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

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    • #6593
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @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
      Admin
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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: 3
        • Replies: 16
        @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: 3
      • Replies: 16
      @glide919
      Hampshire
      GT125

    • #6601
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @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
      Admin
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      @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
      Admin
      • Topics: 45
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      @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
      Admin
      • Topics: 45
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      @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....

    • #6715
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @glide919
      Hampshire
      GT125

      Hi,

      I finally found time to fit the parts ordered as advised above.  The final check is float height. Am I correct that this should be 7mm with the floats resting on the needles as in the attached photo?

      i have followed the advice given so far and will do the same for the final assembly. I will let you know the result.

      Many thanks

      Mike

       

    • #6716
      ♠️ MARCEL
      Admin
      • Topics: 45
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      @mf448kxn6
      North UK
      GTR

      Hi Mike,
      Ignore the fact someone used 7.2mm   (extra .2mm is extreme!), I just used their picture, it is too dark at the moment for me to fetch out a carb and take a picture, please excuse me for that.


      • Just make sure its 7.00mm like this photo shows and you should be good to go…
      • Also, lift the float, and let it go, you should hear the valves under it bounce a tiny bit, only a tiny bit!  Then the spring inside the valve isn’t totally dead. It makes for awkward fueling situations for the float if the valves are dead.

      Hyosung Float Height GT125 Carb GV250 GT250 GV125

      Hope this info helps.

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

    • #6717
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @glide919
      Hampshire
      GT125

      Hi Marcel,

      Thank you for the speedy reply. I have also fitted new needle valves. I will let you know how the bike runs asap

    • #6724
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @glide919
      Hampshire
      GT125

      Hi,

      i removed as much as possible from the carbs except the pilot screws. Everything was cleaned. I fitted the carb rebuild kit, pilot jets and needle jets and float needles. Floats set at 7mm.  I fitted the carbs to the airbox to get a tight seal using the new stainless clamps and then fitted the carbs to the manifold rubbers, again with new clamps plus o rings on the manifolds. Everything was airtight as far as I could tell. Actually the best fit I have achieved to date. I fitted the Laser Iridium Plugs, new coils and leads.

      The bike would turn over but wouldn’t start. There were healthy sparks to both plugs but no fuel. I checked the obvious and then bypassed the fuel pump using a mobile fuel line and the bike started. It idles but dies when revved. I suspect there may be an issue with the pump but clearly I have not solved the carb issues.

      I don’t know where to go from here.

       

    • #6725
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @glide919
      Hampshire
      GT125

      Further update. Both plugs sooty. Carb bowls contain fuel. Bike won’t start. I have started to strip the carb again and this time will remove the mixture screw. I noticed that the rubber manifolds were wet with fuel.

      i want to remove and dismantle the fuel pump but the fixings are difficult to access so no luck so far.

    • #6726
      ♠️ MARCEL
      Admin
      • Topics: 45
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      @mf448kxn6
      North UK
      GTR

      Hey Mike,

      If you can see the filter filling up, that’s good. That means the TAP is on from the tank area.

      Next, take the fuel hose OFF the carb and point it to a bucket , then see if the fuel pump is shooting fuel out  ? The pump is the one that sits near the battery ofcourse.

      • This will check if there is enough vacuum to open the pump so it can do its job, it works in a pulsating nature of the engine intake cycle strokes , and it needs fuel flowing through it to work.  Henceforward check the tank tap is working first by observing if you see the fuel flowing through the filter.
      • Consider the hoses not to be kinked,  the bike does also like gravity assist, so consider how the neat the lines are routed.

      If the pump is good to go, then ofcourse the carb needs addressing again…

      • Take the pilots out, and look in the sky. If you can’t see through them, there is debris inside you will need to clean them harder. It is these jets that will cause the bike to act moody and cut off.
      • Mixture settings all good from my last advise ?
      • Get a clean hose..connect it to the carb, consider your mouth please as i rarely advise this redneck method of doing something but lift 1 float, blow through the hose , and let go of the float.  If you sense air is flowing through with ease.
        Repeat with another float. Then you know your fuel valves do open/close based on the float raising or resting on the needle.

