Addendum to the Mikuni SBN Tuning Manual
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  1. #1
    PWCToday.com Is My Home Away From Home fox river pwc's Avatar
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    Addendum to the Mikuni SBN Tuning Manual

    Addendum to the Mikuni SBN Tuning Manual

    Where to begin?
    Over a decade ago, after having tuned a few dozen Mikuni SBN equipped skis, and after having helped explain to many others the processes I use for tuning, someone said to me, “Hey, you should write your own tuning manual…”

    Apparently, some believe that I enjoy writing ��

    I thought about it, and the idea I came up with would be to write an addendum TO the Mikuni SBN manual that has already existed (since 1994, I believe – at least my original hard copy is dated 1994). After all, the original manual itself is very good. I simply came up with some additional ideas/thoughts/techniques that expand on it.

    With that said, before even bothering to read my addendum, you really need to read and become familiar with the original “Mikuni SBN Tuning Manual”. It’s like, you don’t take Calculus until you’ve taken and passed Algebra. The manual is easy to find. Simply search “Mikuni SBN Manual”. You will find links to the free PDF, which you can download. You will find hard copies you can buy for $5-10. Just get it, read it, read it again, and read it some more, same as I did back in the 90s. No, I won’t post a link. If searching for and obtaining the manual is beyond what you can do, or are willing to do here, then carb tuning should also be out of reach. Just sayin’.

    I don’t recall exactly when I worked on writing the addendum, but I recall the job/office I was working at during that time, so it had to be 2008-2010. I had to dig through old files to find it, which I did. I never really finished it. Some of what I did still needed editing and proofreading. But I do want to share it, so my plan is to drag it into this thread as I go through and edit it, and as I create more content to be able to complete it.

    Everyone is free to ask questions along the way. Everyone is free to offer criticisms or whatever. I can take it.

    Disclaimers:
    1) Everything I’m sharing in this thread is stuff I came up with from my own firsthand experiences. It’s essentially stuff I taught myself in the years before the internet/forums even existed. It was a labor of necessity really. I’ve always been inclined to teach myself stuff as needed, for better or for worse, probably because of how things were during the years I was growing up, LOL.
    2) With that said, I am in no way saying that what I’m presenting here is the ONLY way to tune a ski. I’m not even claiming these are the best ways to tune a ski. I’m simply saying this is how I taught myself and how I do things. In fact, often tuning isn’t easy, and it takes me more time than I might like. So if there are better/faster tricks out there, I’m all ears and I’m not bothered by that.
    3) I repeat, you really need to be very familiar with the original Mikuni SBN Tuning Manual for this thread to benefit you fully.
    4) I’m a tuning geek. You will either appreciate that, hate that, or simply tolerate that. Doesn’t matter to me. I believe part of my geek-ness is because I’m a cheap person of Polish descent. Let me explain. I jokingly say, “It takes 14 dollars’ worth of little brass screws with holes in them to make your ski run absolutely perfectly”. With that said, then why would anyone spend thousands of dollars on pipes and pumps and whatever, to build the modified ski of their dreams, and then NOT take the time to determine what 14-dollar combo of jets will make it run its absolute best???
    5) Much of my addendum will be rooted in the “mindset” of tuning. The philosophy of tuning if you will. Please realize this and take it into account as you read. Sometimes I’m writing to help the reader (and myself) to get their head in the right place to be able to progress.
    6) I’m very much a “big picture” guy. I will often take 17 variables, quickly balance and weigh all of them in my head, and then produce a decision that I believe is ideal. I don’t always take the time to explain all the variables and what I’m juggling/balancing, so I might leave you wondering about something as a result – or if you are focused on just 2 or 3 of the variables you may not agree with the decision I produce. That’s fine. Just ask me to explain further, and I will as time allows.

    Ok, so here we go. My plan is to essentially post/present “segments” of what I believe should be shared. This may take me weeks or months, so please be patient. I will try to keep it as logical and orderly as possible. I promise, you will get your money’s-worth, LOLOL.
    Last edited by fox river pwc; 05-06-2022 at 12:13 PM.


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  2. #2
    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 1:

    The 5 fuel delivery circuits of the SBN.

    To begin to truly understand what you are doing while tuning, you must first understand how and when the SBN delivers fuel.

