Addendum to the Mikuni SBN Tuning Manual - Page 2
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  1. #11
    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

    But I get how the diagram makes it look that way.

    The low speed adjuster and pilot jet circuits are similar in that regard.

    And technically, the main jet starts delivering fuel at 3/8 throttle. I just got in the habit of saying 1/2 due to the 1/2 throttle to WOT test.


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  2. #12
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    Re: Addendum to the Mikuni SBN Tuning Manual

    I'm not looking at the manual I'm looking at the diagram and the physical carb, the high speed jet and the high speed screw can not physically do different things because they are connecting the same area of the carb to the subventuri. The text in the manual saying that they affect different throttle positions is wrong.

    The low speed jet and pilot screw are different because they are not in parallel. All of the low speed fuel goes through the low speed jet and some of it also goes in series through the low speed screw. The low speed screw only affects the progression hole that is downstream of the throttle blade when the blade is fully closed.

  3. #13
    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

    The manual doesn't fully explore signal strength, differential pressure...and how a needle valve performs versus a fixed jet, but even I'm not diving into that here, as I'm more focused on the results of the reality I deal with consistently.

    I can say that when I'm performing the 1/2 throttle to WOT test, and I get an instantaneous gargle, dropping the main jet size is the most effective way to cure the gargle caused by being too rich. In fact, in most cases, leaning out the high adjuster doesn't eliminate the gargle at all and only tends to then hurt top speed/max rpm. And, when I've been tuning and found 1/2 throttle testing to reveal weak throttle response and/or weak pull, caused by the main jet being too small, not only has opening the high adjuster not helped to correct that issue at half throttle, it tends to cause a rich gargle that only occurs above 3/4 throttle. Long story short, I've never found that the high speed adjuster is effective at curing rich or lean issues that occur at half throttle...certainly not nearly as effectively as changing the main jet size is at curing those issues. If your tuning experiences have shown otherwise, I'd be curious to hear about specific examples and the details, and maybe even see the piston wash results.
    Last edited by fox river pwc; 05-07-2022 at 04:53 PM.


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  4. #14
    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

    I guess another way to explain what experience has show me would be:
    Say we have a 701 61x modded/piped ski that is perfectly dialed in (perfect piston wash, plug readings, and performance) with a 145 main jet and the high adjuster 1.25 turns out.

    If you drop the main jet size to a 140 and open the high adjuster to "compensate", it's not going to run the same. I've demonstrated this to myself many times.

    If you raise the main jet size to 150 and close the high speed adjuster to compensate, same deal, it's not going to run the same.

    The differences might be minor, but there are differences.
    Last edited by fox river pwc; 05-07-2022 at 09:17 AM.


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  5. #15
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Thank you for sharing this. Anyone who knows Paul realizes how valuable his info is. No backward castle tabs here. Just pure knowledge from a guy whose done it.

  6. #16
    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

    You're welcome.

    To be fair, I've made my share of mistakes along the way. That's a key part of my learning curve.

    And half the reason I'm finally doing this is I'm assuming someday my ability to pull up all my memory files instantly will diminish, so I can then come back here to refresh my memory as needed, lol.


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  7. #17
    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

    When I get some time I plan to snap pictures of some examples of my written tuning notes, so others can see how I do it.

    Not saying my way is the only way, and my way has evolved over the decades, but it does work for me.

    I probably should scan or photograph all my written notes. Some are so old that my handwriting has changed, and they are stored in a file cabinet in my garage, which is part of my basement, so some of the paper has gotten soggy


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  8. #18
    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 5:

    Pop-off talk. I apologize if this whole thread isn’t going in logical order based on what the readers might be looking for, but much of it I’m simply posting as I finish editing it. And frankly, part of the idea here is that to really tune correctly it helps to know all this stuff. I’m also hoping that the sections can help people to find specific information they are looking for.

    Pop-off can be a tricky thing to dial in, and I will reiterate here that it is often temping to change pop-off AND the pilot jet at the same time. I say do NOT do it, LOL. Resist the temptation.

    As the manual states somewhere, pop-off is essentially related to throttle response, and I will concur that it is generally easier to detect a pop-off that is too high versus one that is too low. Because of this I prefer to start with a pop-off that might be too high and then work my way down.

    Now this is where I wish I could do sound effects, because a pop-off that is too high can be demonstrated verbally – as you snap the throttle open and there is a brief “Baaaah – Waaah!!” sound as you are lean for a millisecond and then the leanness is cured as the slightly late pop-off setting opens up and adds the burst of fuel you need to transition from the low speed screw to the pilot jet. You can sometimes cure this brief hesitation with the low speed screw, to confirm that you need to lower the pop-off setting, but not always. I will say that if you THINK your pop-off is too high, just lower it and do the testing again to see. You can’t cause catastrophic damage with minor pop-off changes.

