GT125RC 2016 Flat spot and False neutrals

[Ryan]

Hi,

I bought my GT125R new and it has had quite a few issues since I got it such as leaky brake, Not wanting to start and what seemed like fuel issues.

The main issues at the moment though are a flat spot between 6k and 7k rpm and also giving me a false neutral between 3rd and 4th.

To help improve the bikes starting and overall running I have upgraded the ignition coils, fuel lines and filter, and upjetted the carb.

I have also ordered a new rectifier and spark plugs from this site.

I was wondering if anyone has any ideas as to what is causing this flat spot aswell as the false neutral as it is not a great feeling while going round a corner. I have tried unplugging the brown plug on the cdi as suggested on the facebook page but that makes the bike cut out when clutch is released.

Also, there appears to be a small hole in the rubber tube going from the carb to the rear cylinder and would like to know if that should be there.

Any help would be greatly appreciated.

Thanks

Ryan.

[♠️ MARCEL]

Hey dude
The CDI cheat never works on a 125 – whoever said so is a faker
“A few”  250 cdis allow 1 wire to be cut – which helps the 250 engines rev slightly better on its STOCK coils.

If you had upgraded the electrics on the bike, it should be very spirited by now.

Sadly, as i’ve experienced fixing customer RC bikes the 4-7K rpm issues is usually caused by either of these:

1. Ignition coil isn’t getting 100% electric juice  (suspect = cdi / magneto )
2. Spark Plug could be sooty  (trust me! – RC bikes do not tolerate sooty plugs more than the old gen bikes & electrical changes = RC bikes have some woeful EFI electrics but i digress!)
   So –> Sooty plug = Intake issues / weak sparking ? / fouled plug (annoyed plug)
3. Air filter needs attention = more air is sucked in as you go faster but the carb likes a clean filter & not restricted to less air flow than before
4. Clutch needs looking at (any RC over 4K must have a new clutch set as the cheap organics wear down fast –> hence the gearbox drama and false N’s)
   Search “Clutch” at the top of the site which explains what kind of power issues you get on a clutch that has worn down a bit (Symptons)
5. However , we can’t rule out a weak spark or a misfire – Make sure the “HT” cap is under 7k ohms  – > RC have an EFI HT CAP for the spark plug that is not that much better than 2007 however too much ohms –> weak sparking -> delayed RPMS
6. Intake Manifolds need investigation – > you need to take out the fairings  and possibly the carbs off – then inspect your intake manifolds – there should be zero holes or zero leaks  (protects piston rings from dirty particles being sucked in too)
7. Battery – If it isn’t 12.8v fully charged when off = inspect magneto , all electric juice starts from there before it feeds the rest of the bike too.
   If you also switch full beam –> engine off -> indicators / hazards on –> if the dash  “DIMMS”   -> check battery is holding up juice / not dropping volts too quickly  (battery should not discharge quickly)
8. intake lines (vacuum) – only lose clamps can cause leaks or  a very old “stock” line that is rotting inside  Vacuum performance is needed for the fuel pump & fuel tap to be effecient in high revs  (4-11k rpm) (more demand = more fuel). Big lines help the vacuum & fuel system to run a bit better (colder air/colder fuel) on a hot engine.
9. CARBS!
   i saved this for last as it’s usually a time consuming process but you may need to inspect your “black CV Sliders” inside the carb and clean them
   CV sliders control most of the carb work, so  as you rev the bike , the sliders go up  , they release more fuel!

   If the bike is hanging at certain rpms or stuck at certain points –> CV Sliders in the carb need cleaning (common) ,  Clogged jets in the carb , float bowls need inspection to make sure they are all at the same height  (not bent floats)

   When you look at your own spark plugs , thats when you get a true story of whats happening to the bike.  A healthy motor = all coffee brown color , no black anywhere.