    You must understand that fuel delivery is directly related to THROTTLE POSITION……NOT rpm, not based on the powerband, not anything else. It’s all about throttle position.

    Then you must understand what circuits deliver fuel and what throttle positions. Once you understand this, you can then determine at what throttle position your engine isn’t doing what it should, so that you can then adjust the appropriate fuel delivery circuit.

    Got it?

    Here are the 5 fuel delivery circuits, in order.

    1) Low speed screw. The low speed screw delivers fuel from zero throttle to WOT (wide open throttle). It is your primary adjustment for getting your best idle and best motor start up performance.
    2) Pop off circuit. The pop off circuit consists of the needle & seat plus the pop off spring. The pop off helps with adding extra fuel and the motor transitions from running off the low speed screw only to also adding the Pilot jet. Pop off has a lot to do with throttle response, especially at lower throttle positions.
    3) Pilot jet. The pilot jet delivers fuel from ¼ throttle to WOT. The combo of pilot jet and pop off has much to do with throttle response as well, as these 2 circuits essentially “overlap”. I will dig into this much later, as there is a lot of confusion with this, even for me sometimes, LOL, but I’ve come up with ways to eliminate the confusion.
    4) Main jet. The main jet delivers fuel from ½ throttle to WOT. More on this later, but a common issue I see with skis that are “gargly” when riding aggressively are due to main jets being too large. Lots more on this later…
    5) High speed screw. The high speed screw delivers fuel from ¾ throttle to WOT. It’s arguably a “non-factor” when it comes to throttle response. It’s primary use it to achieve your max top speed and max RPMs. Screwing (pun intended) with the high speed screw adjuster while trying to improve throttle response is essentially a complete waste of time.

    End Segment 1
    Last edited by fox river pwc; 05-06-2022 at 12:28 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...

  3. #3
    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 2:

    My #1 rule of tuning:
    If you can't make a small jetting change that demonstrates you can make it run worse, then there is no way you can say it is absolutely tuned as well as it can be tuned...because you simply don't know.
    Part of how I taught myself this process/rule decades ago was to make a jetting change on a ski I believed was "tuned great" only to find that I made it even better with said change.

    Another way to explain this is to say that as I’m tuning, I am typically going in a certain direction. For example, I may be increasing the pilot jet size in steps as my testing is showing that motor continues to want more and more fuel while doing the ¼ throttle to WOT test (more on specific testing). As I go along, I will increase the pilot jet size until I can get the motor to gargle during the test – which is the motor’s way of telling me, “Hey dude, that’s too much fuel!!!” So, then I will back the jet size back down to the previous size that produced the best results.

    We could be talking about incredibly minor differences here, but I’m a tuning geek, so these are the differences that matter. The point is that it kept getting better along the way, until it got worse, AND I was able to demonstrate this to myself – so now I KNOW what pilot jet size is now best, and I’m on my way to declaring the ski “tuned”.

    I did say this process takes time. I should also mention that it takes patience. It also helps to have a notebook and a pen (more on that later too).

    End Segment 2
    Last edited by fox river pwc; 05-06-2022 at 12:40 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...

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

    Thanks for taking the time Paul. Great stuff.

  5. #5
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Sub'd!

  6. #6
    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 3:

    My #2 rule of tuning:
    Never change more that one thing at a time.
    I commonly say that making mistakes is great if you never make the same mistake twice. In other words, learn from your mistakes and never repeat them.
    When it comes to this rule, I needed to break it a few times before REALLY learning my lesson, LOL.
    Impatience got the best of me, and I have tried changing more than one carb setting at a time, especially on motors where pulling the carbs, making the change, and reinstalling the carbs can take a good hour. I promise you, from my own failures at this, it isn’t worth it.
    No matter how strong the temptation is to change more than 1 thing, don’t do it.
    Not only will it often keep you from getting your tuning “right”, but it also really keeps you from learning anything valuable and often just sets you back a step anyhow.
    Resist the temptation – period.
    One change at a time. Test. Evaluate. Take notes. Repeat.
    Last edited by fox river pwc; 08-05-2022 at 03:55 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...

  7. #7
    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 PrickofMisery View Post
    Thanks for taking the time Paul. Great stuff.
    Quote Originally Posted by Quinc View Post
    Sub'd!

