Forums 🚥 PiT STOP 🔧 Hyosung Technical Help Hyosung 07 GT125r won't start

  • This topic has 3 replies, 3 voices, and was last updated 2 years ago by Ben.
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    • #3004
      • Topics: 3
      • Replies: 3
      Swansea, South wales

      Hi guys,

      In desperate need of some assistance.

      I bought my GT125R about 8 months ago and parked it up in a garage over the winter. Been getting it ready to put her through its Mot, few jobs here and there, Plugs, fluids, fuel and air lines, lights and pads all over. When I first started the work she would fire up and run no problems.

      All of a sudden, when you hit the ignition switch it does nothing. Battery is perfect. Charged up and holds charge fine.

      Started to some process of elimination and its become quite clear that the previous owner was a bit of a tinkerer as all the wiring it’s taped up and earthed in places I don’t think they should be, not only that but it’s also got a 250 CDI/Ecu unit.

      From what I can gather he’s bodged the wiring so that it can’t start without having the kickstand up, clutch in and bike in neutral. But even with all these in or out. It doesn’t work. (I did find it wierd that the neutral light was always on regardless of gear.

      Can anyone help on why the bike won’t fire? Does anyone have a tutorial on how the ecu warning override  mentioned above should be wired.

      Ps. The starter motor and solenoid is fine as if I bypass the ecu and hardwire it in the bike does turn over. There’s just no sparks. Which I believe to be the fact the ecu is not giving the signals to the spark plugs.

      Any help is much appreciated as it is driving me insane and haven’t been able to ride all summer!

    • #3005
      ♠️ MARCEL
      • Topics: 45
      • Replies: 1,899
      North UK

      Hey Tomas,

      Just read through the post, but to start the bike and get it running we have to go through the following:

      • Battery is 12.8V charged? And isn’t “old”  (ie. a local garage can test the battery for free so you know the cells inside are good , not just a volt check)  Starter motor draws a lot of juice to crank those 2 pistons, so  battery has to be a 14BS type  (12-14ah as per books) but i’ve seen folk use 9ah. They don’t last well.
      • When you press the starter button = Do the lights dimm?
        If yes, then its either the solenoid / starter motor or just a weak battery (though it says 12v!)
        If no, then  check the harness for these:
        Clutch handle bar switch (not really advised to bypass it but they do lol)
        Sidestand switch  (bypassing is highly recommended) it is that annoying.
        Neutral Sensor should be  1pin and the harness 1 pin   or  2pin sensor for 2 pin harness.  Mix matching is a hit n miss some bike looms don’t react well.
        Kill Switch unit , if moisture got to the bike in winter, check inside the switch for clean contacts and ACF50 (a tiny tear drop) on every electrical plug on the bike will kill off rust and keep contacts fresh.
      • Starter Motor =  Remove it’s LONG  black wire,  put your RED METER wire on it.
        BLACK METER wire should be on the frame or battery negative.
        Press START
        See if there is power going there ?
        If yes, the solenoid must be activated by the start button => so starter motor should spin?
        If no,   the solenoid may not be working. (Very common)
      • SOLENOID = If you hear clicks (even faint) when pressing the start button. That outright should tell you that device needs attention or the battery is too weak to crank the bike.
      • CDI Unit = As with carb 125s , the  CDI will check the parts above ^  before sending any sparks to the ignition coils.
        GT250 M815  or  GT125 M795 are the only CDI’s that work on carb 125 bikes (all of em).  So the 250 cdi you have should be fine if it is stamped with the correct numbers i just mentioned ^ .  After all, you said earlier in to the project the bike ran fine with it.   A GV/Aquila cdi just wont work anyway lol.

        The bike will CRANK on the button WITHOUT  a CDI and  REGULATOR/RECTIFIER.  It just won’t obviously spark   but  this last resort of elimination can help check if the harness is good.

      • STATOR =  Part of the ignition system. Bike wont crank if the STATOR trigger sensor isn’t working.     Grab your meter , switch to OHMS   (200 ohms max)  then ..
        Grab the “BLUE & GREEN” stator plug = Probe the plug
        See how many ohms comes out of it? I’ll confirm its good or not.

        While at it,  check how many ohms you also get on the stator 3 YELLOW PLUG  (yellow wires) ====> probe those pins , and see how many ohms you get across all 3 pins

        The 2 images below are cartoon versions of what i’m getting at , this should help you when checking the harness.  You can save the images to blow it up full screen on a phone while working on the bike.


      // Meditation doesn't mean you have to sit still....

    • #3006
      • Topics: 3
      • Replies: 3
      Swansea, South wales

      This is great! I’ll make my way through this checklist! I’ve got through most of part of it already but some checks there I wouldn’t have though about!


      I’ll keep you posted 👍🏻


      Cheers again!

    • #6454
      • Topics: 1
      • Replies: 7
      West Yorkshire

      Any updates on if you managed to get it sorted asI’m having pretty much the exact same issues.

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