Addendum to the Mikuni SBN Tuning Manual - Page 5
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  1. #41
    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

    Apex tuning update:

    With the 85 pilots and 135 mains and 1 turn out on the low adjusters...
    Definitely feeling stronger pull overall.
    1/4 to WOT and 1/8 to WOT test not ideal, stumble/gargle, but wasn't sure of lean/rich...so I went to 1-1/4 out on the lows.
    That made the condition worse.
    So then I went to 3/4 out on the lows.
    That made the condition better.
    So then I went to 5/8 out on the lows.
    That was darn near perfect, but not quite. And piston wash shows I'm still too rich overall.
    All the testing, and taking notes, and really paying attention is only confirming what I suspected early on - that I had started with pilots that were too small, but the mains being too large created the challenge to discover this - as it was masking the pilots.

    Ran out of time to do all I wanted to this weekend, as I had family stuff going on and other vehicle things to deal with.

    Did find time to rejet early Saturday am, but probably won't ride again till one night this week after work.

    Next jetting set up will be:

    132.5 mains
    82.5 pilots
    33psi pop off
    Hi adj, 1-1/4
    Low adj, 3/4

    Yep, breaking the "one change at a time" rule, mainly because I've confirmed that I'm fighting mains that are too big, and I'm only dropping them 2.5...and I now know the 85 pilots are too big. Rolling the dice


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  2. #42
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Are you doing these tests with the ski in the water and on the trailer or actually out riding? I would assume while riding since this would represent how the ski will be used. Thanks

  3. #43
    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 Travis73 View Post
    Are you doing these tests with the ski in the water and on the trailer or actually out riding? I would assume while riding since this would represent how the ski will be used. Thanks
    While riding.


    WHEN IN DOUBT, GAS IT!

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

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  4. #44
    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 Travis73 View Post
    Are you doing these tests with the ski in the water and on the trailer or actually out riding? I would assume while riding since this would represent how the ski will be used. Thanks
    I should add...
    I watched a YouTube vid a while back where someone was "tuning" by pinning the nose of the ski against a dock, fiddling with the screws, and then hitting the gas. Fun stuff. Made me giggle.


    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. #45
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    Re: Addendum to the Mikuni SBN Tuning Manual

    So I'd like to share my tuning saga from yesterday and echo every single thing in this thread. I have a Polaris Pro 785 that I built and it is pretty much a full super stock ski (cylinder and case porting, triple pipes, triple 46mm Novi carbs, etc.) minus the MSD total loss ignition. I had the TL system but pulled it off and put the stock ignition back on because I thought there was a problem with it...but now I realize that I just didn't know sh*t about how to tune the damn thing. The issue was it always seemed to be very soft off the line and had a laggy mid-range. I also couldn't quite get the RPM that I was looking for, which is 8100+ with the mods on this ski.

    Baseline tune:
    120L, 125H, 2.3 spring, 2.3 N/S.

    After reading this thread, I decided that I needed to be VERY methodical about tuning. I must admit...in the past, I used to make drastic and multiple changes in an effort to get the ski to hit off the line and to pull the RPM. I went completely backwards and tried to tune the top end first to get the RPM I was chasing then thought I could clean up the low end and still get the same performance. This is the absolutely wrong way to go about tuning these carbs. After my tuning session yesterday, from now on I'm going to make ONE small change...test...evaluate...and make a correction. Is it tedious and time consuming? Absolutely...but I couldn't be happier after yesterdays tuning session.

    For starters, I could never really figure out if the ski was lean or rich off the line. I always thought it was rich so I would make drastic LS jet changes and after weeks of trail and error, I was just chasing my tail would always leave the lake pissed off and unhappy. So for some reason I decided in my mind that it was rich and would try upping the pop off pressure. I swapped the 80 gram springs out for 95 gram springs and also upped the LS jets to 130. I've read that for every spring change, a general rule of thumb is 4 LS sizes, which is probably a great rule of thumb IF your carbs are correctly tuned. I installed 130 LS jets and would try it out. Well on the trailer after warming the engine up, it would not hit at all when I snapped the throttle. It would just sort of do the whole "punch in the stomach" thing where it just not rev and almost die. After scratching my head, I opened the LS screw up to 1.75 turns and what do you know, it hit pretty hard with the stab of the throttle. Okay, so I'm lean on the LS jet. Installed 132.5 LS jet and tested again...same thing...it would just die when stabbing the throttle unless the LS screw was opened way up. Fast forward though 135...137.5...140...142.5...145...okay, this ski doesn't like the high pop off pressure or in reality the higher spring rate. Let's go back to the baseline tune with the 80 gram spring. I installed the 120 LS jet again since that was my baseline.

