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

How to adjust/tune the carburettor slide needle for improved riding…

---

⚙ What does this do?
This modification allows the bike to reach better cruising speeds , via the slide needle adjustments.  The throttle slide needle is what controls the main jet fuelling as revs go up & down while riding.  This also eliminates some of the odd flat spots that happen during the 4k-7k rpm range , commonly on the 125s.  In other words, for example if the bike starts to pull at almost full twist throttle , this mod will make it pull with less of a twist.  Your bike should run better from 1st to 5th… fuelling wise…

👇  Before we start, This tutorial will assume the following:

• You will do this at your own risk & without rushing.
• You have better spark plugs (ie. Platinums) or older common IX plugs.
• Your ignition coils & HT caps are sparking good
• Your carb has already been upjetted (tap here…)
• You have a performance filter (K&N, BMC, etc)
  or if you have a white-stock cotton one -> cut its snorkel.

---

Tools needed:

JIS type screwdriver ( +philips style )	Large & long flat___  head screwdriver	Wynns Carb Cleaner
or … Allen Key (if using stainless bolts)	Size 7mm socket (sometimes bolts near the front are hex shaped.)
Watchmaker flat screw driver	Clean microfibre cloth

 

 

1. Go to the right side of your bike
You will spend most of your time here. Do 1 pot each time.
Make sure you do not get distracted as we are taking out small parts!

2. Unscrew the 1st diaphragm cover 
Remove the 4 bolts that hold the diaphragm cover
⚠ You need to disconnect your throttle cable also.

3. Remove the spring & the diaphragm
Once the cover is off,  gently remove the piston slide & its spring.

 

4. Inspect the parts for any debris  and clean them.  
Use Wynns Carb Cleaner to clean these parts including the diaphragm cover👍
This will stop your slides sticking between revs / gears & scoring to the internal carb walls.

 

5. Unscrew the plastic plug holding the needle
Use a long good flat blade screwdriver for this and BE GENTLE – ⚠You do not want to damage this plug!

 

6.  Do not forget the order of re-fitting back!
Keep this image in mind, when you refit the needle back. The order of these small bits is crucial too.

 

7. Remove the E-Ring (Circlip) on the needle   (SLOWLY!)  
⚠ This tiny part must be removed gently with a small watchmaker screwdriver only,don’t pull it!

 

8A. See those 3 grooves on the needle?   Read on…  

• A. The pointy end of needle FACES DOWN
• B. Push down the “black spacer” a little bit.
• C. Put the “E-RING (circlip)” in the 3rd GROOVE
• D. Push the “black spacer” back against the E-Ring (Circlip)

8B.  Here is another view for you…

You want your “E-Ring” on the 3rd notch mark like the finished example including the black spacer being pushed back to where it was.
(Lowest groove with pointy end of slide needle facing down.  The Factory set-up is usually the 2nd (middle) or top (1st) near the FLAT end of the needle.)

9A. Grab the throttle slide and look for this hole…
Make sure the hole shown in image is clear  and  proceed to  9B ….

 

9B.  Grab your needle too and follow the image….

9C.  Slot the “Needle => first.”  & its mini-spring on top. 
⚠ Go back to #6 ^  , to remember how the needle spring goes in.
⚠ Remember what was said in 9A & 9B ^

 

10.  Check the needle for free movement (a little bounce back)
Follow the 2 images for instructions/check up…

10B.   Dress up the ring of the throttle slide with a temp oily coat.
It is going back to the carb, so you want to make sure the ring of the slide will sit firmly & create a good seal once fitted. This is to help prevent any unwarranted air leaks / external particle trapping.

 

11.  Put the slide and big spring & close it up.

– Don’t forget to connect back your throttle cable
– Ensure the 4 screws are nice and tight (finger tight)
– Finished?  Go open the next cover…

12.  Repeat at the rear carb, what you did at the front…
Repeat steps 2 to 11^ on your rear carb.

13.   Finished?  🏁  Check your lines before riding…🚦
Final check ups before you start riding , so attention to the following:

• Air box clamps are still tight – check.
• The carb slide cover bolts are finger tight (x8)
• Throttle cable & choke cable are checked.
• check your air lines and tighten them (preventing any vacuum leaks)

---

Done! this should have been a straightforward process ,  if you are unsure of something , just comment on this topic.   Otherwise,  go ride! 🏁

---

FAQ’s / Troubleshooting 🎳

• My bike runs worse after I did this mod, what gives?
  Check again the covers are tight so its not leaking air or check your intake system too (airbox area)
• No air leaks, but it still runs worse?
  Your spark plugs may need to be looked at, if its very sooty, clean it.
  Also remember this mod expects you to have some better sparking than a stock hyosung does
• It has new spark plugs, but still worse…?
  If you have Platinum plugs fitted or  the IX ones , but the plugs appear sooty…
  You may need to investigate your HT Cap & Ignition Coils.
  HT Cap on its own (removed) = 6k ohms on your multi meter it should say.
  Ignition Coil = 6k ohms max  when you measure coil HT WIRE and you bike FRAME. 
• Okay, the HT cap or coils are showing higher values than 6K ohms? Any fixes?
  Replace them both.   The stock ignition sparking parts can’t be user-repaired.
  All Hyosung stock coils and their “GOLDEN” branded caps not great, another main cause of boggy runs on the 125cc bikes especially.
• I changed the coils and HT caps already, its still worse?
  Look at your main jets are clean and also investigate your pilot jets
  The ofricies in the 125cc jets are very small! like this text!
  Tap this link (here…) to learn how to remove the carbs off the bike
• Can I do this on a stock 125cc un-modded bike (ie. brand new machine)
  Not really, it is already running leaner, so raising the needle may help a bit but jetting and uprating the electrics a little will make this mod more effective , because raising the needle makes the mid range a bit richer. (extra fueling)