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Pinned Mikuni BDST 28/26mm Carb Settings/Specs - 3LN / 3YX (Zeal)

Discussion in 'Yamaha 250cc In-Line 4's' started by GreyImport, Jan 4, 2018.

  1. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    BDST 28 carb chart.PNG Jets.jpg Mikuni-Pilot-Jets bleed holes.jpg MikuniEmblem2.jpg Pilot Merge Chamber.png
    CarbScrews_zps361ccc35.jpg
    Sync screws 3LN1 carbs.png
    Mikuni_BDST_AirJet_Fitted.jpg
    Carb Bank details.jpg
     
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  2. cuihailiang

    cuihailiang Active Member

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    thankyou。
     
  3. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    To clarify/correct the settings table at the top for the 3LN1 - there are two possible needle combinations:
    all four carbs with 5CT7 - the inner two carbs (2&3) having a 0.5mm shim to raise the needle - clip position 3rd (middle) on all 4
    or
    outer two carbs (1&4) 5CT7 - inner two carbs (2&3) 5CT9 - all four carbs clip position 3rd (middle) on all 4 - note no shim underneath the clip on the centre two carbs (2&3).

    The reason for this difference is because the 5CT7 and 5CT9 both have the same tapers - 1 degree for the first taper and 5 degrees for the second taper. The difference is that the tapers begin 0.5mm earlier on the 5CT9 so the 0.5mm shim underneath the clip is not necessary for those needles when fitted on the centre carbs (2&3).

    The other reason why I am mentioning this is because 5CT7 needles are only available as part of the keyster kits, so you have to purchase the whole kit for just the needles and emulsion tubes basically as these wear the most rapidly.
    The good part is that 5CT9 needles are available on aliexpress and ebay.

    Where this gets interesting is with a set of four new 5CT9 needles for all carbs you can get the equivalent of the stock 5CT7 needle for the two outer carbs (1&4).

    New setting for 5CT9 needles only
    For carbs (1&4) clip on the 2nd position (higher) and placing the 0.5mm shim underneath - which is equivalent to 5CT7 clip position 3 without shims
    For carbs (2&3) clip on the 3rd (middle) position and no shims.

    This is discussed in this post and elsewhere in this thread
    https://www.2fiftycc.com/index.php?threads/fzr250-3ln3-carb-problem.8770/page-5#post-112020

    peace out
     
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    Last edited: May 13, 2018
  4. driftwood

    driftwood Active Member

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    I'm confused by the megazip parts list - has to be wrong. Item 38 listed as 1HX-1423A-73 #112.5 pilot air intake is nowhere near that big (~0.92mm) and can't be removed on my pair of LN6 and a LN3 set. No p/n is given for the removable one next door, which seems about a #100... the purple arrow in pic above.
    These supply the emulsion tube, so you'd reckon were critical.
     
    Last edited: Jan 13, 2023
  5. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    My Bike:
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    I don't have a carb in front of me, however I do have a jet holder to hand
    Looking at the carb venturi picture, cross referenced with the jet holder, the main jet air [inlet] is on the left and the one on the right [marked with the purple arrow] is the pilot air jet - that's the way they run into the jet holder

    Curious that we don't have a replaceable/adjustable main air jet

    https://en.impex-jp.com/catalogs/mo...ite-sw-00ge-japan-2481/carburator-100662.html

    Looking at the 3LN1 Impex schematic - the same error is there, it shows the LHS jet as removable, which is in fact the non-removable 'main air jet' air intake and the RHS one, which is in the schematic is labeled as part 38 as removable, it's the pilot air jet in fact
    It's got two main jets in the parts list, 35 & 38, however one is the main jet [fuel] part 35 [#102.5] and part 38 [#105] is the pilot air jet - those numbers are correct and correspond with the table that we have
     
  6. driftwood

    driftwood Active Member

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    Oops, it's about 105 (as measured by welding tip cleaners)... but stamped 112.5 AND THERE'S NO WAY!!! Can't force a 1.08mm rod down it. Mikuni item is N100.606 with limited availability (assuming that my 3 sets are all mis-manufactured, I'd have to ship some 115s from the US. [Edit] I can get them from mikuniOz !!!).
    Since running headers I'm more aware of boggy surge on low throttle - not being on the track I'm often stuck behind tourists.
     
    Last edited: Jan 13, 2023
  7. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    OK, I just did a quick google, round Mikuni jet numbers aren't a fractional measurement in mm's of jet size diameter, it's a [fuel] flow number in cc/min to a specific standard of pressure - assuming that fuel and air jets are to the same standard
    ..and it seems there's a difference between round fuel jets and hexagonal fuel jets
    round fuel jet #'s are flow measurements
    hexagonal fuel jet #'s are related to the diameter

    Not sure where that leaves the screw in pilot jets

    That boggy surge on low throttle may just be the inevitable consequence of the large valve overlap and no EXUP
     
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  8. driftwood

    driftwood Active Member

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    With my std LN3 as a comfortably sedate baseline, I remain hooked on the upgraded stablemate. Live in the hills, and nimble handling leaves me up the clacker of some doofus who powers out of corners to stop me passing. If I time it right, they get quite a surprise! Acutely aware of backfire popping on shutoff - most open exhausts do this. But I run a std can to keep back pressure up... the sonic boom comes from the header collector. Which begs the Q? do open pipes just reveal the pops & crackles that'd always been there?
    Dropped in #115 pilot air - WTF not? Air's free, but a petrol afterburner makes no sense! All good. Drilled out outer cyls from this ~1.08mm to 1.2, left inners as comparator.
    Happy thus far. Power comes on smoother thru corners. Plugs 1&4 beige, 2&3 light brown (they had all been russet).
     
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  9. Linkin

    Linkin The Mechanic Premium Member Contributing Member Dirty Wheel Club

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    With an open exhaust, a sudden closed throttle from wide open throttle results in air being sucked into the exhaust, igniting any unburnt fuel in the exhaust. Because of the amount of overlap in the cams (time where the inlet and exhaust valves are open) you will get some fuel going straight out the exhaust, so you're more likely to get an afterfire.

    FZR250 has the EXUP which will keep exhaust pressure up below 10k RPM, the standard cans on these are all noise baffling and increased length (internal length is about double the outside length of the can)

    These bikes are jetted and tuned for performance in general, emissions was a bit of an afterthought in those days, so they didn't care about excess fuel out the exhaust.

    USUALLY an open exhaust results in rich down low, lean up top. The EXUP will negate that fairly well in my experience. It also depends on exhaust header configuration (4 into 1, 4 into 2 into 1, etc)

    From how your plugs look you could probably go up a main jet size on the outer cylinders.

    Another thing to consider, which I did on my ZXR250, was cut the springs for the vacuum slides down slightly to slightly faster throttle action, as all the control over the main jet and needle is done via vacuum. I also modified the throttle tube into a quick turn one. Worked well.
     
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  10. driftwood

    driftwood Active Member

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    Ta. I'm prepared to admit to being delusional, but content at the mo'. Living in the hills, the throttle is always opening up or closing, so leaning the pilot system has improved things a lot. Yes, the EXUPectomy has created a pig below 9000, so it's rev madly and slip it to get away (fun on a dirt road, not!!!)
     
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