    Okay, so now with the lower pop off and the 120 LS jet, it would only hit with the LS screw at 1.5 turns out. Way too much for my liking especially since here in Houston, it is always hot and humid so that leaves me absolutely no adjustability when it gets a little cooler later in the year. So once again I tested...122.5...125...okay, its getting better with the 125 LS jet. The screw had to be about 1-1/8 turn out to hit. Upped it to 130 and damn, it hit hard with the LS screw at about 7/8 turn. To me, that is perfect since it is about 100F out and quite humid. This is pretty much how it will be for most of the riding season so I have some ability to richen it up during the latter part of the riding season. I'd like to iterate all this tuning at this point was on the trailer. For tuning to get that first hit off the line, having the ski on the trailer is more than sufficient to figure out what you need to do.

    Now that it hits off the line, time to do some mid range testing. Well, it was very soft in the mid range and just wouldn't pull hard from buoy to buoy. Definitely seemed like it would hit off the line...sort of lay over...then hit hard again once the HS screw came into play. It seemed lean on the HS jet so I upped it to 130. Man, what an animal. As mentioned before, opening up the HS screw didn't help this at all since that is for top end adjustment. It needed more fuel mid range.

    So my point of all this is you NEED to make ONE SMALL CHANGE at a time and evaluate how it reacted. Do NOT fall into the trap of making big and multiple changes like I did. It will take time and probably annoy the hell out of you each time you pull the carbs to make an adjustment but you'll be so much happier in the long run.

    So I might need one more weekend to make some small adjustments but I think I'm 95% there. Read this thread and read it again...it has helped me so much in the long run.

  6. #46
    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

    Very glad to hear this has helped you.

    Methodical is absolutely the correct approach.
    And yes, it pays off in the long run.

    Getting very close on my Apex now, and the Apex has demonstrated to me, once again, how very critical my note taking is.
    Long story short, what has caused some excess jetting changes on my Apex was that I started with mains (140s) that were simply too big (132.5s now), which I didn't diagnose until the 2nd or 3rd jetting change, and which masked what the Pilots were truly doing. And stopping to read and re-read my notes from last month, that I took the time and effort to take, is a big part of realizing this reality.

    I'm literally thinking I may need to add a new tuning rule = when in doubt, drop the main jet size or just start with smaller mains (than you think or assume are needed), as this isn't the first time I've had too large of mains add to the number of rejetting attempts = plus I'm not afraid to admit that even after 3 decades and dozens of skis, that there is still more for me to learn. And I know/knew that mains that are too small don't tend to mess with your pilot and pop off testing like oversize mains do, so there's that...

    Currently about to test in the Apex:
    132.5 mains
    77.5 pilots
    33psi
    Low adj = 3/4
    Hi adj = 1

    Happy tuning on a happy Father's day!!


    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. #47
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    Re: Addendum to the Mikuni SBN Tuning Manual

    With aftermarket flame arrestors and fuel spraying out of them at wot is that a tuning (I am thinking pop off too low?) issue or just an issue with a/m flame arrestors? Other then having a light film of oil in the engine bay the ski runs awesome.

  8. #48
    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 Quinc View Post
    With aftermarket flame arrestors and fuel spraying out of them at wot is that a tuning (I am thinking pop off too low?) issue or just an issue with a/m flame arrestors? Other then having a light film of oil in the engine bay the ski runs awesome.
    More than likely your reeds are worn/tired or have some damage.


    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. #49
    PWCToday.com Is My Home Away From Home
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Quote Originally Posted by fox river pwc View Post
    More than likely your reeds are worn/tired or have some damage.
    Damn.. They are boyesen rad valves too.. Might have to go back to stock if they are only going to last a few seasons of moderate riding..

    Thank you!

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