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Transfer ports, also known as off-idle discharge ports, are critical components in the operation of carburetors, particularly during the transition from idle to the main metering circuit.


  1. Fuel Delivery: During the transition from idle to higher engine speeds, the throttle plates begin to open, increasing airflow through the carburetor. The transfer ports deliver additional fuel from the idle circuit to supplement the increased airflow and maintain the proper air-fuel mixture ratio for combustion.
  2. Emulsification: At curb idle, off-idle ports serve as an additional air bleed, contributing to the emulsification of the idle mixture. This helps ensure a smooth and stable idle.
  3. Transition to Main Metering Circuit: As the throttle plates open further and engine speed increases, vacuum in the carburetor bore rises. This vacuum draws fuel through the transfer ports, transitioning them from serving as air bleeds to fuel discharge ports. The additional fuel delivered through these ports supplements the main metering circuit, ensuring smooth acceleration and responsive throttle performance.

Components and Operation:

  • Location: Transfer ports are typically located just above the throttle plates in the carburetor’s primary venturi area. This positioning allows them to deliver fuel directly into the airflow path as the throttle plates begin to open.
  • Design: Transfer ports can consist of one or more holes or a single slot. Slots are often preferred due to their simpler design and lower manufacturing costs.


  • Smooth Transition: Transfer ports play a crucial role in ensuring a smooth transition from idle to higher engine speeds by providing additional fuel as needed.
  • Responsive Throttle: Properly functioning transfer ports contribute to responsive throttle performance, allowing the engine to accelerate smoothly without hesitation or stumbling.

Maintenance and Tuning:

  • Adjustment: Proper adjustment of the idle mixture screws and idle speed may be necessary to optimize the performance of the transfer ports and ensure smooth transition and throttle response.
  • Cleaning: Periodic cleaning and inspection of the carburetor, including the transfer ports, are essential to prevent clogging or blockage that could affect fuel delivery and engine performance.

Transfer ports, also known as off-idle discharge ports, play a critical role in the operation of carburetors by delivering additional fuel during the transition from idle to higher engine speeds. Their design and proper function are essential for smooth acceleration, responsive throttle performance, and overall engine efficiency. Regular maintenance and tuning of the carburetor are necessary to ensure optimal operation of the transfer ports and the entire fuel delivery system.


    • Off-idle discharge ports
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