    Thanks guys, and you're welcome.

    Someday, years from now, I might just google a tuning question and find this thread.

    Oh, and I just realized that thanks to bidenflation I need to edit my sayings about "why wouldn't you spend 14 dollars on jets to get your sh1t tuned right...???" I guess I should be saying something more like 64 dollars


    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...

  8. #8
    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 4:

    A properly tuned motor is a healthy motor and lives the longest life possible.

    What do I mean by this? Well, I’ve frequently heard things like “I’m keeping it rich, to be safer…” or some variation of a statement that implies “richer is better/safer”. This is a dangerous belief IMHO. Yes, too lean is bad, but so is too rich. Now, I’m not a metallurgist or an educated Engineer, so I’m not going into major details on this other than to say this VERY broad statement = Lean burns pistons and rich damages bores.

    I’m assuming most everyone reading this has seen pictures of a piston that was damaged by a lean condition. We all know this happens. But I think it is less common to see or possibly know about the damage that overly rich conditions can cause. Too rich, meaning an engine that can’t effectively burn all the fuel it is being fed (and is often gargly sounding), can damage itself as the excess fuel wipes out the lubricating properties of the oil. The damage occurs to the cylinder bores (as wear on the bores) and can be visible as the cross hatching gets wiped out of the cylinder, and you lose compression.

    Point being = tuned perfectly is the safest place to be. Anything in either direction only sacrifices longevity.

    To be clear, I’m not trying to quantify exactly how lean is “too lean” or how rich is “too rich” or even how much worse or better one is over the other. I’m simply saying that getting your motor tuned perfectly will maximize its lifespan, and as a bonus, it also maximizes the power it makes. Crazy hey? You can get ALL the longevity and ALL the horsepower at the very same time with same state of tune.


    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...

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

    Quote Originally Posted by fox river pwc View Post
    Segment 1:

    The 5 fuel delivery circuits of the SBN.

    To begin to truly understand what you are doing while tuning, you must first understand how and when the SBN delivers fuel.

    You must understand that fuel delivery is directly related to THROTTLE POSITION……NOT rpm, not based on the powerband, not anything else. It’s all about throttle position.

    Then you must understand what circuits deliver fuel and what throttle positions. Once you understand this, you can then determine at what throttle position your engine isn’t doing what it should, so that you can then adjust the appropriate fuel delivery circuit.

    Got it?

    Here are the 5 fuel delivery circuits, in order.

    1) Low speed screw. The low speed screw delivers fuel from zero throttle to WOT (wide open throttle). It is your primary adjustment for getting your best idle and best motor start up performance.
    2) Pop off circuit. The pop off circuit consists of the needle & seat plus the pop off spring. The pop off helps with adding extra fuel and the motor transitions from running off the low speed screw only to also adding the Pilot jet. Pop off has a lot to do with throttle response, especially at lower throttle positions.
    3) Pilot jet. The pilot jet delivers fuel from ¼ throttle to WOT. The combo of pilot jet and pop off has much to do with throttle response as well, as these 2 circuits essentially “overlap”. I will dig into this much later, as there is a lot of confusion with this, even for me sometimes, LOL, but I’ve come up with ways to eliminate the confusion.
    4) Main jet. The main jet delivers fuel from ½ throttle to WOT. More on this later, but a common issue I see with skis that are “gargly” when riding aggressively are due to main jets being too large. Lots more on this later…
    5) High speed screw. The high speed screw delivers fuel from ¾ throttle to WOT. It’s arguably a “non-factor” when it comes to throttle response. It’s primary use it to achieve your max top speed and max RPMs. Screwing (pun intended) with the high speed screw adjuster while trying to improve throttle response is essentially a complete waste of time.

    End Segment 1

    One small critique, I would take issue with the separation of high speed jet and high speed screw. They are two parallel paths from the same place to the same place. Not seeing how they can have different effects other than course vs fine adjustment of the same carb circuit.

    SBN high speed.JPG

  10. #10
    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

    Screenshot_20220506-163101_Drive.jpgScreenshot_20220506-163150_Drive.jpg

    Look closer at the manual.
    I would add that leaning out the high speed adjuster has never helped to correct a rich main jet condition effectively.


    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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