Forums 🚥 PIT STOP 🔧 Hyosung Technical Help GT125 2003 Performance Issues

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    • #6593
      Mike Daborn
      Member
      • Topics: 2
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      Oct 3, 2021
      @glide919
      Hampshire
      GT125

      Hi,

      My son has a 2003 GT125 with 13K on the clock. This week he failed his module 1 bike test as he didn’t reach the required speed for the swerve test. This was an issue noted the week before whilst doing his training and I spent a week trying to improve general running and throttle response. The carbs have been off and cleaned multiple times, fuel lines replaced,  new plugs and fuel filter. Spark is good on both cylinders. Bike is running rich (sooty plugs). It starts ok but choke response is erratic whilst warming up as a small movement can send the revs up to 5k or dropping so far the bike stalls. Once warm it idles ok but throttle response is not snappy and revs can take a few seconds to return to idle. I have tried hotter plugs (CR8e) but no improvement. I read that float height should be 7mm and not 17mm as per the manual. Is this correct and is the height measured when the carbs are held so that the floats rest at the lowest point? I was unable to remove the pilot jets but I cleaned them and can blow air through them. The manual says the pilot screw should be set to a certain number of turns but how many? Has anyone got any suggestions what I should try next, and how I can remove the pilot jet? Also is there a better quality photo of a carb showing the location of all the jets as the ones in my manual are not clear?

    • #6594
      ♠️ MARCEL
      Administrator
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      @mf448kxn6
      North UK
      GTR

      Carb, probably the first part we can address, so here’s some pointers to check for:

      • Float height is indeed 7mm when the carb is upside down on the carb , that is the float resting on the fuel valves.
      • Lift the float gently, and see if it bounces a little? – If it doesnt, consider the fuel valves to be replaced under it.
      • Pilot Jets should come out as they are soft brass and the carb body is just as soft too.
      • ultrasonic cleaning really does pay dividents or air compressor session on every hole thats inside the carb.
      • This topic is also a good read in regards to jet sizing, ensure the Pilots are both size 15 each. The factory mains are 87.5 front and 90 rear , upgrade size is 90 and 92.5 optimally
        https://hyoriders.club/forums/topic/tutorial-hyosung-gv-gt-125-250-how-to-swap-carburetor-main-jets/#post-5592
      • Check the slides for wear, and check this topic out below to see if the needles are on “middle setting” or “rich setting” – you don’t want it on the LEAN setting

        [Tutorial] How to adjust Carb Slide Needle (Tuning) – Hyosung GV & GT 125/250

        it should be noted slide needs from the shop are upgrade ones that allow more fine tuning  (5 or 6 grooves) – They always come as a pair for good reason.
        https://hyoriders.club/shop/service-maintenance/jet-needle-carb-diaphragm-hyosung/

      • lastly when the floats come out, the fuel valve assemly has a mesh under it,  it can get clogged with some debris , becareful taking it out as its soft brass
        But you will get new ones anyway , should you end up getting yourself a rebuild kit as precautionary. Who knows how old the gaskets are at this rate.

      Fuel.
      E5 only.  Hyosungs don’t really E10 , even modern cars in general due to the fact:
      – Extra ethanol  doesn’t stay fresh for that long, and can be corrosive towards rubber parts (any thing rubber related including intake manifold pipes) , and can be corrossive towards steel parts (tank etc) ,  and makes carb cleaning annoying after a while of sitting.


      Air Box fitting to avoid leaks

      • Make sure carb is empty first
      • Put the carbs on to the air box
      • tighten the clamps on the airbox hard. but not too much.

      Now then,

      • Hold the airbox and carb together, and carefully drop the whole thing in to the bike until you feel the manifolds swallow the carbs secure.
      • Please… manifolds are very hard to source now, as Korea stopped making them, so try your best not to stress them or cause them to split.  It can be a costly thing to get a new one!
      • make sure the carb clamps are NOT catching linkages of the throttle of carb
      • test the choke is working
      • test the throttle cable is moving freely
      • check your carb to make sure nothing is catching the metal clamps when the throttle is twisted on and off.
      • and ofcourse, secure the fuel line tight , it may get awkward trying to do it with the airbox already there

      or Fit the fuel line on to the carb before carb goes on the airbox,  then the fuel line other end will just join the rear pump when you are ready.

      This is the best way to guarantee there is no air leaks.

      Speaking of air leaks, see below


      Intake Orings.
      I can’t stress it.  Be very patient if you see an old bolt thats on the manifold rubbers and measure their length.  Run to halfords and replace with Stainless steel with a washer.

      Now then, make sure the surface of the engine is very clean where manifolds sit on, and do your best NOT to allow dirt to go inside as valves may get scratched up or cylinder walls of pistons when debris is floating around inside.  So mask the holes!
      Don’t sand anything off,  just use a plastic scrapper for stubborn dirt.

