Forums 🚥 PIT STOP 🔧 Hyosung Technical Help Gt250 Naked 2005 model carb vacuum issues

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    • #3961
      Jacques Giani
      Member
      • Topics: 1
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      Joined:
      Mar 16, 2020
      @jakes
      South Africa
      Hyosung GT 250 naked
      Points: 18

      Hi. Can someone please assist. I bought a GT 250 naked for my son. There is no fuel pump on the bike and no vacuum lines. I looked at the vacuum diagrams posted before. the bike runs without a fuel pump as I bought it, however it runs out of fuel at 8000rpm. Is this due to the fuel pump or vacuum lines? Doesn’t look like there was a fuel pump on the bike and don’t know whether the 2005 model came with a pump or not.

    • #3962
      ♠️ MARCEL
      Administrator
      • Topics: 38
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      Joined:
      Mar 16, 2020
      @mf448kxn6
      North UK
      GTR
      Points: 4,196

      Hey,

      Can you reply with some pictures of what your current fuel pipe set up looks like ,  click “add media” inside the white reply box,  this will upload pictures on here.     My 03 donor has a fuel pump , and an on/off manual tank tap by the left side of the frame.  Upload some pictures so it helps understand if your pipe set up is correct or not

      Then likely attack the  carb next.

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

    • #3964
      Jacques Giani
      Member
      • Topics: 1
      • Replies: 1
      Joined:
      Mar 16, 2020
      @jakes
      South Africa
      Hyosung GT 250 naked
      Points: 18

      Hi Marcel. Bike is standing in storage. There is no tap or pump. There is a tap on the bottom of tank with 1 outlet for fuel. There was another outlet as well on the tap but the tube is gone. I believe this is where the one vacuum line goes to. So from this tap, there is a direct line to fuel intake on carburettor without a filter. That’s basically all there is. I’ve attached vacuum lines to the carb mounting rubbers but closed them off. This makes the bike rev and idle high at 3000 rpm. If I open the vacuum lines the bike idles fine, but back fires. The vacuum lines on top of carb is attached, but also just open. What I see on vacuum diagrams is that some vacuum pipes go to the fuel pump…which of course is not there. I think the issue of the bike running out of revs at 8000 rpm is due to the fuel pump that’s not there as there is not enough pressure to supply fuel to carbs?? Problem is that there is no indication of where a fuel pump would of been and getting spares in South Africa for Hyosung is extremely difficult, even 2nd hand spares. Other option is to install a fuel pump of a car…but not keen on doing that as well.

    • #3989
      ♠️ MARCEL
      Administrator
      • Topics: 38
      • Replies: 366
      Joined:
      Mar 16, 2020
      @mf448kxn6
      North UK
      GTR
      Points: 4,196

      Go to a local bike salvage or shop, look for mikuni DF52  or  SV650 carb rear pump.

      The Hyosung uses a legit mikuni rear pump from 125cc to 650cc (same unit), its single vacuum port with 2 gold liquid ports (IN & OUT)

      Running from the tank -> directly to the carb isn’t ideal.    The 250cc won’t exceed the capacity of the rear pump anyway.     So Tank Tap -> Filter -> Pump -> Carb = In this order.

      You will also make sure the float heights of the carb are 7mm exactly, as that’s fuel control from idle to top gear.

      Beyond 1st gear or 4,000 revs onwards, your slides start to lift and allow main jet fueling , ofcourse the carb will under go extensive cleaning since you said it ran with no filter,  (many holes in the carb!)

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

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