GT125 running issue

[Pete]

Hi guys,

Just joined and looking for a little help, i recently picked up a GT125 and it has some running probs.

Year 2016 carby with full black widow exhaust system

Trouble starting when hot or cold no matter if the choke is on or off.

Up to 1/4 throttle the bike splutters and has no power almost to the point of cutting out, open throttle fully and it will eventually accelerate once the initial spluttering stops (takes 5 or so seconds)

 

What i’ve done

Cleaned carbs and upjetted mains, 90 front 92.5 rear which is much better at higher rpm

Left pilot jet as never heard of needing to replace on a hyo

Made sure no vaccum leaks

Air filters new

New plugs

Checked electrical system, all seems ok

It feels like fuelling but i’m at a loss

Any help would be great

Thanks

 

 

 

 

[♠️ MARCEL]

Hey @sebringmaster

Let’s try another method now,

1. Go to halfords locally, grab a can called BRADEX EASY START
2.  Lift up your fuel tank
3. Spay lots & lots of bradex in to the air filter hole.
4. Fire up the bike, hold the starter for 7 seconds max
5.  WHILE the bike is “cranking” -> keep twisting the throttle multiple times…
6. Once it fires ,  use full choke until it warms up for 10 mins.
   …..failing that….

Run to halfords or eurocarparts or MotoSave again,

1. Grab a pair of NGK CR8E IX  (Iridium plugs)
2. Connect 1 spark plug to the “IGNITION COIL HT LEAD” (HT Cap)
3. Do not FIT spark plug inside engine.
4.  Leave the plug exposed while connected to the WIRE HT LEAD
5.  Switch on Dash.  Put bike in neutral.
6.  Put the SPARK PLUG AGAINST the  “FRAME” or “ENGINE”  (Avoid PAINT)
   (Look for a metal frame part which is not painted)
7.  Press the starter button   (keep the plug on frame)
8.  If you see “SPARKING”   while it cranks ….
9.  That means THAT COIL works.

Repeat this process with another COIL.

What we have done above ^ is checking if your ignition coils front & rear are actually sparking good.

if there is a weak spark.  It is probabaly the coils trying to give up the ghost or behaving erratic  (very very common on stock hyo coils)

Battery:
OFF = 12.8v   / ON = 12.7v max  (it must NOT drop by 1.0 volts)

If a battery says 12.5 or less while on  (without cranking motor) = Investigate the battery , local bike shops can test your battery to see if the CELLS inside actually hold charge or quickly loose their 12v juice  (not matter how charged up it is, the cells must hold it)

Remember = GV125 , GT125 Comet & GTR use identical engine, identical coils….
So please have a read on these links where i gave more detailed response to the others,

1.  https://hyoriders.club/forums/topic/hyosung-gt125r-starting-problems/
2. https://hyoriders.club/forums/topic/2008-gv125c-headlight-running-issue/#post-1293
3.  (Best one) = https://hyoriders.club/forums/topic/hiyosung-125cc-gv-running-slow-when-warm/

 

One thing to always remember =
High Revs / Hanging needle =  Air leak somewhere  / carburators probably messed with.

Boggyness / jerkyness / cuts =  Electrics & Fuel.

Common cuts are usually coils ,  fuel tap , clogged filter & rotted lines , carb has foreign gunk clogging its jets, side stand switch,  clutch lever switch

For more reading on carb 101 , check this out… (I talked about jet sizes & pilots)

[Tutorial] How to swap Carb Main Jets – Hyosung GV & GT 125/250

Report back and let me know how you get on!!

Last tip! – Remove the front spark plug -> spray lots of bradex INTO THE HOLE (in to engine)  – > reconnect the spark plug -> Fire up, and keep the revs up…. till it idles nicely to 1.7k rpm  (best spot)

Go to fuel station. Fill up 4 bars of fuel  (£4-5) , and buy REDEX 2 SHOTS (2 small bottles) -> Fill all of it in to the tank , so it mixes with fuel.

