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Help Alternative carburettors - FZR250 3LN1

Discussion in 'Yamaha 250cc In-Line 4's' started by ajb0235, Mar 12, 2021.

  1. ajb0235

    ajb0235 Active Member

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    I now have my 3LN1 up and running, after an engine strip to replace the piston rings and shim the valve clearances.
    However, cold starting is poor, and idle/pickup is poor. I have read all the carb threads on this forum, and with 33000 km on the clock, and a bike 32 years old, I expect to be replacing emulsion tubes, needles , jets and seals. Needles seem to be unavailable, and I’m now looking for an alternative carb set up which does not have this inherent wear problem on the tubes and needles. So I am looking at the 3LN3 onwards as possible replacements, or carbs off another bike altogether like CBR250rr or the Suzuki and Kawasaki 250 fours.
    I have also seen a video by Alan Millyard where he uses 6 Mukuni copy carbs, on a fabricated inlet manifold on his 374cc. 6 cylinder conversion, made from 2 FZR250 units. Look for RC374 on google
    Failing this I will pursue my original plan to fit fuel injection. Electronics is not my strongest skill!
    I have been looking at a project blog to convert a Honda CBR250RR to fuel injection. Here the original carbs were heavily modified to incorporate injectors.
    http://www.cbr250rri.com
     
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  2. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    You mean like this?
    FizzerCarbs.jpg
    Something happened to the member I was working with just as it was about to get exciting. I have put it on the shelf until I get an FZR250 now.
     
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  3. ajb0235

    ajb0235 Active Member

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  4. gregt

    gregt Well-Known Member Premium Member

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    For a bank of four carbs IMO the cheapest option is four of the Chinese copy Keihins on curved manifolds. Which I assume is Maelstrom's picture.
    The local bucket racer guys say they're pretty good and very tunable.

    I'm in the process of buying two downdraft FCR's - and they're the most expensive carbs I've ever bought.
     
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  5. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    They are Taiwanese OKO's. I will try to fit same to my MC22 just for fun. I would not use the Chinese versions but that's me.
     
    Last edited: Mar 13, 2021
  6. yammiee

    yammiee Active Member Premium Member

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    What model of carbs? I've opened up the carbs in my project fzr and they're completely stuffed. Thinking of going aftermarket.
     
  7. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    ????
    I just answered the same question in the post before yours. Oh well, cut and paste.
    They are Taiwanese OKO's. I will try to fit same to my MC22 just for fun. I would not use the Chinese versions but that's me.
     
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  8. yammiee

    yammiee Active Member Premium Member

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    I read the make but was interested in which series of carbs they are, I assume oko makes more than one type. Thanks anyway.
     
  9. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    If you have the original needles, they seem to be harder than the emulsion tube and don't show wear - they can be checked for actual wear with a razor blade along the edge held up to the light - it's the first few millimetres which goes into the emulsion tube which contacted most

    The keyster kits work, but are slightly excessive in parts supplied - if you search for 5CT9 on ebay there are kits for less $$, then get a seal kit from @maelstrom

    https://www.ebay.com.au/itm/114193337803 - abut $20 each - I've just ordered a set of 4, not yet received, the only crucial thing which interests me is the emulsion tube

    The issue is that the needle diameter is 2.515mm - the hole in the carb slide is very close to 3.0mm - the amount of slop there is huge.

    I have a possible solution in the works for the FZR250 carbs for both the slide and needle retention mechanism which will get rid of the slop in the slide and a retainer which holds them down firmly which also guides the spring...

    For comparison - I've for a set of CBR250rr MC22 Carbs - the needle diameter at the top is 2.9mm and the hole through the carb slide is more than 2.9mm obviously, but it's less than 3.0mm - accurate size TDB, but you can see the difference

    PLUS the CBR250rr carbs have a really good needle needle retention mechanism which also acts as a guide for the spring so that it doesn't slop from side to side as the slide raises - they are a cut above the FZR250 carbs in all respects


    If you were to get alternative FZR250 carbs then you want 3LN6/7 as they have the different needle retention mechanism in the slide which holds it much firmer

    On the alternative side - I've been through the CBR250rr carbs - they will re-rack to the appropriate spacing without major surgery - there's some swapping of butterfly mechanism locations AND one mechanism will have to be cut at the tab and reversed

    The choke slider bar also will need to be modified, again not huge.

    ZXR250 carbs can also be re-racked to get the correct spacing for an FZR250.

    One other issue is the outside diameter of the throat which goes into the rubber manifold boot is 2mm larger so forcing that into the FZR250 carb boots is going to be a struggle, I contend futile exercise.

    Using the CBR250rr boots is possible, BUT they are circular all the way through [tapering], whereas the FZR250 one become oval to shape the flow to the manifold at the cylinder head.

