Reply To: GT125 2003 Performance Issues
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
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.
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