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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.

  6. #6

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    So, I'll add a little more to this thread... Since using the 6.9 x 1.3 o-ring on an OEM needle/seat, which worked well, I decided to try a needle & seat from SBT. At the time, the SBT needle/seat didn't come with an o-ring or filter screen. I measured the SBT and the OEM seats and found they have a different diameter in the o-ring groove.

    OEM is 7.27 mm in the o-ring groove
    SBT is 6.93 mm - 6.96 mm in the o-ring groove

    In addition to the Keihin o-rings I mentioned above, I also ordered some Buna and Viton o-rings from an o-ring supplier in the size 6.5 mm ID x 1.5 mm C.S. (JIS S7) The vitons were like 10 for $2 and the Bunas were like 50 for $2.

    So I tried the SBT seat with a 6.9 x 1.3 and it is working fine in my jet ski. I've probably put about 10 hours on it since then.

    On the bench, I tried the 6.5 mm x 1.5 mm ones for the heck of it and found they were too tight with the OEM seat, but fit right in with the SBT seat.

    Since the SBT needle/seat worked fine, I ordered two more for when I need them in the future. Now, a few weeks later since the first order, they do come with an o-ring and screen. However, the o-ring they come with measures only 1.2 mm C.S. That seems under sized to me since the o-ring groove diameter in the seat itself is smaller than OEM.
    When I put them in eventually, I am going to remove the thin o-ring they come with and use the thicker 1.5 mm C.S. o-ring.

    So far the SBT brand is working fine though.

    That's my update... but, anyone considering the use of this info should do their own research and testing before going forward
    Last edited by FL_Rider; 10-01-2019 at 08:30 AM.

  7. #7
    Quote Originally Posted by FL_Rider View Post
    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.
    The fuel pump I got from Jetskisolutions came with four o-rings. Two for each hole on the back of the pump. They told me to stack them. The reason is the stock o-rings are proprietary and very unusual. Like you said they are a square type o-ring. I've never seen anything like them before. I installed the pump with the two o-rings stacked on top of each other and so far no leaks. I've never stacked o-rings before but they told me that was the only solution since it is next to impossible to find the OEM ones. I checked the existing two pumps I had on hand and their o-rings were dry rotted. One set might have worked but I decided to try the new stacked o-rings instead. Hopefully they hold up.

  8. #8
    Where can I get three of the 2.8mm x 1.2mm o-rings for the pilot/low speed adjustment? What material should they be made of?

    I can't find them anywhere. OEM only seems to sell them with a complete pilot assembly.

  9. #9

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    Quote Originally Posted by beekermartin View Post
    The fuel pump I got from Jetskisolutions came with four o-rings. Two for each hole on the back of the pump. They told me to stack them. The reason is the stock o-rings are proprietary and very unusual. Like you said they are a square type o-ring. I've never seen anything like them before. I installed the pump with the two o-rings stacked on top of each other and so far no leaks. I've never stacked o-rings before but they told me that was the only solution since it is next to impossible to find the OEM ones. I checked the existing two pumps I had on hand and their o-rings were dry rotted. One set might have worked but I decided to try the new stacked o-rings instead. Hopefully they hold up.
    Thanks for the info on the stacked o-rings. I was thinking about getting square rings and stacking them but I didn't know if it would work. I could find square rings, but the height and width were the same size. On the fuel pump, the square ring height needs to be about double the width. Cool...now I know stacking has been tested.


  10. #10

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    Quote Originally Posted by beekermartin View Post
    Where can I get three of the 2.8mm x 1.2mm o-rings for the pilot/low speed adjustment? What material should they be made of?

    I can't find them anywhere. OEM only seems to sell them with a complete pilot assembly.
    I got the Keihin o-rings from newmotorcycleparts dot net.
    Part OR-4619 is the 2.8 mm x 1.2 mm
    Part OR-4623 is the 6.9 mm x 1.3 mm
    They are Buna NBR.

    I got the 6.5 mm x 1.5 mm o-rings from oringsandmore dot com. They have them in Buna and Viton.

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