Different Mikuni Twin Carb set-ups
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  1. #1
    Top Dog aggrovated's Avatar
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    Different Mikuni Twin Carb set-ups

    So, I decided I'd like to upgrade my Stock (800) X2 Gen2. While keeping it light w/twins and on pump gas...i'm changing out some bolt-ons. V-Force 3 reeds w/ Dasa Kawi intake, New Kommander non-girdled head 29mm domes, FPP Wet Pipe, TDR Waterbox, TBM lightened flywheel, Skat Trak set-back pump w/ C75 10/18 prop,SE tilt/drop nozzle and then...and this is where my unfettered brilliance comes in (*not), I hilariously thought to get some 46mm SBN carbs to go with all of it. but the Novi's, Full Spectrums, Bun Freestyle's and Black Jacks, are all expensive. So I decided to check out FB Marketplace for some used ones. The used ones were also expensive. So I decided to do some research, what makes these race carbs so damn expensive?

    After researching, I found out that a lot goes into these race carbs...porting, special jetting and changing out the spray venturi...and even the linkage.

    I thought maybe I can find some old 46's and just re-jet and of course rebuild kits. add some new linkage and be good. but after I purchased some used SBN 46's, they were from and old Yamaha.. I thought, no problem just change out the linkage also.
    But the top holes of the SBN 46's are actually smaller than the tops of my stock 40 I's (48mm) WTF? I have the correct adaptors and the bottom holes are the correct size... it's just the top hole sizing that concerns me.
    and then dumb azz me decided to buy 2 new SBN 44's individuals (just in case).... not knowing that they are different from a Set of dual 44's. (which are also Yamaha based). I think I can still use the Yamaha base carbs... but it's trickier than I expected. I guess I have to drill into the individual carbs somewhere in order to make them dual compatible.. IDK.

    but at this point, i'm wondering if they will actually be better than my dual BN 40 I's?

    BTW, I have perfect jetting specs for my stocker w/ FPP wetpipe as per Jonnyx2's recommendation.
    40 I's w/ Wet pipe
    80p
    150m
    2.5 n/s
    80g (black)

    But if i'm going with the upgraded carbs, I've researched some jetting specs for the 44's or 46's... I found an old thread on here asking about jetting for 44's and 46's, and sifting through all of the answers, I found this for 44's and 46's w/ Wet pipe...Anyone running dual 46s? or dual 44s? (pwctoday.com)

    SBN 44's w/ Wet pipe
    110p
    110m
    2.3 n/s
    95g (silver)
    seems to be the safest specs?

    SBN46's w/ Wet pipe
    130p
    130m
    2.3 n/s
    95g (silver)


    Can someone please give some advice towards my predicament?
    Also, what advantage do intake spacers have?
    Last edited by aggrovated; 03-01-2021 at 10:21 PM.

  2. #2
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    Re: Different Mikuni Twin Carb set-ups

    If you bought a set of Yamaha OEM dual carbs, they wouldn't be 46's, yamaha never had oem dual 46mm sbn's, they did have 44's on 760's. I suspect that is the case because I have a set of 40i's and a set of 46's and they use the same flame arrestor adapter.

    I'm not sure what you mean about drilling into the carbs, you might be talking about drilling the return restrictors, you don't really need to do that to run them as dual carbs (Yamaha didn't), I have a set of yamaha 44's on one ski and a set of mikuni 46's on another ski and didn't do the drilling of the return restrictor on either, I'm not claiming to be any kind of master tuner but they seem to run pretty well.

    I have no idea what is the best set to run, the "straight from mikuni" sbn44's that I think you're refering to as the "new individual 44's" some people say those are a little better than the yamaha oem 44's, I have no idea, haven't tried them. I know that I like the oem linkages better than the aftermarket stuff.
    Last edited by Storbeck; 03-01-2021 at 10:35 PM.

