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Help More FZR carb problems

Discussion in 'Yamaha 250cc In-Line 4's' started by tryin from China, Mar 22, 2021.

  1. tryin from China

    tryin from China Active Member

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    Hi all,

    I bought an FZR 250 3LN 1 about a year ago. It ran but stalled and bogged down at low RPMs. I did the valve clearances, new plugs, oil, filter, new fuel pump and cleaned the carbs but it still had a problem in the idle circuit (as I never could get the Pilot jets out). I minced that with a screw extractor and well, now the jet housing is toast.

    I was able to purchase a fully functioning set of second-hand carbs in perfect condition here in China.

    Unfortunately, they were the 3LN 6/7 variant.

    I would like to know if it's possible to fit them to the 3LN 1 motorcycle. And what kind of float height adjustments/jet sizing would be required since I can't just put the jet housing in the 3LN 1 carbs.

    Any advice would be helpful.


    WeChat Image_20210322141106.png WeChat Image_20210322141117.png WeChat Image_20210322141132.png WeChat Image_20210322141137.png
     
  2. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    You're lucky - the 3LN6/7 carbs have a better emulsion needle support design to prevent them from wearing out along with the emulsion tubes, which is the cause of the low speed bogging and stalling in many cases

    They are functionally the same as 3LN3/5 carbs otherwise. The only difference between 3LN3/5 and 3LN6/7 is reduced engine redline from 18,500rpm to 16,500rpm due a change in Japanese law regarding 250cc bikes max output, from 45HP to 40HP.

    My advice is to jet the carbs to 3LN3/5 specification.
     
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  3. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    Put the good carbs on and let her rip

    carb settings.png
     
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  4. tryin from China

    tryin from China Active Member

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    Cheers guys, I’ll let you know how it proceeds.
    give it a crack
     
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  5. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    If, and this is a big if, the carb throat diameters are the same between the sets, definitely keep the 3LN6/7 sets complete.

    If however the carb throats are smaller on the 3LN6/7, you can transplant all of the jets and definitely the slides as they keep the needle more still and don't suffer emulsion tube wear of the previous models, and as @Linkin said, jet to 3LN3/5 specs.

    I for one am interested to know if they are the same throat diameter in both instances

    Note the fuel level differences which go hand in hand with the various different jet types and sizes between 3LN1 -> 3LN3/5/6/7
    Transferring the entire float assembly also should suffice if you're not keen on messing with float height/fuel level settings
     
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  6. tryin from China

    tryin from China Active Member

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    Sorry guys, the light of day revealed an error on my part: my perfect condition carbs are from a zeal, not a 3LN 6/7. I realized this after inspecting the needles and jets. Also, the slide housing is bigger and cannot accommodate one from a 3LN1 or 3LN 5, which makes it impossible to seat the 5CT7 and 5CT9 needles as the diameter of the plastic shims are larger than the ones on the 5CL2 needles.

    Is it ok to change them (the plastic shims) and continue jetting for 3LN5 specs or just leave the zeal carbs as they are?

    the internal carb throat diameter was measured at 28mm using my trusty metal ruler with the 3LN 1 carbs right beside it. Seems moot however since it’s zeal carbs though. Sorry.
     
    Last edited: Mar 27, 2021
  7. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    The plastic shims, if the same thickness, then they can be changed - also the slide housing slides out, I do actually have a set of Zeal carbs also, but mine don't have the more effective needle retention mechanism, so I would only contemplate swapping parts between the sets, IF they are different throat diameter AND the Zeal carbs have the better needle retention mechanism

    I suspect that Yamaha/Mikuni used a bit of a mix & match of parts on some cross overs due to availability

    Interesting that your Zeal carbs measure at 28mm, the ones I have measured up at 26.5mm from memory, but had 3LN1/3/5 slides with no effective needle retention mechanism

    Definitely jet for 3LN3/5 if you have the 5LN3/5 Emulsion tube/needle jet - note that it is different from 3LN1 -> 3LN3/5 - apparently provides better atomisation
     
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  8. tryin from China

    tryin from China Active Member

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    having some issues uploading photos, they seem to be going outside of this conversation automatically, not sure if that is because it’s “new content “ or some bug related to iPhones, sorry if they appear elsewhere unintentionally.


    This is the more effective (zeal) “needle retention system” you referred to where the needle is screwed into the diaphragm slide.

    image.jpg

    image.jpg
     
    Last edited by a moderator: Mar 28, 2021
  9. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Those Zeal carbs are an excellent purchase - I purchased some for just that reason - to get the needle retention mechanism, BUT my Zeal carbs are the same as 3LN1/3/5 unfortunately, which is why I think Yamaha/Mikuni did a mix and match of some parts probably to use up excess left over parts
     
  10. tryin from China

    tryin from China Active Member

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    Hey guys quick update: zeal carbs jetted to 3LN 3/5 settings with mixture screws 3 turns out.

    Bike will only idle with the choke on and stalls if I turn it off or blip the throttle (choke on or off is the same for blipping).