Ride your bike until the fuel level drops to 1 bar…. We have to see if redex can help attack all the gunk that travels on the lines, filter,  and also attack gunk in carbs, so that once it hits the piston, the get burnt off by the plugs.

Personally once a month, I use LiqyMoly (german)  , same as redex but for bikes with extra additives,  the principal is the same.  It means i have less maintenance work to do with carbs.

 

Good luck!

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

[Pete]

Morning,

Thanks very much for your detailed reply, unfortunately almost everything on your list has been checked with the exception of the coils (both cylinders/plugs) are firing however i will check for a weak spark.

 

You mention the lines could be dirty which is something i didnt think about however after i gave the carb a good clean it didnt run any better and the settings are stock apart from the main jet replacement which wouldnt affect the running at 1/4 throttle.

 

Brand new battery has also been fitted and working as it should.

I’ll take the carb off again and have another look, always a possibility that there is gunk in the tank thats pulling through, as the bike is only 18 months old i’m at a loss 🙁

[♠️ MARCEL]

For the coils,

1. Grab a multi meter
2. Red wire on the HT LEAD
3. Black wire on the spade-terminal of the coil.

You must get 5.0K – 6.2K Ohms maximum. That’s a healthy coil, any values beyond that, the coil needs replacing as they can’t be repaired.

5K = 5,000 ohms.

Like my example here..
(Black wire from your meter must test both the “terminal” spade and “fins” (where bolt slots through)

 

Take out your spark plugs , this image below was my laser iridium plugs after 2 years (20k miles) .
COFFEE BROWN =  Healthy fuel system including your carbs (optimal)
BLACK CHARCOAL = too rich or carb issues inside (gunk?)
BLACK & WET =  fuel smell = coils are not working or plug itself is shot.
BLACK & OILY = piston rings (God forbid!)
WHITE CENTER  = too lean… that will kill piston rings very soon! (Suspects = carbs , coils & plugs)

In all cases the last 2 are critical and no one want’s to be there… better to be richer than leaner…

 

 

You said you only had it 18 months. How many miles is it done so far ? & does it have history of service inspections  concerning the check for plugs ,  air filter clogged , and possible check on coils ?  if it was behaving erratic in the past before ?

If it was a new bike, the dealer would “have” to fix back to standard again as part of the warranty.

 

Pilots & Starting Jets  (2 other tubes next to main jet) control starting & idling , slow running until main jet takes over from 2nd gear ish or full open throttle.

Get a CLEAR hole , while cleaning , like my example below:
Pilots are VERY notoriously easy to gum up! –

A big fuel filter helps a lot prevent gunk going to the carbs in the 1st place.

A clean air filter is also important! – Some clogged particles do get sucked in!

 

“STARTER JET” is the jet that can’t be removed, but compressed air with a straw can blow hard through it or a carb cleaner is best.  The hole is much more tiny than the pilot!

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

[Pete]

Thanks again for the quick reply,

I’ll give the coils a check at the weekend.

 

As for the plugs i swapped them out when i bought the bike along with a new filter, will need to check the colour now.

Bikes done 3k since new and unfortunately i have no history in terms of previous issues, it’s had difficulty since i got it.

I did notice the pilot jets were gummed up when i cleaned the carb so they are ok now.

The starter jets didnt get cleaned and you describe almost perfectly the issue i have, i’ll strip the carb at the weekend and report back, i may owe you a beer 🙂

 

Thanks again

 

[Pete]

Sorry it took so long for an update, thanks again for all the above suggestions.

All fixed, below is what was required.

Carb had a sticky choke valve so was drawing more fuel than it was meant to, fixed and still running like crap.

Pilot jets 1 size up

Main jets 92.5 front, 95 rear

Moved needle 1 slot for mid range for more fuel.

Tested over a few weekends with plug chops and plugs are the perfect colour now.

So much for a plug and play exhaust system 🙂

Bikes never ran this good even when stock.

Now that it’s running perfectly i can work on enhancements.

 

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