How to tune & modify carburetor performance

Written by cassandra tribe
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How to tune & modify carburetor performance
Basic carb showing the adjustment screws (George Szappanos)

Carburettors are still the equipment of choice for modified racing vehicles because of the ease and economy of modifying their performance capabilities. Unlike fuel injection systems, you can tune and modify carburettor performance without expensive system upgrades or specialised tools. With a basic understanding of how to tune a carburettor and what conditions will affect your carburettor performance, you can make your modifications quickly and easily.

Skill level:
Moderately Easy

Things you need

  • Flat head screwdriver

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  1. 1

    Locate the two tuning screws: idle and mix. If you have the manual for your carburettor, look there to find the screw locations. If you do not, the screw protruding from body of the carburettor nearest the choke assembly will be your idle adjustment screw, and the one located near the fuel inlet (but near the bottom of the carburettor) will be your mix.

  2. 2

    Using a flat head screwdriver, turn the screws until they are both "seated" in the body of the carburettor. In other words, turn them clockwise until they are completely tightened.

  3. 3

    Turn both screws counterclockwise two full turns and one half-turn. Your carburettor is now considered "factory set" at sea level.

  4. 4

    Start the car and turn the idle screw in or out by half-turns until the idle is smooth and even.

  5. 5

    Turn the mix screw in or out by quarter turns until the engine begins to miss and threaten to stop running; turn the screw a quarter turn in the opposite direction until the engine runs evenly again. Your mix is now correctly adjusted for the altitude that you are currently at.

  6. 6

    Let the engine continue running for 15 minutes so the engine and the carburettor are at operating temperature. Adjust the idle screw again until the idle is running smoothly but slightly faster than before. This sets your running idle. All of these adjustments are made by turning the screws and listening to the engine. You will be able to hear the changes in the speed as you tune and modify your carburettor performance.

Tips and warnings

  • If you have access to a tachometer, connect it to your ignition coil and monitor the actual RPMs of your engine as you make your adjustments. A normal cold idle RPM for any car is 500; for a racing vehicle you will want the cold idle set to 700 to 900 RPM.
  • Some carburettors have a third screw above the mix screw near the fuel inlet. This screw should be fully seated and never loosened, as it is the fuel jet control and if "adjusted" will destroy the calibration of your carburettor.

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