  3. #3
    Top Dog aggrovated's Avatar
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    Re: Different Mikuni Twin Carb set-ups

    Yeah, I bought the Yamaha 44's used, but when I measure the bottom hole.. my meter says 46mm. so just to add to the confusion. the oem Yamaha 44's ... are the same as oem aftermarket/individual 44's? just different linkage?...and aftermarket meaning replicas.. they're not oem mikuni's?
    and yeah, i meant drilling holes for the return restrictors.. thanks for that info.

    i bought two of these... are they aftermarket.. or oem? I thought they're oem from the number.

    Mikuni Super BN Series 44mm Carburetor BN44-40-8067 [PB089RY9] - $88.23 : hototin.com

    and here's my used yamaha 44's..


    I already got repair kits and jetting for them both, I guess I have to replace all the linkage also

    but what is weird, is that my kawasaki oem 38/40 I's have a bigger venturi
    Last edited by aggrovated; 03-01-2021 at 11:23 PM.

  4. #4
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    Re: Different Mikuni Twin Carb set-ups

    The terms "oem" and "aftermarket" get a little tricky when talking about sbn 44's.

    There are:

    1. The carbs that came on a Yamaha 760. Some people call these "oem 44's" because they were used by Yamaha as oem carbs.

    2. The carbs you can buy from Mikuni. Some people also call these "oem 44's" because they are straight from mikuni they haven't been modified by NOVI or Full spectrum or whatever, but some people call these "aftermarket 44's" because they didn't come on a stock jet ski on the showroom floor, you can only buy them from Mikuni as an "aftermarket part"

    3. The carbs you buy from NOVI or Blackjack or Full spectrum or whatever that some people call "aftermarket" because they are modified from what Mikuni sells.

    This is further complicated because there are also mikuni 44's that came on Yamaha single carb 701 and single carb 650 and other brand skis and those are different from any of the above. Assuming you have carbs from a Yamaha 760 that came as dual carbs from the Yamaha factory, don't worry about the "Yamaha single carb" carbs but just be aware they are different and there are variations among them, they're also not all the same.

    The carbs that come on a Yamaha 760 from Yamaha are 44mm, there is a slight step so the diameter at the very bottom mounting surface might be slightly bigger than 44mm but the size at the throttle blade is 44mm. They are the same size as the #2 carbs above but they do have some subtle differences. The most significant difference is the "progression holes". If you look at the little holes inside the carb bore that the throttle blade uncovers as you open it from fully closed, the number and size of those is a little bit different on the "straight from Yamaha" carbs than the "straight from mikuni" carbs. The Yamaha carbs have three holes, one is significantly bigger than the other two, the straight from Mikuni carbs have four holes all about the same size. Some people distinguish them by saying "three hole" carbs or "four hole carbs". There is also a difference in the angle of the throttle blade when it's fully closed.

    The effect of those differences is that how the mixture changes as you open the throttle past those holes is different between the "Yamaha" carbs and the "straigh from mikuni" carbs. People say that the "Yamaha, three hole" carbs have a spot that is a little lean. I've never felt like mine have a problem, but again I'm no master tuner, and I haven't tried the "straight from mikuni, four hole" carbs to see the difference.

    So, among the sets you have, most people would probably say that the "straight from mikuni" are the most desirable because of the progression hole thing.

    Myself, I have a ski with Yamaha 760 carbs because they were cheap, and another ski with Mikuni SBN46's because more bigger more better. Have never run them on the same motor so couldn't tell you if the bigger 46's actually make a difference. Might try it this summer.

  5. #5
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    Re: Different Mikuni Twin Carb set-ups

    Yeah the picture is definitely yamaha 44's (#1) and the link is "straigh from mikuni" 44 (#2)

  6. #6
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    Re: Different Mikuni Twin Carb set-ups

    If you look down in the 40i's, the narrowest spot where the throttle blade is has a smaller diameter than the 44's, they just flare up to a big opening at the top.

