or damage. A damaged gasket can cause the engine to run only with the choke on. Replace it if necessary.

Problems with Air Intake Boot and Vacuum Line

Check for any signs of damage in the air intake boot or vacuum line.

A split in these parts can cause problems. 

Replacing these parts is usually straightforward, but ensure they're fitted correctly to avoid common mistakes like improper fitting.

Clogged Transition Circuit or Main Jet

If your engine runs with the choke closed but dies when the choke is opened, debris could be partially blocking the transition circuit or main jet.

Clean these parts thoroughly to ensure there's no debris left behind.

Remember, always refer to your owner's manual or consult with a professional if you're unsure about any step in the process.

Also, ensure the engine is off and has cooled down before starting any maintenance work to avoid accidents.

Related: Smooth Carburetor Running: Solving the Choke Mystery

Additional Carburetor Problems

While the aforementioned issues are the most common, you might encounter other problems that cause the engine to run only with the choke on.

These could include:

Carburetor Choke Problems

The choke spring tension may not be tight enough and the butterfly may open too soon.

Confirm the choke is operating properly when cold and warm, as per the manufacturer's specifications.

Damaged Carburetor Gasket

If this gasket, which prevents excess air from entering the engine, is damaged, it could disrupt the optimal fuel-air mixture.

More Complex Issues

If adjusting, cleaning or replacing a carburetor doesn't resolve the issue, the underlying cause might be more complex, such as a weak fuel pump, plugged fuel filter or fuel line, or even bad gas.

By following this guide, you should be able to troubleshoot and resolve most common carburetor problems.

Always remember that it's not just about the ride; it's about the journey and how smoothly you get there.

Preventive Maintenance Tips for Carburetors

Proper maintenance of your carburetor is crucial for ensuring optimal engine performance and longevity.

Here are some essential preventive maintenance tips to keep your carburetor functioning well:

Routine Checks

• Visual Inspection: Regularly examine the carburetor for signs of wear, damage, or fuel leaks. Look for cracks, loose connections, or deteriorating gaskets.
• Air Filter Check: Inspect and clean or replace the air filter as needed. A clogged air filter can disrupt the air-fuel mixture and cause poor performance.
• Fuel Quality: Use fresh, clean fuel and avoid letting fuel sit in the tank for extended periods. Old or contaminated fuel can clog the carburetor.

Cleaning

• External Cleaning: Use a carburetor cleaner spray to remove dirt and grime from the exterior. This helps prevent debris from entering the internal components.
• Internal Cleaning: Periodically (every 1-2 years or as needed) perform a thorough internal cleaning. This may involve disassembling the carburetor and soaking parts in a cleaning solution.
• Jet Cleaning: Pay special attention to cleaning the jets, as these small openings can easily become clogged. Use compressed air or a fine wire to clear any blockages.

Adjustments

• Idle Speed: Check and adjust the idle speed according to your vehicle's specifications. An improper idle can lead to stalling or rough running.
• Mixture Screws: Fine-tune the air-fuel mixture using the idle mixture screw. This adjustment can improve fuel efficiency and engine performance.
• Float Level: Verify and adjust the float level if necessary. An incorrect float level can cause flooding or fuel starvation.

Additional Tips

• Use Fuel Stabilizer: If your vehicle sits unused for long periods, add a fuel stabilizer to prevent fuel degradation and carburetor clogging.
• Warm-Up Period: Allow your engine to warm up properly before driving, especially in cold weather. This helps the carburetor function more efficiently.
• Professional Service: Consider having your carburetor professionally serviced annually or bi-annually, especially if you're not comfortable with DIY maintenance.

By following these preventive maintenance tips, you can significantly reduce the likelihood of carburetor-related issues, including the problem of the engine only running with the choke on.

Regular maintenance not only improves performance but also extends the life of your carburetor and engine.

Remember, if you're unsure about performing any of these maintenance tasks, it's always best to consult with a qualified mechanic.

They can provide expert guidance and ensure that your carburetor is properly maintained for optimal performance.

In Summary

If your carburetor only works with the choke on, it's likely due to one or more of the issues we've discussed.

It's critical to diagnose the problem accurately and make the necessary adjustments or repairs to get your engine running smoothly again.

Remember, it's not just about the ride; it's about the journey and how smoothly you get there.

FAQs

Can I drive my vehicle with the choke always on?

While it may be possible to drive a vehicle with the choke always on, it's not recommended. Running the choke continuously can lead to a rich fuel mixture, which could cause engine damage over time, reduce your vehicle's fuel efficiency and increase exhaust emissions.

How often should I clean or service my carburetor?

The frequency of carburetor cleaning or servicing depends on several factors, including the age of the vehicle, its usage, and the type of fuel used. However, as a general rule of thumb, a carburetor should be cleaned or serviced at least once a year. Always refer to your vehicle's maintenance schedule for specific recommendations.

Are there any signs that my carburetor needs adjusting?

Yes, there are several signs that your carburetor may need adjusting. These include a rough or uneven engine idle, difficulty starting the engine, a noticeable decrease in the vehicle's fuel efficiency, and black smoke from the exhaust. If you notice any of these signs, consider checking your carburetor and adjusting it as necessary.