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How to rejet the GT650 with photos

Bretto · 52 · 77499

Offline Bretto

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Re-jetting Hyosung GT650 carburettors for Korider

As with all modifications all care but no responsibility has been taken
I recommend you read the GT650 service manual and also read the link at


before attempting, they help a lot; if not sure get a professional to do it for you.

If you are still game, this is how I did it. I used info from the above links, info from other motor cycle mechanics in town and other stuff I found on the web as well as looked at and purchased kits around. One of the easiest kits I found and good value for money was Larry Cooks kits, comes with couple of jets (some kits have too many to choose from), drill bit and instructions. So if you want to do it your self but don’t want to mess around as they have done in the SV650 rejetting notes grab one of Larrys kits. This re-jetting DIY assumes that you have a free flowing exhaust already fitted.

Position your bike so you have access to both sides with room to move

Remove seat and disconnect fuel sender unit plug and remove 12mm bolt at rear of tank.

Raise tank and remove fuel hose, vacuum lock hose and tank breather hose from base of tank.

Slide and wriggle tank back from bike, watch front rubber grommets as they can fall off from frame of bike.

Remove lower fairing (this makes it all a lot easier – I have done it with fairing on and removing is the quickest and best option)

Remove air filter screws and air filter

Lift air box up from carburettors, pushing rubber seals inside box down at same time,

Using long nose pliers remove AIS hose’s (RH side) and crank case breather hose’s (LH side) from air box.

Remove vacuum line from front carburettor to AIS systems, fuel line from fuel pump to carburettors and vacuum line from rear carburettor to fuel pump.

Loosen retaining clamps from base of both carburettors.

Pop up idle adjustment cable to holder (RH side engine) and feed back to top of engine area.

Remove two screws from LH controls and separate halves to remove choke cable.

Remove choke lever from assembly and remove cable and feed back into engine area.

Wiggle carburettor assemblies side to side and up, loosen and remove throttle cables (easier with carburettors up out of inlet tubes) remove from bike checking that all cables are clear and free.

Place clean rag over intake tubes and place carburettors on clean bench and inspect for any damage or wear.

Drain fuel from float bowls, catching fuel and inspecting for rubbish.

Complete 1 carburettor at a time.
Remove 3 screws from float bowl, these lock in really tight (could be caused from 2 different metals aluminium and steel and different heat transfer rates causing them to lock in) use a gripping compound, a really good screw driver or a No 2 bit in a ratchet set up, I have had these apart several times and even after 100 klms of riding they become tight again. Remove bowl and inspect.

Ensure all is clean and remove and replace main jet, pilot jet and check float level (7mm as per service manual).

Main jet used in rear carburettor 140 and front 142.5 (Split jet set up)

Pilot jet size 22.5

Check and set float height (7mm) and refit float bowl and screws.
Complete same procedure to remaining carburettor.
Turn carburettor assemblies over and remove slide cover screws (these are also very tight)

Remove cover and spring

Remove slide assembly with rubber diaphragm (be careful of the diaphragm and needle). Take note of position of vacuum port.

Remove needle assembly from slide – be careful of parts – retainer, spring, washer and needle. Use long nose pliers to remove retainer.

Remove C clip from needle with small screw driver and reposition down one groove (4th from top) this raise’s the needle.

Drill out smallest hole in bottom of slide with 1.2mm drill bit.
Refit washer and place spring over end of needle and refit to slide, refit retainer and ensure locks into place.
Refit slide, check needle as you slide it back into the carburettor, also ensure rubber diaphragm is positioned correctly – vacuum port.

Replace spring, cover and screws.
Complete same with other slide.

While you have to carburettors out it is a good time to complete the AIS block off, I took the hose from one AIS to the air box and joined the two AIS’s together and blocked the vacuum hose from the front carburettor. The second AIS hose I used to block of the ports at the air box.

Position carburettor assemblies back into bike and fit and adjust throttle cables, wiggle and push down on carburettors until they are seated into the intake tubes, tighten clips and feed idle and choke cables back through. Refit idle cable into holder and refit chock cable into LH control assembly.
Check operation of throttle and choke cables.
Refit fuel line from carburettors to fuel pump and vacuum line from rear carburettor to fuel pump.
Refit air box over carburettors and feed through rubber seals from top. Run finger around inside of seals to ensure a correct fitment. Move air box slightly and check sealed correctly. Refit crank case breather hose’s and clamps.
Drill 2 x 25mm holes in air filter and clean filter, refit to box. New jets and free flowing exhaust need more air in.
(After a bit more testing I have upped the main jets sizes and added 4 3/8 holes to the air filter)

Refit fuel tank and hose’s, check operation of throttle cable and choke cable and recheck all work.
Start engine and warm up, set idle speed 1300 – 1500 rpm.

Next step is to check the balance of the carburettors as if out of balance the engine will get different amounts of fuel and air and vibrate. I am in the process of a DIY carburettor balance using a home made manometer. When I checked my carbys they were out and the differnece when balanced was worth the trouble to balance them.

After a couple 100 klms remove spark plugs and check condition, I found mine was slightly sooted up, only just and I drilled another smaller hole in my air filter to allow more air in. Learn how to read your plugs, they will tell you a lot about your engine.
If your plug is showing signs of being too lean you can just tape back over the small hole, you can do this for change’s in altitude, fuel quality and air temperature.
Good luck and have fun.
This took more time to write up then it took to complete so if I have missed something or got something wrong go easy on me :?

« Last Edit: August 06, 2011, 09:30:05 PM by Bretto »
2006 GT650R, derestricted, rejet, mod airfilter, Screaming demon exhaust, stainless braided brake lines, Metal Gear brake pads, Carbon fibre look mirrors with inbuilt blinkers and brake light modulator.

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