    Since all jets are new and carbs are spotless; fuel issues have been bypassed by way of a water bottle connected directly to the carbs; and I’ve fished out spark related issues which led to the main fuse connection shorting on me... what am I missing here? Is it starving for fuel?

    I’m pretty sure I got fuel levels correct with float heights at about 15.1mm (on my bike).

    I’m a bit stumped here about what to do. Any guidance would be helpful.

    Cheers.
     
  11. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    OK, just to check - when you say all jets replaced, that includes the needle and needle jet (emulsion tube) - yes?

    If you're at 15.1mm float height, measured with the carbs off the bike and propped at the right angle obviously, then your fuel height is 10.1mm, 0.4mm low, but definitely not enough to have the effect which you are describing.

    Checking the actual fuel level with pieces of clear tube is very easy to do, that would be my first step

    It's a PITA, but do you have the fuel pump in the line for the fuel supply?
    A longer piece of fuel line from the tank to the tap will allow you to just use the tank propped off to the side

    How does the water bottle allow for air to go back into the bottle when it supplies fuel to the carbs, if there's vacuum there, the float bowls may not be filling correctly

    EDIT - is there a difference in the slide cutaway between the two sets of carbs?
     
    Last edited: Apr 11, 2021
  12. tryin from China

    tryin from China Active Member

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    I used the zeal 652 needle jet holders, instead of the 3LN 3/5 638’s. I suspect you’ll tell me to swap them immediately.

    I settled for 15.1mm float height because it delivered the closest fuel level setting (bench mounted at 38 degrees using the fuel and a tube off the bowl drain plugs), which took several repeats and any attempt close to 14.7 was resulting in fuel levels being much too high. Real PITA. The math you guys have described earlier just wasn’t working for me but that may be my lack of finesse.

    The water bottle may be creating a vacuum as I didn’t cut the end off. Might be shooting my self in the foot there.

    Thanks for the input.
     
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  13. tryin from China

    tryin from China Active Member

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    P.s. needles are 5CT7’s and 9’s.
     
  14. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    and your brass choke tubes are not leaking?
     
  15. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    OK, so the needle jets are different between because the zeal uses an O-4 needle jet, whereas the 3LN series uses an N-8 needle jet - there's a couple of hundreths of a millimeter in it, but it's a lot when considering the needle diameters which are different

    an N-8 needle jet is 2.590 millimetre diameter, whereas an O-4 is 2.620 millimetre diameter

    Now from memory I know the thickest part of an 5CT7 or 5CT9 needle is 2.515mm - so the clearance is 0.075mm with the emulsion tube

    I cannot recall the thickness and taper of the 5CL2 needles, but looking at the above, you can appreciate the issues a sizing mismatch would make

    Quick way would be to put zeal needles with zeal emulsion tubes - they will be wrong for the tune of that engine all told but it may rev somewhat correctly
    But to do it properly 3LN needles AND emulsion tubes, yes, sorry you get to take the float bowls off again

    I'll find the spreadsheet which I have somewhere with all of the needle jet and needle diameters later on - the clearances with the throttle closed are tiny, any mismatch will cause havoc

    As for the float height, you're very close, leave it be till the needles/jets are correct

    EDIT - so this is where I get to correct myself a little - 5CL2 has the same largest diameter as the 5CT7 and 5CT9 - the BIG difference is in the needle jet itself - interesting to see such a big difference in the way the zeal carb is tuned versus the 3LN's

    my reference is below, which IIRC is also available either on this site in the resources section or at https://www.litetek.co/ - all thanks to @maelstrom in either case

    http://kawatriple.com/carb/jetneedlearea.xlsx

    The 5CT7 and 5CL2 are the same largest diameter, and beginning taper point, they differ in the taper angle to the tip 5 degrees for the 5CT7 versus 3 degrees for the 5CL2

    Here's the numbers on the needle and needle jet cross sectional area differences between the 3LN and Zeal - the BIG difference is in the jet area - all numbers in millimetres square

    Needle area mm2 - 5ct7/5ct9/5cl2
    4.96782009807562

    Emulsion tube area - N-8 - Orifice 2.590mm
    5.26852941988642

    Jet area 5ct7/5ct9/5cl2 - N-8 Needle Jet
    0.3007093218108

    Emulsion tube area - O-4 Orifice 2.620mm
    5.39128715282544

    Jet area 5ct7/5ct9/5cl2 - O-4 Needle jet
    0.42346705474982
     
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    Last edited: Apr 12, 2021
  16. tryin from China

    tryin from China Active Member

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    Thanks. I’ll go find my old friend JIS once more.
     
  17. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Despite what needles and emulsion tubes you have, you should be able to open the throttle when stationary.
     
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  18. tryin from China

    tryin from China Active Member

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    Let me clarify bike behavior:

    Bike will not start by itself.

    Bike will not start with full open throttle.

    Bike WILL START with full choke.

    After a minute of idling on choke (the rpms do drop as it warms), if I slowly release the choke (deactivate it), the bike stalls.

    if I repeat slowly easing off choke and blip throttle, it stalls immediately.

    if I blip throttle with choke on, it stalls immediately.
     
  19. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    Use something thats vented and I would run it via the fuel pump
     
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