Addendum to the Mikuni SBN Tuning Manual - Page 3
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  1. #21
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Segment 8:

    The order of tuning.

    The basic and broad statement here is that you need to tune from the bottom up. In other words, you don’t start with determining which main jet or what high speed adjuster setting is best. You start with getting the idle as good as it can be, via the low speed adjuster and the idle stop screw, then move on to determining the best pop-off, then the pilot jet, etc…

    The reason for this is because these “lower in the throttle position” fuel delivery circuits all affect the overall fuel delivery at higher throttle settings. A simple example would be that a pilot jet change COULD then affect performance at half throttle and therefore require a subsequent main jet change, but changing the main jet isn’t going to affect how much fuel is delivered at throttle.

    Now, it is still critical to realize that if you are starting off with main jet(s) that is/are WAY out of the range of what will ultimately turn out to be ideal, well, you’re in for some additional struggles. This is where things get nitpicky. For example, when doing the tests I defined in Segment 6 that you REALLY need to pay attention to what the motor does when you INITIALLY grab that throttle. We’re talking milliseconds here.

    Let me explain with an example. Let’s say you’re doing the throttle test to determine your ideal pilot jet size, but the pilot jet is slightly too small, and the main jet is currently much too large. When you grab WOT, you may just get a millisecond of barely discernible hesitation followed by a rich gargle. This can be very difficult to read. I’ve been there. You may not figure this out until you’ve made multiple pilot jet changes – which can be a clue, but there must be a better way, right? This is why I talk about motors running gargly-rich (during “playful” riding) are commonly due to the main jet(s) being too large. If your ski meets this description, drop the main jet size and continue tuning your lower settings, hopefully much more productively.
    Last edited by fox river pwc; 05-11-2022 at 11:16 AM.


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  2. #22
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Fantastic thread so far and you've basically confirmed some thoughts I've had while I still seem to struggle with my own tuning! I did want to comment on the HS jet vs. HS screw thing that throws people into a loop.

    The reason why the HS screw handles 3/4 and up is because the flow characteristics of a fixed orifice (HS jet) versus a needle orifice (HS screw) are different. A fixed orifice will flow about the same regardless of the signal strength whereas a needle orifice will flow more and more fuel with increasing signal.

    As expected, as the throttle valve position is increased, the signal to the LS circuit will initially drop until the air velocity through the carb increases and starts providing adequate signal to the HS circuit (transition circuit). This means the signal (or vacuum to the carb if you will) to the HS circuit will increase as the RPMs climb. There comes a point where the HS needle circuit starts to flow more and more fuel into the HS circuit since the signal is increasing all while the HS jet has been providing nearly the same fuel flow throughout the range the HS circuit is active.

    This all boils down to the fact that the HS screw is in fact a needle valve. It is known that needle valves will flow more with increasing differential pressure. This is exactly what signal is....the differential pressure between the inner venturi vs atmospheric pressure.

    This is my own understanding of why the HS screw is used for the upper throttle opening tuning. I'd like to hear another argument but I have yet to find one other than "I think the Mikuni manual is wrong."

  3. #23
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Quote Originally Posted by hyoctane23 View Post
    Fantastic thread so far and you've basically confirmed some thoughts I've had while I still seem to struggle with my own tuning! I did want to comment on the HS jet vs. HS screw thing that throws people into a loop.

    The reason why the HS screw handles 3/4 and up is because the flow characteristics of a fixed orifice (HS jet) versus a needle orifice (HS screw) are different. A fixed orifice will flow about the same regardless of the signal strength whereas a needle orifice will flow more and more fuel with increasing signal.

    As expected, as the throttle valve position is increased, the signal to the LS circuit will initially drop until the air velocity through the carb increases and starts providing adequate signal to the HS circuit (transition circuit). This means the signal (or vacuum to the carb if you will) to the HS circuit will increase as the RPMs climb. There comes a point where the HS needle circuit starts to flow more and more fuel into the HS circuit since the signal is increasing all while the HS jet has been providing nearly the same fuel flow throughout the range the HS circuit is active.