      If the slide needle is on the 3rd grove pointing towards the tip, place it on the 4th groove facing the tip, that lifts the fuel up sooner as bike revs increase.

      Check the slide cap isn’t leaking any air by ensuring the slide caps were flat and screwed in well, as if there is a chance of air leak, the slides won’t lift efficiently , thus the bike dies when revs increase.

       

      I haven’t address the air intake system (orings, manifolds, airbox) as you have said you installed them as religious as you could.

       

      I hope after this you can finally have results and test ride the bike yourself to see how it is behaving.

      if the rpm needle drops fast ….and the revs stay at 1700 rpm (ideal) hot,  the bike is OK.
      if the rpm needle drops really slowly … you either have a pilot jet clogged that’s hunting fuel or there is an unknown air leak.

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

    • #6729
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @glide919
      Hampshire
      GT125

      Hi Marcel,

      i will test the fuel pump tomorrow.

      i removed the pilot air screw and it was set at  3 turns front and 3 1/4 at the back. Should these be 2 1/4 – 2 1/2 front and 2 3/4 back?

      i have attached a photo of the new pilot jet  next to the old one. The holes in the new one are much smaller. Will this be a problem? I also attached a photo of the new and the old needles and the new ones are much fatter do is it the fact that the old ones are worn?

    • #6730
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @glide919
      Hampshire
      GT125

      Hi,

      I have stripped and clean the carbs again. I installed the pilot air screws and opted for 2 1/2 and 2 3/4 turns front to rear. The pump appears to work but when I attach the pump to the carbs the bike dies. If I go direct from a mobile fuel unit to the carbs the bike idles and revs  like a dream without the airbox fitted or the carbs clamped to the manifolds. I checked the plugs and both are very sooty so I guess I need to later the pilot screws maybe a quarter to half a turn each?

      I don’t understand why the pump appears to work but the engine dies when it is connected.

    • #6731
      ♠️ MARCEL
      Admin
      • Topics: 45
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      @mf448kxn6
      North UK
      GTR

      Hey,

      The as long the pilot jet says it is size 15, it is OK.  The pilots are working by the fact you said it idles and revs up like a dream.  I would leave the mixture screws at 2.5 turns each and leave it there until i advise when to reduce or increase it based on how it behaves on its upcoming road test ride.

      If the pump was opened and there was nothing faulty in it ?  Have you connected the pump back to the fuel filter.

      The hose that leads to the carb, take it off and see if fuel shoots out and does it flow it or comes out like a trickle drop drop ?  If it’s the later,  investigate the manifolds are fully sealed and the carb is fully sealed to the manifolds to improve the vacuum system to operate the pump.

      Both rear and front manifolds should have nipples to send vacuum to the rear pump as it does need both engine cylinders cranking to draw it open.

      Failing that, it may be time to replace the pump. It could be its original one and never investigated in the bike’s life time.  They rarely fail , but it does happen & isnt uncommon.

       


      The slide needles both look good to me, can’t see any obvious bending. However, i will say new is better than old.   Remember the advise of setting to the 4th grove towards the pointy end or revert back to the 3rd groove  , if it behaves lean,  5th groove it is.

      The needles take full effect when the bike revs go up and you’re riding as its part of “main jet” fuel metering.


      I am glad the bike is running at least, so i think our last stop is the diagnosis of the fuel pump and vacuum system.  I got your email , i’ll reply to that one too shortly!

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

    • #6733
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @glide919
      Hampshire
      GT125

      Hi,

      Fuel pump checked and works fine.

      Last part will be to fit the K&N filter and bigger jets.

    • #6736
      ♠️ MARCEL
      Admin
      • Topics: 45
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      @mf448kxn6
      North UK
      GTR

      Hi, Fuel pump checked and works fine. Last part will be to fit the K&N filter and bigger jets.

      That’s good news! – One less thing to worry about 🙂

      See how it goes with the carb, also an excuse for one final major clean,  (we can’t clean once can we,  sometimes twice or thrice is good!)