    Thinking on a solution for that currently which might use the remnants of FZR250 boots and the metal which joins the cylinder head with the CBR250rr boots over the top.

    Everyone, myself included would love to do fuel injection

    I did a post where basically GSXR [and other manufacturer] ECM/ECU's can be reprogrammed with new fuel, timing and tables etc - they are incredibly powerful and do on the GSXR's both sequential and staged

    It would involve grafting the ECU and loom and various sensors, gauge set off of a donor GSXR - not that expensive to obtain

    Google GSXR1000 ECU reflash - fair warning though, it's a rabbit hole - there's quite a few very informative threads on gixxer.com

    I am planning to do fuel injection on a YZF600 as a stepping stone

    https://www.2fiftycc.com/index.php?...t-whered-2-months-go.11199/page-5#post-142810
     
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    Last edited: Nov 19, 2021
  10. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    My Bike:
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    So I bought 4 of these - as mentioned above
    https://www.ebay.com.au/itm/114193337803

    I've got some pin gauges graduated in 0.01mm increments - 2.50mm - 2.60mm

    EDIT - for correct information - Chinese kit emulsion tube
    Turns out the emulsion tubes in these kits has an orifice which is ~2.52 millimetres, whereas the N8 OEM emulsion tubes are supposed to be 2.59mm

    I don't know if these emulsion tubes are the same as supplied in the Keyster kits.
    I did have 4 new Keyster kits - unopened - which were in a small plastic tub which went missing from the back of the ute - bummer.

    If anyone is intending to purchase Keyster kits for their FZR250 - let me know as I'd love to measure their emulsion tubes, I'm not to keen to spend ~$200 on something which I am not going to use

    EDIT - for correct information - Chinese kit emulsion tube
    The fact that these emulsion tubes are 'too small' - the orifice is slightly larger than the 2.515mm of the needle, and one which I just checked does take a 2.52mm pin gauge.

    This isn't an issue for me as I am planning something else, but it's something to be aware of if purchasing the kits - the needle is such a a clean fit that the tubes will wear rapidly at idle if the needle slop in the slide isn't remediated.

    That's the source of the problem - stay tuned
     
    Last edited: Nov 25, 2021 at 11:41 AM
  11. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Good work Ruckus, about time some more light was shone on these Chinese kits. Keyster are Japanese, so I expect that they will be quite good. I will get a smaller set of pin gauges soon so I can check jets. I expect that the Chinese ones are going to be all over the shop.
     
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    Last edited: Nov 25, 2021 at 12:03 PM
  12. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    The Chinese pin gauges that I bought are damned accurate, I am very impressed

    FWIW - I've done a visual comparison between the 5CT9 needle and an OEM needle out of a CBR250R carb.

    From all of the charts on jets-r-us pages:
    The essential point in measuring Keihin needles is 2.515mm diameter and the length from the top where the needles are that diameter

    CBR250RR needle is single taper, whereas the FZR250 is dual taper shallow then steep - the taper points [first for FZR250 needle] commence at similar lengths down the needle.

    Essentially at idle [nil lift of the slide] the needle is almost a plug of the emulsion tube

    Why am I mentioning this, because although the main jet numbers on the Mikuni FZR250 carbs differ from the main jet numbers on the Keihin carbs by number - the numbers themselves aren't equivalent between the two brands, when the conversion between the numbers is done, they are almost identical.

    So all things being equal, if the CBR250RR carbs and OEM needles are too rich in the middle - single steeper taper commencing earlier, there is the possibility to fit Mikuni needles from an FZR250 to the CBR250 carbs without too much hassle
     
  13. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    @ruckusman Put a copy of your post in the
    Needles and needle jets/emulsion tubes - FZR250 Mikuni BDST28 Carurettors
    thread
    Imagine how many -"My Fizzer runs like crap, and I even put new jet kits in it" threads will continue into eternity and all caused by completely wrong parts.
     
  14. ruckusman

    ruckusman White Mans Magic Master Premium Member Dirty Wheel Club

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    Will, do, shortly - I've got a stage 1 repair for the needle slop in the slide forthcoming

    What I really want to do is correct one or two of the errors on that page - new one in it's place - as well as well documented float height - fuel level settings AND the emulsion tube wear with the slide housing fix

    EDIT

    OH, and I am going to do some micro photography of the throat of my OEM emulsion tubes to show the wear, under a loupe it is quite simply catastrophic - it's no wonder the things don't run properly at idle and it sux that these bikes have a bad reputation and are undervalued because Yamaha cheaped out on the carburettor slides
     
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    Last edited: Nov 25, 2021 at 12:50 PM

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