  7. #7
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    Re: Different Mikuni Twin Carb set-ups

    I don't see any reason why you would have to change the linkage on the Yamaha carbs. The Kawasaki spacing is 2mm wider than Yamaha, but the yamaha linkage probably would still work fine. If you're going to replace the linkage I would definitely not bother with the Yamaha carbs, their advantage is low cost and nice linkage, if you replace it they no longer have either of those things going for them.

  8. #8
    Top Dog aggrovated's Avatar
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    Re: Different Mikuni Twin Carb set-ups

    Storbeck! Thank you for this in-depth information. This is what I needed to know. I don't know anything about yamaha's. so again, Thank you.

    I'm gonna check for the 3 hole/4 hole setup.. when the 'straight from mikuni's' finally arrive.
    So, I already bought new linkage, which i'll connect to the 'straight from mikuni's' to set-up as duals of course...and keep the linkage on the yamaha mikuni's in tact with them.

    and based from all of what you said... I guess the new 'Straight from Mikuni" carbs will be my best choice to use (assuming they have the four holes).

    I hope these carbs will be sufficient to help power the bolt-ons I got.
    Last edited by aggrovated; 03-02-2021 at 12:26 AM.

  9. #9
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    Re: Different Mikuni Twin Carb set-ups

    Well done Storbeck, all this info was hashed out in bits and pieces but your writeup captures it all. - Hats off to you!
    Quote Originally Posted by Storbeck View Post
    The terms "oem" and "aftermarket" get a little tricky when talking about sbn 44's.

    There are:

    1. The carbs that came on a Yamaha 760. Some people call these "oem 44's" because they were used by Yamaha as oem carbs.

    2. The carbs you can buy from Mikuni. Some people also call these "oem 44's" because they are straight from mikuni they haven't been modified by NOVI or Full spectrum or whatever, but some people call these "aftermarket 44's" because they didn't come on a stock jet ski on the showroom floor, you can only buy them from Mikuni as an "aftermarket part"

    3. The carbs you buy from NOVI or Blackjack or Full spectrum or whatever that some people call "aftermarket" because they are modified from what Mikuni sells.

    This is further complicated because there are also mikuni 44's that came on Yamaha single carb 701 and single carb 650 and other brand skis and those are different from any of the above. Assuming you have carbs from a Yamaha 760 that came as dual carbs from the Yamaha factory, don't worry about the "Yamaha single carb" carbs but just be aware they are different and there are variations among them, they're also not all the same.

    The carbs that come on a Yamaha 760 from Yamaha are 44mm, there is a slight step so the diameter at the very bottom mounting surface might be slightly bigger than 44mm but the size at the throttle blade is 44mm. They are the same size as the #2 carbs above but they do have some subtle differences. The most significant difference is the "progression holes". If you look at the little holes inside the carb bore that the throttle blade uncovers as you open it from fully closed, the number and size of those is a little bit different on the "straight from Yamaha" carbs than the "straight from mikuni" carbs. The Yamaha carbs have three holes, one is significantly bigger than the other two, the straight from Mikuni carbs have four holes all about the same size. Some people distinguish them by saying "three hole" carbs or "four hole carbs". There is also a difference in the angle of the throttle blade when it's fully closed.

    The effect of those differences is that how the mixture changes as you open the throttle past those holes is different between the "Yamaha" carbs and the "straigh from mikuni" carbs. People say that the "Yamaha, three hole" carbs have a spot that is a little lean. I've never felt like mine have a problem, but again I'm no master tuner, and I haven't tried the "straight from mikuni, four hole" carbs to see the difference.

    So, among the sets you have, most people would probably say that the "straight from mikuni" are the most desirable because of the progression hole thing.

    Myself, I have a ski with Yamaha 760 carbs because they were cheap, and another ski with Mikuni SBN46's because more bigger more better. Have never run them on the same motor so couldn't tell you if the bigger 46's actually make a difference. Might try it this summer.
    Never argue with an idiot, they will drag you down to their level then beat you with experience.

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