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Help FZR 250r 3LN1 rev issues

Discussion in 'Yamaha 250cc In-Line 4's' started by Spen, Mar 15, 2020.

  1. Spen

    Spen Member

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    The bike has severe misfires above 12 thousand rpm, I've get new spark plugs and new could, cleaned the pick up points on the rotor, can't see any spark leaking from the ht leads to the block or head, not sure where to look next. Help.
     
  2. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    Check that the EXUP valve is working properly - there are other things to check which are more involved, however that's a good starting point

    https://www.2fiftycc.com/index.php?...-powervalve-information-and-maintenance.1099/

    Then if you can let us know what other maintenance &/or parts changes have been done I have another idea, actually several ideas, but I don't want to swamp you with things to check straight up

    Let's keep it simple to start
     
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  3. Spen

    Spen Member

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    Exup valve is set up perfectly and operates as expected when reving, opens little by little ect. The misfire was found whilst dynoing it still makes 45 hp at 15000rpm but past that the graph looks like a set of hills in the Welsh vallies... the jetting is perfect as is all the pilot jetting and needle settings, stock air box and filter and carbs balanced too.
     
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  4. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    Those numbers are very good - OK this is one which gets overlooked, did you perhaps put a Keyster kit through the carbs?

    By rights it shouldn't rev past ~12K RPM if the main jets are backwards, but here's a thread where the incorrect jets - '2 & 3' and '1 & 4' cause issues at high RPM

    Start at about page 13 and the solution comes at about page 16
    https://www.2fiftycc.com/index.php?threads/the-second-fizzer.9044/page-12
     
  5. Spen

    Spen Member

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    I translated the Yamaha manual to English as I speak Japanese and the main jets for 1&4 are 102.5 and 2&3 are 100. When we dynod it the needles were at the middle setting ( clip 3) but that was rich as hell. Dropped the to the leanest setting the 5 clip and it runs perfect according to the a/f ratio machine the Dyno had.
     
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  6. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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  7. Spen

    Spen Member

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    Carbs are set to this apart from the needles but they were at the factory setting and it was way too rich, plugs gapped to 0.70 and I've get new could as of yesterday
     
  8. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Misfiring does not sound like fuel issue. If all plugs, leads and caps are good, I would go for the TCI unit. @my67xr wrote a good thread about repairing them.
    New ones are available here https://www.lixianda.com/
     
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  9. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    My next suggestion was going to be to check the fuel level if it's running really rich, then definitely onto the electrical system

    Also to edit the above table, if the carbs have 5CT7 needles across the board, then there is supposed to be a .5mm shim below the two centre carb needles

    This post explains the slight confusion in the above table as it's a mix of two different configurations which won't get you to the right place with the needles

    https://www.2fiftycc.com/index.php?...settings-specs-3ln-3yx-zeal.9787/#post-112136
     
    Last edited: Mar 15, 2020
  10. Linkin

    Linkin The Apprentice Premium Member Dirty Wheel Club Contributing Member

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    The larger main jets should be in the centre cylinders with the corresponding needles and emulsion tubes, which should also be checked for wear and ovality.
     
  11. Spen

    Spen Member

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    Couldn't find a post to fix the tci, had a look but can't seem to remove the circuit board without completely breaking box, I can remove the front cover though, it seems odd that the misfire only happens under high load, something must be on the way out.
     
  12. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    Larger jets go on the outer cylinders, it's in the service manual - check the post I linked to above - page 16, incorrectly installed it will cause issues at WOT over about 12K RPM
     
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  13. Linkin

    Linkin The Apprentice Premium Member Dirty Wheel Club Contributing Member

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    That makes me think that the needles and tubes are set up incorrectly, because inner cylinders have less cooling capacity than the outers, they need more fuel. It wouldn't be the first time a manual is wrong either, my R6 service manual lists the 152's as being in the outer pots and 148's in the centre, it ran like a dog and was clearly wrong. Difference is on the mikunis you can move the emulsion tubes / jet housings, you cant on keihins as they're pressed in.
     
  14. GreyImport

    GreyImport Administrator Staff Member The Chief Contributing Member

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    My 3LN1 carbs are setup exactly per those specs above and after the weekend at the drag strip I can assure u it runs just fine at WOT and redlining to the limiter
     
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  15. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    Yes, I remember when there was confusion about the piston cooling because of the two inner pots being hotter - that may or may not be true however, but the real outcome on the fueling is that the intake tracts on the inner pair are hotter leading to less dense air which requires less fuel, therefore different main jets.

    The FZR250 runs the same emulsion tubes across all 4, and it CAN run the same needles, 5CT7, across all 4 with a shim below the centre pair OR two 5CT7 on the outers and two 5CT9 on the inners - I explained the differences and the error in the table above in this post as it combines two different possible setting in the one table as being one possible setting.

    https://www.2fiftycc.com/index.php?...settings-specs-3ln-3yx-zeal.9787/#post-112136

    Now we've got leaner mains on the centre pair, BUT we've not got the needle at one half richer which may yield the equivalent of say 101.25 mains for the centre pair - speculation. I think this is likely a fine tuning method to give an interim step in the jet selection

    The R6 may be a different beast if it has entirely different emulsion tube and needles between pairs, which sound like what you were dealing with
     
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  16. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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  17. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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  18. Spen

    Spen Member

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    I've checked the pick up, ignition coils, spark plug caps and tci .all have correct resistance readings. not sure where to go next
     
  19. maelstrom

    maelstrom LiteTek Staff Member Premium Member 250cc Vendor Contributing Member

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    Resistance readings are mostly useless and the fact that it is breaking down under load makes them more useless.
     
  20. ruckusman

    ruckusman Well-Known Member Premium Member Dirty Wheel Club

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    I'd be inclined to open the TCI box, replace the capacitors as a matter of course because they're 3 decades old and electrolytic caps dry out.
    Then if you can replace both of the ignition transistors AND if you feel inclined replace both of the signal transistors behind them, that part isn't a great strategy unless you do it in stages, ignition transistors first then test and continue to replace the signal transistors.

    I have a list of potential substitutes for the OEM ignition transistors - from this post - BU941ZT - note they are untested, so it's only a suggestion at this stage

    https://2fiftycc.com/index.php?threads/coil-on-plug-conversions-cop.9631/page-4#post-118294

    The good part is that it's a quick job, relatively speaking

    This post has the component locations and values/ID's
    https://2fiftycc.com/index.php?threads/coil-on-plug-conversions-cop.9631/page-4#post-118309

    Past that if it fails to remediate the issue, you could consider doing the Coil on Plug conversion from that thread which will eliminate the possibility that the coils are an issue, contingent upon getting four good working coils for a good price
     
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