Addendum to the Mikuni SBN Tuning Manual - Page 7
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  1. #61
    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

    2 hours on the water today on the RN.

    Starting point:
    20psi
    120 pilots
    110 mains
    Hi adjusters 1 turn
    Lo adjusters 1 turn

    Back it in my launch to set the idle.
    Can immediately tell it's too rich.
    Turn the lows 3/4 = better.
    Turn the lows to 1/2 and idle is great.

    Time to ride and test with the lows at 1/2 and the highs at 1.
    Overall runs fantastic. It's not even the same ski I rode last year. Not even close. Hit is instant and strong. Pulls everywhere. Top end is strong - twitchy/sketchy fast. Most riders I know would have said "done" and called it a day. That's not how tuning is done, lol.

    Plugs are a tad light, but I've only run maybe one or two quarts of fuel through it, so that's not all that meaningful.

    But I do want to see how much fuel it will tolerate on top, so i turn the highs out to 1.25. Pull is similar...maybe a tad stronger in the middle during the 1/4 to WOT test. Time for more fuel.

    I turn the highs out to 1.5. Now the 1/4 to WOT test reveals an instant gargle, and holding it at WOT keeps the gargle going.

    So now we know how much fuel is too much on the hi adjusters. We learned something via focused testing.

    I take the highs back to 1-1/8 keep riding/testing/paying attention. Only thing that isn't 100% perfect is it might have a tiny lean stumble when riding aggressively and coming off idle, but it's incredibly minimal.

    So I turn the lows out to 5/8 and keep riding/testing.

    With the lows at 5/8, trying to take the highs out to 1.25 produces the gargle again. Again, I'm uncovering how much fuel it will tolerate.

    Finish riding, blow it out. Idle is beautiful. Throttle response is beautiful. 1/2 tank burned and the new plugs are a beautiful carmel color - just slightly on the dark side. Piston wash is looking correct.

    This is with the hi adjusters at 1-1/8 the lo adjusters at 5/8.

    I would say that the effective range for the adjusters on this RN would be:
    Hi = 7/8 to 1-1/8
    Lo = 1/2 to 3/4

    I'm leaving the current jetting and pop off, as I couldn't get this ski to do anything to suggest that either jet or the pop off is wrong. It idles great. It starts easily at all temps without a struggle. It has no lean hesitations anywhere and no gargles and never loads up with fuel. It's what I would consider "dialed in". At this point the owner should simply ride and monitor it. If it were to start fouling plugs prematurely, for example, then the highs could be turned in a bit. If it were to feel slightly lean or rich right off idle, then the low adjusters could be moved an 1/8th turn accordingly.

    Now I need to plop my Apex in the water and go ride and enjoy my day off from work (my real job). As great as this RN runs, I just don't enjoy SJs anymore...Bwaaaaahahahaha!!!!


    WHEN IN DOUBT, GAS IT!

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

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  2. #62
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Thank you for taking the time to write such a detailed report on tuning. So many moments of clarity ..
    Last edited by Klong; 08-05-2022 at 06:29 PM.

  3. #63
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    Re: Addendum to the Mikuni SBN Tuning Manual

    Another appreciation post... More of a lurker these days although still have 2 skis in the toy fleet, but documenting experience like this is important with the folks who have countless hours of time. Information is invaluable.
    89 650 X2: 2" Chop, Shaved Head, Lightened Flywheel, Timing Advanced, SBN44, Tau Ceti, Stock Mod Pipe, Drilled Waterbox, 8/16 Impeller, Pump Cone, Mariner Grate, Dcut Ride Plate, Bars, Finger Trim (New Project)
    SOLD 86 750 X2:
    Big pin 770, PJS pipe, 650 Elec, R&D Head/Intake Manifold, 46 SBN, MMR Waterbox, Bars, OP Ride Plate, PJS Grate, UMI Finger throttle.

    SOLD 88 650 X2: Stock with Oil and Crankcase Drain Blockoffs

  4. #64
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    Re: Addendum to the Mikuni SBN Tuning Manual

    This is incredibly valuable knowledge. But I have to ask, how much do air temperatures vary in your area? How can you “pick up where you left off” day to day if air temp changes????

    I’m struggling to tune in a 750x2, I’m only turning 6500 rpm’s. (I know that’s a vague statement) Where I am it could be 70 degrees and cloudy one day, and the next it may be 95 degrees and humid as hell. It’s very hard for the ski to run consistent.

    Also I really need to know what you’re running for an exhaust on your ski. Do you have a wet pipe or dry? If it’s wet how do you take the sonic wave into consideration while tuning? Also are you running a stock ignition or zeel/msd?

    Maybe these are irrelevant questions to pure carburetor tuning, but bolt ons to the engine should effect jetting?

  5. #65
    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 Klong View Post
    Thank you for taking the time to write such a detailed report on tuning. So many moments of clarity ……..
    You're welcome.
    Sometimes it seems like I'm permanently stuck in a moment of clarity...and it can be hell to deal with, lol.


    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. #66
    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 jplasse View Post
    Another appreciation post... More of a lurker these days although still have 2 skis in the toy fleet, but documenting experience like this is important with the folks who have countless hours of time. Information is invaluable.
    You're welcome.
    On an internet full of nonsensical jibberish, I figured I should at least try to add something that might help


    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. #67
    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 js3000 View Post
    This is incredibly valuable knowledge. But I have to ask, how much do air temperatures vary in your area? How can you “pick up where you left off” day to day if air temp changes????

    I’m struggling to tune in a 750x2, I’m only turning 6500 rpm’s. (I know that’s a vague statement) Where I am it could be 70 degrees and cloudy one day, and the next it may be 95 degrees and humid as hell. It’s very hard for the ski to run consistent.