   Spark plug color hints  = (Intake system / Fuel System / Sparking Electric system conditions) 

 

So it’s a case of verifying what i said above & most times some TLC on the intake system & carbs wakes it up.   Electrics are common on RC but not impossible to fix,  if you have upgraded electrics , that’s a good thing Less drama down the road!

Let us know how you get on, or if i missed something too.👍

Also have a look at this , as some of my replies can apply to your situation too.
https://hyoriders.club/forums/topic/gt125r-no-power-between-4k-8k-revs/

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

[Ryan]

Hi,

Thanks for the very detailed reply,

 

The bike is only 4 months old as it was a pre reg and has only done 3577 miles. If it is the clutch what should I replace it with?

The carb appears spotless inside and  the spark plugs seem to be a dark brown colour.

The place i bought it from has adjusted the carbs once or twice as he claimed they were too lean from the factory. However since then i have changed the jets and dont know if it will need altering again.

I dont think the gouge in the rubber goes all the way through under the carbs  and the lights do not dim when everything but engine is on.

Thanks.

Ryan.

[Ryan]

Also, the baffle from the exhaust is very black and covered in soot when removed. Is this normal or is the fuel too rich?

Thanks

[♠️ MARCEL]

Ahhhh,

Carbs hence i left it as last haha. There is some discussion about this too on the FB group too.

The service manual actually forbids workshops or dealers from tinkering the air mix screw.

Hyosung never revealed how to mess the air screws simply because it would be the abyss to get it right again!

(Lots of plug chopping & re tuning)

If the carb was lean all he had to do was increase main jet 1 size up.

90 n 92.5  so bike is less lean more rich without messing the air screw.

Stock is 87.5 n 90

Dark brown is good!   Should be lighter brown all round , suggests to me that you may need more air (filter upgrade due?)

More fuel = more air to mix 👍.

If you search *carb* at the top ,there is a tutorial on jet swapping which mentions more detail on sizes and why we dont need to change *Pilot Jets*

Just *Main Jets* usually only upgraded thats all.

Sooty exhaust may indicate a weak sparking behavior from magneto/cdi?   –  or else you need more air  or  the jets are too big   (or air/fuel screw ratio off?)

Your mech owes you to fix the carb since he tinkered with it so it must give us a good result?

A little soot is a ok but caution ofcourse how much soot is there.  Too much aint good  (hint = bike spits raw fuel on cold warm up)

If the carbs are cleaned thats brilliant  but did the mechanic clean the CV sliders (the black tubes)  they are the mother of the carbs , defo must be zero signs of particles then they slide & retract nicely

(Slider behavior -> Up n down based on your revs => they release JET needle up n down attached to it)

Usually touching carb sliders.  Push up by hand and you will see them retract very fast  & same pace , no delay or getting stuck =  thats a hint of health there.

 

So battery works.  No dimms.  Seems to hold well.👍

Did you strip down the bike plastics? And then look at every nook n cranny near the intake system n carbs to really be sure its leak free.

Whatever bolt is on carb n engine must be *tight* =  hot engine can make things flex like rubber boots  (like taking a gulp of air) the clamps must be tight so it doesnt leak out even a small hint of air.

 

Last not least,  confirm with yourself that all exhaust bolts are tight.  We loose power on a leaky exhaust system too.

So to put in short, we are checking  intake system , exhaust system for leaks.   Re investigate the carbs , jets , air filter and including the black CV sliders in carb bounce nicely (critical)

 

Keep us updated ,   we can do all we can to help the bike 👍

 

Good luck 🔧

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

[♠️ MARCEL]

Also, as the RC is 3.5K

Stock clutch may be ok but probably due soon but if youre looking for a better clutch ,  this site does EBC UK plates & race springs so get more out them and a good torque boost on hills too guaranteed.

Its at /shop/ section of this site.    I know no one at all doing race clutches  for 125s except myself ,  our own local UK dealers only do expensive & woeful china plates.  £20 a piece!

 

Id stick with the trusted EBC brand & Suzuki bits 👍

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

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