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  1. #1

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    Ultra 150 carburetor o-ring sizes

    Anyone know the factory specs/sizes for the needle seat o-ring and the idle mixture screw o-ring in an ultra 150? I ordered a couple different sizes for both things, but it would be great to learn the sizes the factory used and eliminate any guesswork. I measured them with a digital caliper, but they may have shrunk or deformed over the years.

    Here's what I ordered:
    Needle seat o-ring:
    6.9 mm x 1.3 mm
    5.7 mm x 1.3 mm

    Idle mixture screw o-ring:
    2.8 mm x 1.3 mm
    2.8 mm x 1.2 mm

    Thanks

  2. #2
    steve45's Avatar
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    Welcome aboard, FL_Rider!

    They're proprietary. The fuel rail O-rings also fit the needle seat. Don't know about the mixture screw O-rings.

    I've been running some Viton O-rings that are slightly smaller. So far, they seem to work OK, but I want to test them some more before I recommend them.
    Last edited by steve45; 08-26-2019 at 09:40 PM.

  3. #3

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    Thanks for the info on the fuel rail sizes. If the o-rings I ordered fit right and don't leak, I'll post up the sizes that worked.

  4. #4

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    Ok, here's the update.

    The 6.9 mm x 1.3 mm fit perfect on the fuel rail and the needle seat. When the parts are installed, they go in snug without too much resistance.
    The 2.8 mm x 1.2 mm fit perfect on the mixture screw and the screw installs nicely. Using the 2.8 mm x 1.3 mm, it goes in with too much resistance.


    After searching the internet high and low, this is what I found. Apparently, there are a only a relatively low number of o-ring sizes used in all Keihin carbs dating all the way back to the 70's for watercraft and motorcycles. It appears the fuel rail and needle seat o-rings sizes are used in a bunch of other applications than just the ultra 150. And, it appears that the Keihin carbs use NBR Buna o-rings.

    So, the needle seat o-ring passed the pressure test and the hour long test ride on the water today. I didn't take apart my running carbs any further that, so the fuel rail and mixture screw o-rings haven't been tested yet on the water. I did however check the fit on my extra set of carbs, and the fit was good.
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  5. #5

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    Anybody know how to get the o-rings for the fuel pump? It looks they are square o-rings or U-rings. I checked a site that sells o-rings for Keihin and Mikuni carbs and they have some U-rings that seem to be in the size ballpark, but I don't know. The ones I took out look squared off on both sides.


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