1992 750sx 44 SBN mikuni jetting with factory pipe - Page 2
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  1. #11
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Quote Originally Posted by Thorny View Post
    Hello all,

    The boat runs fine except from 0-30% throttle. Not as bad if I roll throttle on slowly

    but if I stab it it basically hesitates for a few seconds and then goes.
    Your "stab it" test is not a valid way to test any carburetor. We see similar
    threads once or twice a year. You are creating a massive lean condition that
    the carburetor can not instantly respond too.


    Bill M.
    Horsepower == Speed, RPM != Speed



  2. #12
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Quote Originally Posted by wmazz View Post
    Your "stab it" test is not a valid way to test any carburetor. We see similar
    threads once or twice a year. You are creating a massive lean condition that
    the carburetor can not instantly respond too.


    Bill M.
    Hi Bill. I’m not sure if you watched the video I posted. In my opinion this is not normal or acceptable. As far as my test goes, this is straight from the Mikuni manual.
    C9A9AB5B-4AA5-407D-98E1-B5DF0F260C52.jpeg
    Last edited by Thorny; 12-03-2021 at 11:24 AM.

  3. #13
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    I would rather explain how to power tune your ski while on a trailer.

    1. Throttle response should be crisp, with no hesitation.
    2. Ride the boat at a constant 1/4 throttle opening for about 1 minute
    and then quickly open the throttle fully, there should be no hesitation and
    the engine should not show signs of being loaded up. If it hesitates, it's
    lean; if it's loaded up, it's rich. The first test is to check pop-off pressure,the
    second test is for checking the correctness of the low speed jet size.
    There are limits to #1

    #2 does not apply to you.


    Bill M
    Horsepower == Speed, RPM != Speed



  4. #14
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    I don't remember the FPP 750 limited pipe, but at PJS we had a
    real ugly canister pipe we used on the PJS Pro Women's 750. That
    pipe was probably a FPP pipe. If it is the same, your Solas 11/16
    might be a good impeller and a 10/18 may be too large. Some
    pictures would be helpful.

    I don't like test #2 because it is operating in the high limits of the
    low speed circuit. The transition from the low speed to main jet has
    some overlap, but there is a lean spot during that transition. That lean
    spot occurs on many different types of carbs.

    The SBN-i carbs were designed to improve that lean transition, and
    it would be less noticeably on 38, 34mm sbn's.


    You also mentioned:
    As you modify or change your watercraft's flame arrestor to a less
    restrictive type you will most likely start to experience a lean hesitation
    caused by a decrease in manifold pressure. This change will require an
    adjustment in pop-off pressure to regain crisp throttle response. Because
    most aftermarket flame arrestors are less restrictive than stock, you will
    need to decrease pop-off to compensate.
    This doesn't apply to your 44, nor does it apply to the stock Keihin carb
    because the 750 had a free flowing flame arrestor.

    While I was at PJS, the SBN had just came out and we had a stack of those
    manuals in the parts department. I read, and tried many of the tips in that text.
    Within a few years after its release (~1991) technicians and dealers learned
    what worked and which portions of the manual to ignore.



    Bill M.
    Horsepower == Speed, RPM != Speed



  5. #15
    PWCToday.com Is My Home Away From Home fastgtfairlane's Avatar
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Quote Originally Posted by Thorny View Post
    Ok thanks. Iím running that exact jetting but Iím running 2.5 N/S with 80 gram spring. I have a 1.5 N/S. Iíll try it. Iím running a Solas 11/16 Concorde for prop.
    What flame arrestor are you running? I'm running a 750 sp in a x2 with a fpp pipe as well. I'm running a 2.0 n/s with a 95g spring. Measures 25psi. If you change the popoff it's also a good idea to verify the pressure with a gauge. The video sounds like a lean hesitation off idle, right in the popoff circuit. Ideally, you want the popoff pressure as high as possible with no lean hesitation for best throttle response. Different ways to accomplish this. Some go with small n/s and lower spring, some will use a larger seat with stiffer spring. I prefer stiffer spring as engine vibration can cause fuel to dribble with a lower spring. I tried a 80g and 95g spring on mine and liked the 95g better. I'm running a oem single Yamaha flame arrestor with 1 screen.
    96 superjet- rockered, -4, wide tray, adjustable footholds, kv148 mag setback pump, skat 7/14 impeller, rrp adjustable pole, blowsion ovp steering, 62t/62t, ada head, zeel A02T cdi, dual 44s, boysen power reeds, TNT SS chamber

    93-x2, newmiller ported SP750, milled head, R&D single 46 intake, sbn46, fpp blaster chamber pipe, hooker 9/15, umi, rhaas squirrel cage, dakine foot straps

