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Thread: What is the name and purpose of this carburetor part?

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    Default What is the name and purpose of this carburetor part?

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    What is the purpose of the little diaphragm on the side of the carb body? What is its correct name (Yamaha parts fiche calls it a diaphragm set)

    Also if it fails or if the holes leading to it are plugged, what will be the symptoms?

    Has this part been known to fail on ventures.

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    Default Re: What is the name and purpose of this carburetor part?

    Yes, they somtimes fail. Some have used stuff like Yamabond to repair holes in the diaphragms. Vacuum against this diaphragm moves a fuel metering rod in and out of the jet that feeds the engine above idle. If there is a hole in the diaphragm or the metering rod is stuck, the engine will not run right above idle speed. #not a mechanic
    2007 Yamaha Royal Star Venture 23k miles
    1989 Yamaha Venture Royale 6k miles
    1985 Kawasaki ZN700 4cyl shaft 5k miles

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    Default Re: What is the name and purpose of this carburetor part?

    Quote Originally Posted by jfman View Post
    https://www.venturerider.org/forum/a...hmentid=118151

    What is the purpose of the little diaphragm on the side of the carb body? What is its correct name (Yamaha parts fiche calls it a diaphragm set)

    Also if it fails or if the holes leading to it are plugged, what will be the symptoms?

    Has this part been known to fail on ventures.
    It is variously called the "coasting enrichment valve" or "air cut-off valve" or "transient enrichment valve".
    When the throttle is closed, there would a transient lean condition, causing popping upon deceleration (coasting). This device enrichens the circuit to prevent popping.
    The diaphragm can be damaged by aggressive carb cleaners and the diaphragm can also develop holes. The result will be a vacuum leak and popping.
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ID:	118154Air Cut-off how it works.docx
    Kevin M. Campbell Hidden Content
    1974 Honda CB750, 1991 Venture Royale "the Tuxedo"
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    Default Re: What is the name and purpose of this carburetor part?

    Quote Originally Posted by BlueSky View Post
    Yes, they somtimes fail. Some have used stuff like Yamabond to repair holes in the diaphragms. Vacuum against this diaphragm moves a fuel metering rod in and out of the jet that feeds the engine above idle. If there is a hole in the diaphragm or the metering rod is stuck, the engine will not run right above idle speed. #not a mechanic
    I think you have it mixed up with the diaphragm from the main slide/needle.

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    Default Re: What is the name and purpose of this carburetor part?

    If you pull the carbs apart to clean them, the only way to get cleaner and air through the passages leading to the air cutoff circuit is to depress the small ball that the rod on the diaphragm presses on. I just used the old cutoff diaphragm that I was replacing to depress it when I cleaned mine.

    Ron

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