« Back to Dictionary Index

Function and Purpose:

  • Automatic Control: The thermostatic coil choke is designed to automatically control the choke butterfly valve in the carburetor, which regulates the air/fuel mixture during engine startup and warm-up.
  • Heat Sensing: It uses the heat from the exhaust manifold to adjust the choke valve, ensuring optimal engine performance as the engine temperature changes.

Components and Mechanism:

  • Thermostatic Coil (Bimetallic Spring): This coil is made of two dissimilar metals bonded together, which expand and contract at different rates when heated. This differential expansion causes the coil to wind or unwind.
  • Choke Butterfly Valve: A valve located in the throat of the carburetor that restricts airflow, enriching the air/fuel mixture for easier starting.
  • Mounting Position: The thermostatic coil is typically mounted on the carburetor, with a linkage connecting it to the choke butterfly valve.

Operating Principle:

  • Cold Start: When the engine is cold, the thermostatic coil contracts, closing the choke butterfly valve. This restricts the airflow, creating a richer fuel mixture necessary for starting a cold engine.
  • Warm-Up Phase: As the engine warms up, the heat from the exhaust manifold warms the thermostatic coil. The coil expands, gradually opening the choke butterfly valve to allow more air in, leaning out the mixture to match the engine’s operating temperature.
  • Normal Operation: Once the engine reaches its normal operating temperature, the choke butterfly valve is fully open, allowing the carburetor to mix fuel and air in the correct ratio for optimal combustion.


  • Ease of Starting: By automatically adjusting the air/fuel mixture based on engine temperature, the thermostatic coil choke simplifies the starting process, especially in cold weather.
  • Improved Fuel Efficiency: Proper adjustment of the choke helps prevent overly rich mixtures, which can waste fuel and cause carbon buildup.
  • Reduced Emissions: By ensuring the engine runs with the correct air/fuel mixture, the thermostatic coil choke helps reduce exhaust emissions during the warm-up phase.

Maintenance and Troubleshooting:

  • Common Issues:
    • Sticking Choke: If the thermostatic coil becomes dirty or worn, it may stick, causing the choke butterfly valve to remain closed or open incorrectly.
    • Heat Transfer Problems: Poor contact or insulation issues can prevent the coil from sensing the correct temperature, leading to improper choke operation.
  • Symptoms of a Faulty Choke:
    • Hard Starting: Difficulty starting the engine, particularly when cold, can indicate a malfunctioning thermostatic coil choke.
    • Rough Idle: An engine that idles roughly during warm-up may have an improperly functioning choke.
    • Poor Fuel Economy: Excessive fuel consumption can result from a choke that does not open properly as the engine warms up.

Replacement and Repair:

  • Inspection: Regularly inspect the thermostatic coil and linkage for signs of wear, dirt, or damage.
  • Cleaning: Clean the coil and linkage to ensure smooth operation.
  • Adjustment: Some thermostatic coils are adjustable; refer to the manufacturer’s guidelines for proper adjustment procedures.
  • Replacement: If the thermostatic coil or choke mechanism is damaged or worn, it should be replaced to maintain proper engine performance.

The thermostatic coil choke is a critical component in carbureted engines, providing automatic control of the choke butterfly valve to ensure optimal air/fuel mixture during different phases of engine operation. Proper maintenance and timely repairs are essential for the efficient performance of this system.


  • Choke
« Back to Dictionary Index