  6. #16
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Quote Originally Posted by fastgtfairlane View Post
    What flame arrestor are you running? I'm running a 750 sp in a x2 with a fpp pipe as well. I'm running a 2.0 n/s with a 95g spring. Measures 25psi. If you change the popoff it's also a good idea to verify the pressure with a gauge. The video sounds like a lean hesitation off idle, right in the popoff circuit. Ideally, you want the popoff pressure as high as possible with no lean hesitation for best throttle response. Different ways to accomplish this. Some go with small n/s and lower spring, some will use a larger seat with stiffer spring. I prefer stiffer spring as engine vibration can cause fuel to dribble with a lower spring. I tried a 80g and 95g spring on mine and liked the 95g better. I'm running a oem single Yamaha flame arrestor with 1 screen.
    Makes sense. Thanks for sharing. I’m running the tau ceti flame arrestor I tried the 1.5 N/S that was recommended earlier in the thread and my ski wouldn’t even run. To be fair it was an old one I had laying around so maybe the oring was worn.
    30C12364-C800-4D5F-945D-B740D6920D89.jpeg
    Last edited by Thorny; 12-04-2021 at 12:24 PM.

  7. #17
    PWCToday.com Is My Home Away From Home wmazz's Avatar
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    JonnyX2 had good single 44mm, and 46mm jetting specs.

    The Mikuni manual's method (riding the ski to test the low jet) does not give you
    more than a rough idea what to change. The Mikuni manual makes tuning more
    difficult than it needs to be.


    A single 44 is an easy power tune, and during the power tune you will know if
    the low jet is rich or lean before you test the acceleration.


    Bill M.

    Reusing the carb base gasket is a bad practice when you are trying to
    determine the jet specs. It can cause poor results that have nothing
    to do with your carb specs.

    Make your own thick gaskets with felpro 1/16" ruberized felt.
    Last edited by wmazz; 12-04-2021 at 07:04 PM.
    Horsepower == Speed, RPM != Speed



  8. #18
    PWCToday.com Is My Home Away From Home fastgtfairlane's Avatar
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    I always use a thin layer of assembly goo on all gaskets so if I have to remove something, the gasket wont tear. Since you have the open filter style arrestor, try the 2.0 n/s and a black 80g spring.
    96 superjet- rockered, -4, wide tray, adjustable footholds, kv148 mag setback pump, skat 7/14 impeller, rrp adjustable pole, blowsion ovp steering, 62t/62t, ada head, zeel A02T cdi, dual 44s, boysen power reeds, TNT SS chamber

    93-x2, newmiller ported SP750, milled head, R&D single 46 intake, sbn46, fpp blaster chamber pipe, hooker 9/15, umi, rhaas squirrel cage, dakine foot straps

  9. #19
    Attention ***** PWCToday.com Is My Home Away From Home JonnyX2's Avatar
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Good known start-point jetting for a single 44 w a pipe is 2.0NS 80G (black) 120P 120m start with both at 1-1/4 turn open. Tune from there. Both the 650s and the 750s used identical jetting, tho the 650s wanted a larger main for whatever reason.
    Quote Originally Posted by Firebird A/C&Heating View Post
    OMG.....Rules for 550 vintage ski class...550 ski riders do not conform to any kind of rules. That is why you are riding a 550 ski in the first place. Rules suck....
    PS...the rule book will be in my 550 pump
    Quote Originally Posted by WB1994
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  10. #20
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    Re: 1992 750sx 44 SBN mikuni jetting with factory pipe

    Quote Originally Posted by JonnyX2 View Post
    Good known start-point jetting for a single 44 w a pipe is 2.0NS 80G (black) 120P 120m start with both at 1-1/4 turn open. Tune from there. Both the 650s and the 750s used identical jetting, tho the 650s wanted a larger main for whatever reason.
    Ok thanks. I’ll give that jetting a shot. I tested the ski again today. I’m wondering if my problem has to do with the waterbox loading up with excess water at slow speeds. Maybe I need to divert some water away from the headpipe to a second pisser. The bogging/hesitation is much worse when I get the boat off plane and ride at a slow speed and then try and gas it. It is especially noticeable after I come in from a ride and idle back to shore, then
    the next time I go back out for a ride it takes a good bit to clear it out and get it to respond.
    my old 93 750sx did not have this issue so I’m puzzled by it. I dug up some video of me doing the same stab the throttle test with instant acceleration and throttle response in 1994.

    Last edited by Thorny; 12-05-2021 at 03:10 AM.

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