      Make sure the metal surface area of the intake pipe is sanded off (use fine or extra fine sand paper)

      Get racing white grease (high temp motorbike grease) such as Putoline i use.

      Get new orings below (they come as a pair)

      Intake Pipe O-RING (Inlet Boot Seals) x2 – GV125 GT125 RX125 GV250 GT250 Hyosung

      Now, everything might be perfect ?  But if orings are old or fishy ,it will cause half the issues you have described above too.

      Often overlooked, but just as crucial as you sealing the airbox too!


      Mixture Screws = This will be address as a last resort when we are sure we have diagnosed every part of the bike first.  But please, they are literally the most easily rounded off , ensure the area there is soaked with carb cleaner and flush out the dirt before attempting to turn them.

      Mixture screws do have orings too,  (for a reason) under the spring so nothing leaks fuel or air otherwise and can make fine tuning annoying, new screws already come with the orings anyway  as you saw at the shop.

      But make sure the mixture screws HIDE under the black bungs when done or see this product here for clarification

      Carburettor Carb Air/Fuel Mixture Rubber Plug (Adjuster Screw Bung) – GV125 GT125 R RC GV250 GT250 Hyosung

      Make sure the screw driver is “exactly” fitting, or it rounds off easy. Could be a day job trying to extract the old screw out!


      Have i helped so far ?  Let us know how you get on!

      Plus, i did not see the picture of your fuel tap, so please upload on this forum if you can –  Click the “Add Media” button above reply box you write in.

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

      • #6614
        Mike Daborn
        Member
        • Topics: 2
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        Joined:
        Oct 3, 2021
        @glide919
        Hampshire
        GT125

        Thank you Marcel.

        I have ordered parts according to your first recommendations below and will see how this goes before going further

    • #6600
      Mike Daborn
      Member
      • Topics: 2
      • Replies: 5
      Joined:
      Oct 3, 2021
      @glide919
      Hampshire
      GT125

    • #6601
      Mike Daborn
      Member
      • Topics: 2
      • Replies: 5
      Joined:
      Oct 3, 2021
      @glide919
      Hampshire
      GT125

      Hi Marcel,

      Thank you for your comprehensive reply. I have posted a photo of the fuel tap that I need as requested.

      From your suggestions I have a plan and a list of things that I need to check and to buy. I will start with the more essential fuel related parts and then review. I have some questions:

      1.The manual says not to touch the mixture CO screw so does this definitely have to be left alone?

      2. How do you remove the pilot jet?

      3.How many turns is recommended for the pilot screw?

      Thank you again and I now feel that I have a more methodical approach following your advice

       

       

    • #6603
      ♠️ MARCEL
      Administrator
      • Topics: 38
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      Oct 3, 2021
      @mf448kxn6
      North UK
      GTR

      You can remove the tap, and soak it in carb cleaner to ensure all debris is gone.
      Also you can unscrew the dirt trap that’s under the tap (it’s a cap  than needs a socket/spanner to unscrew it off)

      Then it is pretty much ready to go.  You may not need to replace it unless there is a leaking concern with it (ie. orings)

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

    • #6604
      ♠️ MARCEL
      Administrator
      • Topics: 38
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      Joined:
      Oct 3, 2021
      @mf448kxn6
      North UK
      GTR

      Hi Marcel, Thank you for your comprehensive reply. I have posted a photo of the fuel tap that I need as requested. From your suggestions I have a plan and a list of things that I need to check and to buy. I will start with the more essential fuel related parts and then review. I have some questions: 1.The manual says not to touch the mixture CO screw so does this definitely have to be left alone? 2. How do you remove the pilot jet? 3.How many turns is recommended for the pilot screw? Thank you again and I now feel that I have a more methodical approach following your advice

      1. & 2 = We will get to the mixture screws and adjustments when the bike feels like it needs to or when you replace the mixture screws (with orings)
        Korea generally discourages it as they have done it spot on from the factory,  some GV Aquilas use the same engine and carbs as the GT , but they tune it different sometimes but not “that much” , as to keep things uniform with 2 cylinders in harmony with each other They also make the rear slightly richer as it’s the hottest (due to less air flow going to it)

        Either way i will advise accordingly , privately , i’ve known someone to copy my mixtures and didn’t turn out very well on their bikes because i have modified the bike differently to theirs & different parts on different bikes etc .  General rule of thumb is that beyond 3 full turns out has no effect on most bikes with a CV carb like yours .

        3. A screwdriver that’s flat will slot in the pilot jet. And turn it left gently, it should just unscrew out pretty easy , unless dirty , allow the carb to be soaked in carb cleaner for sometime to agititate the dirt.

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

    • #6617
      ♠️ MARCEL
      Administrator
      • Topics: 38
      • Replies: 545
      Joined:
      Oct 3, 2021
      @mf448kxn6
      North UK
      GTR

      Thank you Marcel. I have ordered parts according to your first recommendations below and will see how this goes before going further

      No problem fella.

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

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