Information regarding Yoshimura needle upgrade for Keihin CR carburetors
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Information regarding Yoshimura needle upgrade for Keihin CR carburetors

This is a discussion on Information regarding Yoshimura needle upgrade for Keihin CR carburetors within the Technical forums, part of the Caferacer.net Forums category; So while digging through the japanese money pit known as https://japan.webike.net I came across this kit from yoshimura: https://japan.webike.net/products/21290830.html KEIHIN CR SPECIAL Knife φ 29 ...

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  1. #1
    Senior Member CaTacL1sm's Avatar
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    Information regarding Yoshimura needle upgrade for Keihin CR carburetors

    So while digging through the japanese money pit known as https://japan.webike.net I came across this kit from yoshimura:

    https://japan.webike.net/products/21290830.html

    KEIHIN CR SPECIAL Knife φ 29 Carburetor YOSHIMURA original "MJN MultiPullJet Nozzle" It is Kit that can be incorporated.
    "MJN" It adopted high performance and high combustion efficiency.
    CR SPECIAL KnifeCarburetor tends to severe setting around Idling because there is no Idol port, but incorporating MultiPullJetNozzle makes it easy to set around Idling, and Accelerator operation also becomes Smooth.

    [What is MultiPullJetNozzle]
    It is unique mechanism of YOSHIMURA which realized ideal combustion efficiency because fuel particles are fine and less uneven than ordinary Jet needle type, realizing Large width output improvement and Linear Throttle response.
    Ok, so it's some sort of needle replacement kit for Keihin CR carbs that's been developed by Yoshimura.

    I did some initial googling because my curiosity was piqued and didn't really come up with much that wasn't in japanese.

    I did manage to find some photos of the directions however:







    Additionally, I found close up pictures of the "needles" and ancillary parts:









    And how the yoshi kit compares to the stock needles:





    I reached out to Yoshimura America to see if they had any more info or data or english documentation that could give me more information on this kit. Unfortunately they were unable to find anything. They referred me to a Yoshimura Japan POC who, while unable to show before/after dyno charts, did provide me a link to a japanese article about the technology that was written in english:

    YOSHIMURA MJN: THE GAME CHANGING CARBURETOR ???????

    Here are some interesting snippets from the article"

    Conventional carburetor draws fuel through the gap between tapered jet needle and cylindrical needle jet. Jet needle slides up and down through needle jet to adjust the amount of fuel entering from the main jet in the float chamber in response to throttle control. Pilot jet (slow jet) in the float chamber meters fuel for low rpm range including idle speed, through its extremely small hole.
    The gap between jet needle and needle jet is located at the bottom of carb throat where the vacuum pressure / air velocity is low. Although the jet needle can adjust the amount of fuel with its tapered section, it cannot deliver fuel to the center of carb throat (the middle of the Venturi) where the vacuum pressure / air velocity is highest. Atomization in this manner is also far from optimal since fuel-air mixture tends to flow close to the bottom of the carb throat when the throttle valve opens during the transition from part throttle to full throttle.

    MJN is a unique device made out of a straight hollowed tube which is made in-house, that replaces conventional jet needle. Through the multiple holes on its sides where the most vacuums are created and sprayed, MJN ejects fuel throughout the carb throat and achieves improved atomization and higher charging efficiency by taking advantage of the high vacuum pressure.

    P.S. The conventional jet needle type carb does draw the fuel through its gap between needle jet and Jet needle to main bore when even throttle valves are fully closed vs. with MJN nozzle fuel drawing is very minimum. Let’s image, you are entering corner with throttle fully closed, with needle jet type carb which draws fuel into combustion chamber to the point where spark plugs could get almost flooded vs. MJN such effect is very minimum due to again using straight hollowed tubes. This effect would make your transient accelerating throttle work much smoother.
    One of the merits of the MJN is the ease of conversion. You can transform your conventional carburetor into a MJN conversion, just by replacing the jet needle with the MJN and the needle jet with the MJN guide, without altering its basic construction. The simplicity saves cost and thus provides reasonable price.
    “With a MJN, you can use a smaller main jet than normal because of the better atomization ---- which leads to better fuel efficiency and mileage. The transient response will be better also. It will definitely run smoother and will provide better throttle response at upper-throttle range. There's really no negatives to it.”
    and an image of the "atomization"





    This kit is interesting but I question just how worth it it is, given the ~$250 price tag.
    I'd love to see some before/after dyno charts as well as before/after jetting and fuel mileage data to back up the claims that this kit makes. I don't know if I want to be the guinnea pig for that experiment though, unless someone buys it for me (then I'll take care of the dyno portion ).
    Quote Originally Posted by alwhite00 View Post
    Fuck all of you guys. Get into your little circle jerk and have fun. Thought this may be a pretty cool message board but damn, you guys are assholes.

