Air/fuel mixture screw
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Thread: Air/fuel mixture screw

  1. #1
    Junior Member liberty74's Avatar
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    Air/fuel mixture screw

    Hi guys I was just wondering about the air/fuel mixture screw. Does turning it in add more fuel or does it add more air? Thanks

  2. #2
    Super Moderator J-T's Avatar
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    We would need to know specifically what bike you're talking about to give you the definitive answer, however, as a general rule the mixture screw regulates the amount of air/fuel mixture that is introduced to the air stream going through the throat of the carburetor at iidle. Usually (there can be exceptions), turning the screw counter-clockwise (out) results in a richer mixture. Turning the screw clockwise (in) results in a leaner mixture.

    There are differences among carburetors, but, again as a general rule, the mixture screw is used to adjust the mixture only at idle. It's effect on the mixture diminishes as the throttle is opened until it is pretty much irrelevant at WOT.
    JT
    81 CM400T

  3. #3
    Junior Member liberty74's Avatar
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    What I have is a 74 cb200. As we've been tuning we were wondering about which way was making it leaner or richer. A friend was asking about it, because like you said that some exceptions were different. Thanks

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    Member Guylr's Avatar
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    Quote Originally Posted by J-T View Post
    We would need to know specifically what bike you're talking about to give you the definitive answer, however, as a general rule the mixture screw regulates the amount of air/fuel mixture that is introduced to the air stream going through the throat of the carburetor at iidle. Usually (there can be exceptions), turning the screw counter-clockwise (out) results in a richer mixture. Turning the screw clockwise (in) results in a leaner mixture.

    There are differences among carburetors, but, again as a general rule, the mixture screw is used to adjust the mixture only at idle. It's effect on the mixture diminishes as the throttle is opened until it is pretty much irrelevant at WOT.
    The rule does not read quite like that. If the screw is in the bellmouth area between the slide and air filter then it adjusts the air reaching the pilot jet circuit and is typically know as an idle mixture screw. Turning that screw out will add air and make the idle mixture leaner. Screws in the carb throat area between the slide and the cylinder head are usually called fuel screws and adjust the amount of already mixed pilot circuit fuel entering the engine. Turning a fuel screw out will add more mixed fuel to the engine and richens the idle and to a lesser extent, overall mixture.
    Guy
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