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

    • #6760
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @glide919
      Hampshire
      GT125

      Hi,
      One further clean of the carbs and the bike runs fine without the airbox fitted. Revs well, idles well and response is snappy. I fitted the upgraded K&N filter plus jets and all good without the airbox fitted so filter not being used at this point. Fitted airbox and it will idle and rev but not so well. Will sometimes die after hard revving. Sometimes the revs take a few  seconds to drop.  All the connections between manifolds and carbs and carbs to airbox are all tight. If I remove the air filter it seems  to idle better. After a few revs it starts to increase revs on its own.

      I did notice that the rear cylinder took much longer to warm up than the front one.

      I don’t know where to go from here. Totally lost and a bit fed up.

    • #6767
      ♠️ MARCEL
      Admin
      • Topics: 45
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      @mf448kxn6
      North UK
      GTR

      Hey Mike,
      Happy holidays!


      Sorry I delayed,  I didn’t think we would hear back from you so soon since i think the K&N filter was the last thing the bike needed before its road test.

      Has it been through its road test yet to see how it behaves or will it just cut out if you put it in first gear ?

      If you are adamant these are okay below:

      • Clutch Switch was bypassed by previous keeper and disconnecting it cuts the bike off  (as a test) then put it back on.  or the switch is not cut off but it’s working.
      • SideStand has been bypassed  and waterproofed well as rusty terminals will make it behave temperamental too. The CDI always “wants” a complete circuit   (either from the switch itself or by bridging the wires  aka bypass

      If the bullet points check out OK above ?
      I will ask again, if the bike idles fine

      • Can it take a short ride down the street without an airbox or will the engine cut off once in first gear ?
      • If you fit the airbox and free flow filter back on and take a ride , does it seem better as the speed increases ? Ie. from 5mph to 20mph local street speed  (basically putting more load on the bike to see how it behaves ?

      Please try these so that i can have a more clear cut answer as in regards to your post just before i responded now.  You got this….

      What you have been through just now , i have experienced it myself on a donor bike that’s also an 03  and a 66 plate donor bike ,  but they had different causes. So i want to be sure what i can advise you next … Do the tests and report back if you can, it’s a warm week 🙂

      I will watch this topic

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

    • #6778
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @glide919
      Hampshire
      GT125

      Hello,

      I now know what the issue is: the rear manifold has delaminated from the metal base plate and was moving when the engine was revved. I have separated it from the plate and hope that I can rejoin them.

      I also need to place an order for the screws that hold the needle jet base in place as one sheared in half tonight.

    • #6789
      ♠️ MARCEL
      Admin
      • Topics: 45
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      @mf448kxn6
      North UK
      GTR

      Hello, I now know what the issue is: the rear manifold has delaminated from the metal base plate and was moving when the engine was revved. I have separated it from the plate and hope that I can rejoin them. I also need to place an order for the screws that hold the needle jet base in place as one sheared in half tonight.

      It sucks when that happens, don’t forget to make sure the “orings” are in good spec otherwise we generally replace them anytime the manifolds are off.

      Please save that manifold, if there is a hyosung part that is in major scarcity , it would be those rubber inlets and korea stopped making them a long time ago. You don’t want to spend 3 figures asking MotoGB/Sinnis to give you one  (real Hyo UK)  (not the fake spares uk one , they don’t have it.)

      I won’t digress as i will assume you know what you’re doing and have seen loads of youtube videos on restoring classic bike intake inlets or cars that used to run carbs in the ancient days.

      Oh yeah, i have manifolds also with base plates disintergrated, blame korea for the poor sealing, i just didnt get time to fix them up yet before i flog em away.   I may just keep them as salvage for when my cnc guy eventually makes me half-rubber/half-metal ones , since he already does cnc stuff for hyos on my repquires  (ie. rearsets, clip on bars etc, making cables of any on demand)

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

    • #6832
      Mike Daborn
      Member
      • Topics: 3
      • Replies: 16
      @glide919
      Hampshire
      GT125

      Hi,

      Manifold repaired and all air leaks stopped. New filter fitted. Bike starts, idles, revs and goes as it should and MOT passed. Success!

      Thank you for all the help.

    • #6844
      ♠️ MARCEL
      Admin
      • Topics: 45
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      @mf448kxn6
      North UK
      GTR

      💯❤🏍 Happy for you!  – I like stories like this.  Glad it’s all good and now the rider can enjoy the bike for what it is!

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

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