    Also I really need to know what you’re running for an exhaust on your ski. Do you have a wet pipe or dry? If it’s wet how do you take the sonic wave into consideration while tuning? Also are you running a stock ignition or zeel/msd?

    Maybe these are irrelevant questions to pure carburetor tuning, but bolt ons to the engine should effect jetting?
    Thank you.
    I'm in SE Wisconsin.
    I've ridden when it was 38 and when it was 95 degrees, but most of my riding is in the 60-85 range.
    I don't typically actively tune during cold weather, but when I ride in the cold I will intentionally more slowly warm up the ski, and I might also adjust (richen) the hi adjuster and/or limit how much water is I injected in the pipe.

    In my experience, only very heavily modded (aggressive porting, annular discharge modded mikuni carbs - like red tops) are noticeably affected by the weather swings you're referencing.

    I've tuned pretty much every wet pipe that's been around, and a few dry pipes. Most of what I deal with personally is rec skis, so mostly wet pipes. Wet pipe tuning is almost another science in and of itself, and since I'm no longer a racer looking for that last 50 rpm at all costs, I tune stuff to be fun and reliable - in other words, I run stuff on the wet side, so I'm not fighting loss of bottom end. As a very general approach to something like a b-pipe, for example, I will set the bottom screw before it is installed (cuz messing with it later is ridiculous, lol), close the middle screw, and then use the upper screw to achieve the pipe temp I want - and this is also a way to adjust for temp changes. As for the bottom screw, I normally set it at 1/8 to 1/4 turn open, depending on compression and dual or single cooling (see my demonstration thread on the b-pipe adjustment screws). As for ignition, I'm dealing with mostly MSDs and stock stuff. Haven't messed with zeels yet.

    Not sure what bolt ons you're referring to in your last sentence?

    I hope this helps. It sounds like you may be fighting a combo of variables. We can talk privately on the phone sometime if you'd like.
    Last edited by fox river pwc; 08-09-2022 at 09:38 AM.


    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. #68
    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 RN I finished tuning yesterday got picked up last night, and I'm expecting a ride report from the owner after he rides it today.

    And now I have a modded 2016 RN in its place to evaluate and tune. Might get started on it this weekend. Got some family plans this aft/eve, and I really wanna go jump some boat wakes at my place today on my Apex. Bunch of my neighbors will be out plowing , gotta chase wakes when the chasin is good....


    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. #69
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    Re: Addendum to the Mikuni SBN Tuning Manual

    How much does aftermarket FAs effect jetting vs OEM? Reason I ask is I have been fighting (can’t say tuning after reading your post LOL) Dual I40s on my 750BP swapped 550 kawi for a while now.
    Some background: The motor is stock with exception of a cut head (180PSI) and running a 650 manifold and pipe with an extended expansion chamber and 2” pipe. I don’t think the exhaust is too restrictive but obviously plays a role so I thought I’d mention it. I was using Blowsion tornado FAs and could not get it to run right. Starting with standard settings, NV, spring jetting etc, the POP off was clearly too high. If I got the pop off low enough it would start dribbling in the Venturi at idle and load up due to vibration and low POP off I guess. I assume it is vibrating at idle due to higher comp? I don’t know but motor is rebuilt, checked alignment etc. With POP off set high enough to cure the dribble it would again experience a lean hesitation comping off idle and fall flat on its face. Turning my LS adjusters out far enough confirmed the lean bog.
    I went back to an OEM style FA ( due to tight space I had to Frankenstein an FA using the 750 OEM base and two SX650 covers - pic attached)and got close to good, keeping POP off high enough to eliminate dribble and going larger on the Pilot to compensate. It pulls well from idle, well from all throttle openings however it gargles a bit if I run at a constant 1/4 throttle. Piston wash is a tad rich. As the gargle is at 1/4 throttle I am not thinking main Jet at this point.
    I believe Standard settings are 1.5 NV 80g, 67.5 pilot 147.5 main at sea level. I am currently at 2.0 NV
    / 115 spring (technically same POP off as a 1.5nv / 65g spring - I figured the heavier spring would help fight vibration better) 75/152.5. LS is 3/4 turn out and HS 1 turn. If I go out another 1/8 on the LS she gargles even more and acceleration is slower. I tried going back down to 72.5 on the pilot but the hesitation came back and I had to turn out the LS to compensate. I have not been as methodical as you prescribe but have started from the bottom with POP off and worked up from there making countless changes to NV, springs, Jetting etc in the right order. In my search for the holy grail of tuning answers I did stumble across some information regarding the transition from high pressure to high volume during throttle plate opening and how less restrictive FAs can effect this. Any thoughts, advice or comments on what I may have missed would be greatly appreciated. Thanks again.
    Attached Images Attached Images
    Last edited by Klong; 08-08-2022 at 12:04 AM.

  10. #70
    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

    Aftermarket FA vs OEM is a great question...and I can't give you a one size fits all answer, LOL. Too many variables, but I have seen examples where the difference, jetting-wise, was minimal at best.

    I've had restrictive exhausts make tuning impossible. Great example would be the JS701 Sleepersaki I built for Jet Blast 2018. Trying to run the 650LX exhaust and stock kawi waterbox, on an otherwise stock 701 61x just wouldn't work well at all.

    From your details, I'd suggest trying a higher pop off with larger pilot jets than you've been running...and also maybe smaller mains. Yeah, changing more than one thing at once is against the rules, but you're at a point where it sounds like progress isn't being made. I think you're currently fighting a pop off that is too low.


    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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