  2. #2
    Senior Member brad black's Avatar
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    i thought they were a different carb as such, and the prices for them seem pretty eye watering.

    wonder if you can get a twin carb fcr kit, cheaper and ducati friendly.

    i usually look at the gale speed wheels on the webike site. cheapest ones for ducati, and they make a really nice retro-ish sport classic wheel.

  3. #3
    Senior Member CaTacL1sm's Avatar
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    They make their own carbs with this technology.

    I was primarily looking at info regarding the retrofit kits.
    Quote Originally Posted by alwhite00 View Post
    Fuck all of you guys. Get into your little circle jerk and have fun. Thought this may be a pretty cool message board but damn, you guys are assholes.

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  5. #4
    jcw
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    Senior Member jcw's Avatar
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    Quote Originally Posted by brad black View Post
    i thought they were a different carb as such, and the prices for them seem pretty eye watering.

    wonder if you can get a twin carb fcr kit, cheaper and ducati friendly.

    i usually look at the gale speed wheels on the webike site. cheapest ones for ducati, and they make a really nice retro-ish sport classic wheel.
    Me, too. Almost a thousand cheaper for a set I wanted. Unfortunately, It is always not available. (the gale speed SB-1)

    It almost feels like bait and switch.


    Anyway, I ended up going with the company that bought carroz (sp? so I just stopped trying) wheels. Core moto. They have a new lightweight aluminum set for a good price.


    I've seen the jets before somewhere but don't remember anything else about them...

    EDIT may have been here
    http://www.zrxoa.org/forums/showthre...s-amp-Journal&


    I have a feeling if these really worked that much better, we would have seen them around by now in one form or another. Who knows...
    Last edited by jcw; 01-29-2019 at 07:20 AM.

  6. #5
    Senior Member CaTacL1sm's Avatar
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    That last picture showing the "needle" at WOT helps clarify how the kit works.

    Where the needle functions like a plug and allows fuel to flow around it, the tube sucks fuel up like a straw.

    Where the needle measures fuel with the diameter and taper, the tube does so by the number (and size) of holes exposed as the slide opens
    Quote Originally Posted by alwhite00 View Post
    Fuck all of you guys. Get into your little circle jerk and have fun. Thought this may be a pretty cool message board but damn, you guys are assholes.

  7. #6
    Senior Member Geeto67's Avatar
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    honestly, at the kind of money you are spending and the "advantages" being touted, I'm kinda leaning toward why don't you build your own fuel injection and run that instead.

    this was great tech 30 years ago when carbs were still viable, and had the OEMs invested in that tech it might have become more cost effective and common, but FI offers all the same benefits, plus reliability, plus tuneability, fuel mileage, engine longevity, and they can pass emissions (not that it matters on old bikes). These days injectors are cheap, throttle bodies are cheap, in tank fuel pumps are cheap, crank triggers and O2 sensors are cheap, and you can get an inexpensive setup like a megasquirt to run the setup. I've seen at least one person make an FI setup for an older KZ using almost the whole setup off a 1983 GPZ1100 and using a megasquirt to power it. I've seen replacement throttle bodies for weber DCOE setups that are like $30 a TB, so the stuff is out there to do something way more interesting on a budget.


    that is....if your charging system can take it.
    Ever tried. Ever failed. No matter. Try Again. Fail again. Fail better.
    - Samuel Beckett
    A tool is just an opportunity with a handle
    - Kevin Kelly

  8. #7
    Senior Member XB33BSA's Avatar
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    reading between the lines those spray bar deals were used to smooth out the power delivery of a pretty radically tuned ,cammed to where they were not even streetable,outdated heater

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