    This all boils down to the fact that the HS screw is in fact a needle valve. It is known that needle valves will flow more with increasing differential pressure. This is exactly what signal is....the differential pressure between the inner venturi vs atmospheric pressure.

    This is my own understanding of why the HS screw is used for the upper throttle opening tuning. I'd like to hear another argument but I have yet to find one other than "I think the Mikuni manual is wrong."
    Thanks for the comment and clarification.
    Glad you are enjoying the thread thus far.
    I do see some value in understanding the "why" in the fact that the high speed adjuster and main jet behave as they do = affecting the fuel delivery at different throttle positions, but for the purposes of this thread, I am more concerned with "how" to effectively utilize both adjustments to achieve your ideal state of tune.


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  4. #24
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Addendum to the Mikuni SBN Tuning Manual

    As for the struggle of tuning, I'm hoping to hit on many more topics to help with that. Not saying I can make it easy for everyone all the time, but I think some of my ideas will help to minimize the struggle for others as they have for me.


    WHEN IN DOUBT, GAS IT!

    Yeah, I'm an @sshole, but I'm not a complete @sshole.

    http://badgerlandjetpilots.com/

    I don't come here to make enemies - only to identify them...

  5. #25
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Funny side note on the whole "HS jet vs. HS screw thing throwing people for a loop..." is that I literally had completely forgotten about that "loop" for at least the past 20 years, until I was reminded in this thread. Gotta remember that most of my "book learning" on tuning came before the internet and well before forums, but I recall having phone discussions about that back in the 90s, and once I effectively got deep into the realities of firsthand tuning - which have repeatedly demonstrated to me that the manual has it right...I just stopped thinking about it. It was a bit of a trip down memory lane to think about it all over again.
    Last edited by fox river pwc; 05-11-2022 at 02:06 PM.


    WHEN IN DOUBT, GAS IT!

    Yeah, I'm an @sshole, but I'm not a complete @sshole.

    http://badgerlandjetpilots.com/

    I don't come here to make enemies - only to identify them...

  6. #26
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Quote Originally Posted by fox river pwc View Post
    Thanks for the comment and clarification.
    Glad you are enjoying the thread thus far.
    I do see some value in understanding the "why" in the fact that the high speed adjuster and main jet behave as they do = affecting the fuel delivery at different throttle positions, but for the purposes of this thread, I am more concerned with "how" to effectively utilize both adjustments to achieve your ideal state of tune.
    I may have not gotten everything 100% correct in my post but I believe the gist of it is correct. I suppose my mind struggles to apply tuning knowledge or procedures without knowing the "why" certain things do what they do. At least to me, it is just as valuable to know the reasons behind why making certain adjustments have a particular outcome so I can see if that real outcome of that adjustment is actually as predicted. Perhaps it is outside the scope of your thread but it was for sure an enlightening moment to me when I discovered the correlation and I was able to explain it in my mind. I can certainly delete my post if you'd like as to not muddy up your thread.
    Last edited by hyoctane23; 05-11-2022 at 05:03 PM.

  7. #27
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Addendum to the Mikuni SBN Tuning Manual

    No worries at all.
    It's all good.
    This thread was never gonna be short anyhow, lol.


    WHEN IN DOUBT, GAS IT!

    Yeah, I'm an @sshole, but I'm not a complete @sshole.

    http://badgerlandjetpilots.com/

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  8. #28
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Had a few people bugging me about this thread, and the pause in the progress, LOL...I promise I haven't forgotten about it. There is much more to come. I'm working on it as time allows between my real job and trying to get ready for JB22.


    WHEN IN DOUBT, GAS IT!

    Yeah, I'm an @sshole, but I'm not a complete @sshole.

    http://badgerlandjetpilots.com/

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  9. #29
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Thank you, Paul.. Great info.
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  10. #30
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Quote Originally Posted by Yamabooeey View Post
    Thank you, Paul.. Great info.
    You're welcome, Norm.


    WHEN IN DOUBT, GAS IT!

    Yeah, I'm an @sshole, but I'm not a complete @sshole.

    http://badgerlandjetpilots.com/

    I don't come here to make enemies - only to identify them...

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