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Yamaha YZ 125 Help

Discussion in 'Technical and Troubleshooting Torque' started by Tenny, Sep 13, 2013.

  1. Hello fellow bike riders :)

    Unfortunately my yz 125 has been having a problem upon cold starts, but it does not affect the bike in any way other than when it's cold.

    What happens is when the bike is first started of a day the engine will rev uncomfortably high until it has warmed up after about 5 minutes or so.

    There isn't a starting problem or anything like that, but I will usually turn the bike off for fear of damage. Turning the bike on and off a few times normally reduces the revs enough for me to feel comfortable to leave it idle like that till it warms up.

    Once the bike is warm / hot the bike will idle fine after, even after sitting to cool down over 2 - 3 hours.

    I've done a little bit of research and I think it's possible it could be an air leak or perhaps the carb is worn?

    It's a 2002 model with a .25 over sized bore, to my knowledge.

    Anyway, I'm not the greatest with mechanical things so any help on what to do or what it could do would be greatly appreciated [​IMG]
  2. Sounds like idle speed or choke just needs adjusting. Just take it to a mechanic, should be a 2 min job I would have thought. How long has it been doing it? Since you got it or only recently? If only recently, have you changed anything or had anything done to it?
  3. It's been doing is pretty much the whole time i've had it, bar the first few weeks which would be around a year.

    Nothing has been changed on the bike, its been stripped, air box, sub frame etc all taken off and it still has the problem after being put back together.

    I don't think its the idle screw because once the bike is warmed up it runs really nice with a good idle.

    I don't have any idea how to adjust the choke like you've said so i'll have to look into it.
  4. Yeah but cold idle speed with the choke on could be diff to once it's running. I have no actual idea if thats the case, if you can change it if it is or isn't or even if thats possible but just throwing it out there.
  5. Does it rev up with the choke on, or with the choke off, or both?

    Two strokes tend to rev up with a lean mixture. This could be caused by either an air leak, or by a fuel restriction.

    Starting a two stroke cold without choke can sometimes cause this, as they run lean when cold without the chokes richer mixture.

    If it revs with the choke on, then this may or may not be normal, depending on how high it revs. It is normal for them to rev up a bit on the choke, but revving way up is not normal. If it does rev excessively then the fuel supply circuit for the choke could be blocked. Need to strip & clean carbie.

    This is all assuming that the engine is mechanically sound. Worn piston & rings, leaking exhaust header, reed valves not sealing etc, can all play a part in running issues.