    When it comes to the pop-off being too low, I have found the easiest way to detect this is if your motor, when warm, is initially blubbery (gargly) upon start up and even requires opening the throttle to start it – same as when you’re trying to start a flooded engine. This is because the pop-off is opening too soon, or possibly even instantly when you hit the green button. This won’t tend to cause the blubbery running when cold, because the cold engine likes the extra fuel when first starting. In fact, a pop-off that is too low (or even a needle & seat that is leaking) may help a cold engine to start and even eliminate the need to choke or prime it. So really pay attention to how your WARM engine behaves at start up, as that initial gargle or blubber could be telling you that your pop-off needs to be raised a bit.
    Last edited by fox river pwc; 05-09-2022 at 09:40 AM.


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  9. #19
    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 6:

    Specific testing definitions. Here I am going to simply list and describe the actual tuning/riding tests that I do. Later on, I will dig in much more deeply about how I use these tests to learn what I need to learn along the way through the tuning process, but I will say now that actual tuning is not simply making changes/adjustments and then simply riding around for a bit to see if you like them or not. You must be patient and diligent, do the actual structured tests, and then record the results, so you can then determine any changes that need to be made.

    Idle test: Allow the ski to actually idle, with zero throttle input from you, for 5 seconds, then instantly grab WOT.

    High idle test: Hold the ski at high idle, 1/8 throttle, for 20-30 seconds, perfectly steady, then instantly grab WOT.

    throttle test: Hold the ski at throttle for 20-30 seconds, perfectly steady, then instantly grab WOT.

    throttle test: Hold the ski at throttle for 20-30 seconds, perfectly steady, then instantly grab WOT.

    Throttle response test: For this you’re basically just riding at low and medium throttle settings, stabbing the throttle, and paying attention to the response you get. It helps to do so in corners as well, to better load the motor, as loading it can help to reveal a main jet that is too small.

    With all these tests it is critical that you’re paying attention to the result you’re getting the instant you pull on the throttle. This is because once you get the throttle up into the position of the next fuel delivery circuit the next circuit can essentially mask the condition of the previous fuel delivery circuit. Make sense? Another way to say this, as I stated earlier, I believe, is that it is always critical to know the exact throttle position you are at when the motor behaves as if it is rich or lean, as that is how you can best decide what fuel delivery circuit to adjust to correct the rich or lean condition.

    Yeah, much of this can take some practice, but I promise it will pay off in the end.


    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...

  10. #20
    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 7:

    Other factors and variables to consider.

    I should clarify, I’m writing this addendum from the standpoint of “assuming” that all of the other variables with the ski are “correct”. I know the Mikuni manual mentions some of this, but it is probably a good idea for me to explain a bit more.

    I’ll say it like this…
    You can’t tune a ski with ignition problems, whatever they may be. If your ignition isn’t correct, trying to tune the ski will be impossible.
    You can’t tune a ski with low or uneven compression.
    You can’t tune a ski with cylinder porting that is poorly designed or executed.
    You can’t tune a ski with an exhaust system that simply isn’t suited for that ski/motor.
    You can’t tune a ski a hood that restricts airflow too greatly.
    You can’t tune a ski with water routing and/or water injection that is poorly thought out.
    You can’t tune a ski with a clogged waterbox or kinked exhaust hoses.

    And the list goes on…but you get the idea. To be clear, I’m making vague statements here, and I don’t plan to dive into detailed explanations, but I just want to make sure this was stated.

    Then you also have the variable of pump set up. My very basic overview statement on pumps is that how the pump loads the engine is also a factor in tuning, and an improperly or less than ideal pump set up can make tuning difficult to impossible – especially if the pump loads the motor too greatly. This even includes the weight of the rider because that is also a factor that affects how the pump loads the motor. The fact of the matter is that pump set up is yet another “tuning” discussion in and of itself, and I’m not personally educated or experienced enough in it to provide incredibly detailed training on pump set ups. I’ve messed with pump cones, impeller pitches, nozzle diameters, pump stuffers, etc… enough to recognize the changes occurring as I make them, but again, a technical discussion of all that should be a standalone discussion separate from carb tuning, IMHO. The same goes for a discussion of water routing, cylinder porting, exhaust systems, and even ignition types/curves…all truly very critical subjects to consider when building and tuning a ski, but they still need to be discussed on their own. I may touch on some of them as I go along here, but I won’t be digging deep.


    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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