Mikuni SBN Hi Speed circuit question
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  1. #1
    I dream skis jetskidude's Avatar
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    Mikuni SBN Hi Speed circuit question

    Just a discussion and would like others opinions.. I usually put the recommended jets and tweek the hi speed adjuster to get it tuned. But looking at the Hi Speed circuit of the Mikuni SBN flow diagram, both the Hi Speed jets and the Hi Speed Adjuster flow out the same port of the Inner Verture. Wouldn't you get the same results by using a smaller Hi Speed Jets and open the Hi Speed Adjuster more and vise verses using a bigger Hi Speed Jets and close the Hi Speed Adjust? Never tried it. But, looking at the flow diagram, the Hi Speed Jets don't seem too critical as you can tune it with the Hi Speed Adjuster. what makes the Hi Speed adjuster for adjusting 3/4 to Full throttle when you can just put a bigger Hi Speed Jets and totally close the Hi Speed Adjuster. Am I missing something?
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    Last edited by jetskidude; 05-28-2021 at 02:48 PM.
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  2. #2
    Top Dog JC-SuperJet's Avatar
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    Re: Mikuni SBN Hi Speed circuit question

    Seadoo installs Main Jets so that the High Speed Adjuster is Closed (Zero Turns).

    But this means you can't reduce the Fuel Amount on very Humid and Hot Days where the Oxygen is reduced, and will then run Rich.

    Adjustment Screws are Fine Tuning Tools and allow optimum performance for many Weather (Air Density) conditions.

    Be grateful these Adjustments Screws are a Standard Feature.

  3. #3
    I dream skis jetskidude's Avatar
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    Re: Mikuni SBN Hi Speed circuit question

    Thanks JC-SuperJet. that is kind of what I thought. the adjuster is a fine tuning for the Hi Speed circuit. for example: I have 130 HS Jets and only 1/2 a turn on my HS Adjuster. I was thinking of trying a maybe 120 Jets and turn out my adjuster more like 1 or more turn as they all flow out the same inner venture port.
    Last edited by jetskidude; 05-28-2021 at 03:22 PM.
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  4. #4
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    Re: Mikuni SBN Hi Speed circuit question

    Going a little off topic here but hoping that maybe can someone can clue me in a bit. I Ride in Southern California at the beach at Sea level. Every summer I go to the high sierra mountains and sit at 8,000 ft above sea level. This makes my jet ski run not so great. I'm wondering if adjustments can be made via these screws to help the situation or if I actually need to take the carburetors out and put in a different pilot and Main jet. If anybody has experience in this stuff I would really appreciate some feedback. Thank you!

    By the way, I have a pair of 96 wave ventures

    Sent from my GM1917 using Tapatalk
    Last edited by Delightfor; 05-28-2021 at 03:40 PM.

  5. #5
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    Re: Mikuni SBN Hi Speed circuit question

    Quote Originally Posted by Delightfor View Post
    Going a little off topic here but hoping that maybe can someone can clue me in a bit. I Ride in Southern California at the beach at Sea level. Every summer I go to the high sierra mountains and sit at 8,000 ft above sea level. This makes my jet ski run not so great. I'm wondering if adjustments can be made via these screws to help the situation or if I actually need to take the carburetors out and put in a different pilot and Main jet. If anybody has experience in this stuff I would really appreciate some feedback. Thank you!

    By the way, I have a pair of 96 wave ventures

    Sent from my GM1917 using Tapatalk
    In your case you want to power tune the main jet so the top adjustment screw ends up 1.5 turns out in the ocean. You should be able to close it enough to work at high altitude. The process of getting this right will make you a jetting expert. Not many jetski operators can do this.
    What exactly does the IJSBA do?

  6. #6
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    Re: Mikuni SBN Hi Speed circuit question

    Quote Originally Posted by kweasel View Post
    In your case you want to power tune the main jet so the top adjustment screw ends up 1.5 turns out in the ocean. You should be able to close it enough to work at high altitude. The process of getting this right will make you a jetting expert. Not many jetski operators can do this.
    Thank you Kweasel,

    It sounds like I will not have to take out the carburetors to get better performance at 8,000 ft. Is this top adjustment screw accessible from below the seat? In other words a screw that I can easily access from under the hood so to speak? Is it in the picture above in this thread? Thank you!

    Sent from my GM1917 using Tapatalk

  7. #7
    Top Dog JC-SuperJet's Avatar
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    Re: Mikuni SBN Hi Speed circuit question

    Here's a Guide to adjusting the High Speed Circuit for Altitude changes . No need to change the Main Jets.

    Carb Altitude Adjustment.jpg

  8. #8
    I dream skis jetskidude's Avatar
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    Re: Mikuni SBN Hi Speed circuit question

    Quote Originally Posted by kweasel View Post
    In your case you want to power tune the main jet so the top adjustment screw ends up 1.5 turns out in the ocean. You should be able to close it enough to work at high altitude. The process of getting this right will make you a jetting expert. Not many jetski operators can do this.
    With whats been said in mind... what is the ideal Hi Speed Adj setting or is there any? I'm currently running #135 HS Jets and with only 1/2 turn out HS Adjuster on my SJ with 38's and a Bpipe pretty well tuned in cold weather but a tad rich in the summer. Should I go down to maybe #130 so I can open up the HS Adjuster for more room to play with?
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  9. #9
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    Re: Mikuni SBN Hi Speed circuit question

    The carb does not care if you have a smaller high jet with the screw further out or larger high jet with screw further in, if the total fuel flow is the same it makes no difference. Most people shoot for about a turn out on the adjuster because that puts it about the middle of its range.

    The low speed screw is a different story.
    Last edited by Storbeck; 05-29-2021 at 04:10 PM.

  10. #10
    I dream skis jetskidude's Avatar
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    Re: Mikuni SBN Hi Speed circuit question

    Thanks for all the comments, I feel the same as both the hi jet and hi adjuster works together as one circuit. The reason I post this question is because the Mikuni SBN Manual tuning steps shows them as 2 separate circuit which I feel is incorrect.
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