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Thread: OMC Orifice Plugs

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    Default OMC Orifice Plugs

    Can someone explain the difference between a "C" & "D" hi speed jet for OMC motors. There is also a hi speed jet with no letter designation. Those have the jet size running all the way through the jet, whereas the "C" & "D" hi speed jets have the jet size just part way through the jet. There is visually no obvious difference between the "C" & "D" hi speed jets. Help!!!

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    the letters tell which way the fuel flows through them.

    Part one of "Secrets of the OMC Mod 50" : http://www.boatracingfacts.com/forums/showthread.php?t=2052

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    Quote Originally Posted by Fast Fred View Post
    the letters tell which way the fuel flows through them.
    Hi Fred, Yes, I know the "C" & "D" has to do with flow direction, but what is a 1/4"-20 hi speed jet with no letter designation? There are three: Ex: 73D, 73C, & 73. To straight 73 has no counter bored hole in the jet. Just the .073" all the way through. I just used the size 73 as an example.

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    off hand i would say no letter would mean air jet, 73 seems big for air

    Part one of "Secrets of the OMC Mod 50" : http://www.boatracingfacts.com/forums/showthread.php?t=2052

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    How important are these "air jets" on OBs? I accidentally left one of a carb on my Merc triple and it ran fine.

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    not a small merc guy my self, but, air jets for the most part are for low speed, pulling air in to the idle mix wile the throttle plate is closed or almost closed. the idle circuit flows through the full curve of the moda, if it is rich or lean, it will influence the top end burn.

    Part one of "Secrets of the OMC Mod 50" : http://www.boatracingfacts.com/forums/showthread.php?t=2052

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    Had an air jet coming loose in my OMC 56ci -92, I didn't notice any change in top end but the idle suffered and the boat struggled to get on plane at all. For me those jets made a huge difference.

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    Thanks! Very interesting.

    I wonder which way they effect: a larger air jet makes for a richer
    idle mixture (or visa versa)?

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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    Quote Originally Posted by Fastjeff57 View Post
    Thanks! Very interesting.

    I wonder which way they effect: a larger air jet makes for a richer
    idle mixture (or visa versa)?

    Jeff
    The larger the air jet, the leaner the mixture. You are adding more air to the same fuel delivery.

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    Gotcha! Thanks.

    Jeff
    "We live at the bottom of an ocean of air." - General Marvage Slatington

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