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Help 1986 XT250

Discussion in 'Yamaha 250cc Singles' started by LouisS, Sep 3, 2018.

  1. LouisS

    LouisS Member

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    hi all,

    never had a 250 before, bought a 1986 XT250 for a daily rider and am struggling to tune it. It's got stock exhaust and air box.

    Had been sitting for years. I've pulled the carbs three times now for cleaning and have a rebuild kit on the way.

    When I clean the carbs I have removed all jets, needles, floats etc and used copper wire through all tiny holes as well as carb cleaner. I have checked the float heights and they are spot on.

    It has always idled fine. Used to die with ANY throttle when I first bought it. After cleaning the carbs it will rev fine up until about 2500 rpm then start to struggle and die, lots of popping and backfiring until eventually dies. With the choke on it will rev up fine til about 4500 rpm then die same as above.

    When I pull the plug it is black and sooty, but most of the other symptoms suggest it's running very lean???

    I've checked for airleaks around the manifolds and can't find any.

    Any suggestions for what to try next?
     
  2. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    Welcome to the forum
    Do you have an XT250 4V like mine
    Does it have a Teikei Carby?
    Have you checked what the fuel level's are in the carby, and how many turn's out is your mixture screw?
     
  3. LouisS

    LouisS Member

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  4. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    Ok, my XT250 is the 4V model with the Teikei, the vacuum secondary carby so a bit different.

    I'm guessing you cleaned out the tiny hole's across the emulsion tube ?
    Part number #35 in the pic below
    Also did you check the filter on the fuel tap is clean, and inside the tap has no blockage's, might pay to pull the needle and seat out the carb too if you haven't already done so
    Is the standard airbox still on with a clean filter and the air inlet hole/snorkel hasn't been modified or opened up

    ya4490_11.gif
     
    Last edited: Sep 3, 2018
  5. LouisS

    LouisS Member

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    Yep cleaned the needle tube very thoroughly. Removed the needle and seat last night and cleaned those passages too.

    Cleaned the petcock also and am confident in it distributing fuel.

    No modifications to airbox. As for the air filter, it seems clean to me, it's just a spongey material. Are the cleanable? I've also tried to tune the bike with both air filter in place and removed, with no difference that I could tell.
     
  6. Andych

    Andych Well-Known Member Premium Member

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    Seems you have covered the fuel system reasonably well.. maybe give the Plug lead a trim and check the resistance of the cap etc. I know there is a link here somewhere for details on doing this but I cant seem to find it.. I am sure one of the Guru's will chip in with it soon :)
    You may well have had multiple issues.
     
  7. LouisS

    LouisS Member

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    Would leads give you a stumble at particular revs? It seems to be that mid range is the drama.

    Appreciate the help
     
  8. Andych

    Andych Well-Known Member Premium Member

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    Crook leads can show up in multiple ways.. and with the mid range stumbles is that under load (being ridden) or just as you are trying to tune it..
    You might want to check jet sizes as well... As long as your jets are OEM and not worn or damaged dont change them to aftermarket items.. It is often very hard to know what has been done previously..
     
  9. GreyImport

    GreyImport Administrator Staff Member The Big Cheese Contributing Member

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  10. LouisS

    LouisS Member

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    No just in the shed trying to tune it, not under load.

    Okay, I won't replace the jets. The main is 165 (or it might say 155, the writing is TINY and worn) with the genuine mikuni mark on it. Pilot I'm not sure about, but idle is fine anyway?

    Will trim my spark plug leads tomorrow after work.

    Any other suggestions before I come back and update on progress?
     
  11. GreyImport

    GreyImport Administrator Staff Member The Big Cheese Contributing Member

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    If its an old style plug cap it will come apart .... look for corrosion inside

    Maybe check the coil resistance as well

    IMGP1636 (Medium).jpeg
     
  12. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    Main jet should be a #165, going by the US Part's Manual
    Pilot jet is a #17.5
    Needle jet #Y217
    Needle and seat #2.0

    Maybe check the resistance on the ignition coil
    For the primary test set your multimeter to 200 Ohm's
    and put the red lead from the meter to the positive terminal on the ignition coil and the black probe from your meter to the ignition coil earth or terminal
    Write down your result's
    Then test your secondary resistance, set the multimeter to 20K Ohm's (20,000),
    place the red lead from your meter to the positive terminal on the coil and the black lead into the end of the spark plug lead (with spark plug cap removed)
    Post up your result's
    And yeah check the spark plug cap resistance too, set your meter to 20K Ohm's


    Check the ignition coil is mounted to clean/bare metal on the frame, use some dielectric grease on the metal to prevent corrosion, also use it on the positive terminal.
    Check and clean all connection from the Ignition box to the main harness and grease them too, as well as your kill switch connection's, and maybe spray some wd40 or crc into the kil lswitch past the on/off button and switch it on and off a few time's
     
    • Well said! Well said! x 2
    Last edited: Sep 3, 2018
  13. LouisS

    LouisS Member

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    No problem, will test coil and trim leads.

    But would spark issues such as above explain the symptoms I have?
     
  14. Wozza

    Wozza Active Member

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    Can do...
    You always need to start with the basic's...compression, fuel(right air fuel ratio) and spark...
    so check compression...check valves are opening the correct amount...new spark plug..up to operating temp..then carb settings.oh and make sure you have fresh fuel, sometimes bad or contaminated fuel can send you off on a tail chasing exercise
     
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  15. my67xr

    my67xr Bike Enthusiast Staff Member Premium Member Contributing Member

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    How did you check for air leak's ?

    I found some info on the XT250K Ignition coil too
    Primary should read 1 Ohm
    Secondary should read around 5K Ohm's
    Spark plug cap should read 5K Ohm's
     
    Last edited: Sep 3, 2018
  16. LouisS

    LouisS Member

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    Thanks.

    Air leaks were checked for by spraying carb cleaner around the boots at idle and listening for any rises in revs.

    I don't know how to set valve clearances, so using process of elimination, I'll keep going with the fuel and spark causes and if this doesn't improve the running condition it will be time for some help.
     
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