GV125C Tuning frustration

[Darkvader]

Hello everyone!
I’ve seemingly hit my skill ceiling, when i rebuilt the carbs the other month I had a running issue and so had to put things on the back burner while I got my new jets and did my full A mod 1 (which i’ve just passed!).
At the minute the bike is behaving like there is a vaccumm leak, it has quite a bad rev hang and the front cylinder will drop out at idle, however I can’t find one.
I’ve gone over everything three times now, i’ve changed the o-rings and checked them twice, i’ve swapped plugs over to rule out spark etc etc. unfortunetly having no luck at the minute at all.
One thing that IS better, the bigger jets (to 87.5) has made the high speed portion of the bike much happier. I also did the slide mod on the rear cylinder and things are much happier with that cylinder, but the front cylinder is still causing me grief, i’ve even been getting backfiring (though that has been just today, yesterday it wasn’t doing it and it may be because i had the tank off).
What would you do in this situation?
I am thinking new pilot jets but haven’t a clue where to go from here.
thanks everyone

[Darkvader]

oh i need to elaborate, At high speed the front cylinder does come back and will run perfectly, but throttling off will cause the bike to rev hang. Also the bike doesn’t surge at idle, it just seems to not want to fire on that front cylinder (doesn’t get as hot)
i pulled the plugs after a ride and the front cylinder has a light tan to it and the rear has a moderate tan, basically perfect… if it wasn’t for the idle issue.

[♠️ MARCEL]

Hello 🙂
I hope you had a nice weekend and you was able to make some free time to make any further diagnosis ?

And also it sounds stupid , but i feel like i need to start here first
– can you make the engine hot for 5 mins idling
– plugs out , both of them
– full throttle open
– put a compression tester in front head
– crank the bike for 5seconds , and see how much PSI the engine makes at full throttle

Then repeat for rear head , report back here.

Please make sure to use only the “metal part” of the compression tester  (it is a straight metal bar with a rubber end that needs pushing down hard)

Don’t use a “Flexi Flexi” hose attachment ,  so to avoid false readings as flexi hoses bulge anyways

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

[Darkvader]

🙂 i did thank you, hope you did as well! I sadly didn’t get chance to try sort the bike again but I did manage to sort the garage out, so when i next have a chance it wont take as long to find things.

I haven’t yet got a compression tester but I’ll be ordering one shortly, hopefully by sunday next week i’ll have had a chance to do a compression test.
I’m not sure if i’ll be able to idle for that long but what i can do is have a quick back and forth down the road if that works? it’s quiet here so aside from the strange looks it wouldn’t be a problem if that’s better haha.

I’ll update you on the measurements when i get chance to do them
Thanks Marcel!

[Darkvader]

Hello again! Sorry for the delay. I’ve had some other bigger focuses come out of nowhere in the last few months so I haven’t had any time. On the plus side I passed my full A licence test 2 weeks ago so the L plates are off.

I did a compression test earlier today and i can confirm both the front and rear cylinders read the same value of 140ish psi. I had no luck with the hard tube but the screw in hose worked well enough.

Now here’s where things get interesting. When I pulled the plugs and before I ran the test i spun the engine over for a split second and it spew out a considerable amount of raw petrol out of the cylinders. I’m amazed it didn’t hydrolock! This now means that there’s something else going on. It would also explain why the front cylinder wasn’t firing, it was probably getting fouled something chronic.

So my next bet is to pull the carbs off again and re-do everything. float heights etc. Plus change plugs and do oil again.

Knowing I double checked the floats and seats before i tried getting the bike running i’m surprised but hey at least the bike has good (consistent) compression.

Any further advice would be useful. Thanks again!

[Darkvader]

oh forgot to say, I didn’t get the engine hot for this test, it wasn’t idling properly at all but with the raw petrol being shot out I reckon if i pushed it i would have caused some harm.

[♠️ MARCEL]

Congrats on your test! Stay in the hyo family lol.   Keep the 125 for winter or short runs as i own one myself for just that or as a work horse sometimes (if car isn’t being used haha)  , but keep an eye for a GT650 or GV650 🙂

On your GV125C – Few pointers if it helps;

• Float height 7mm sitting down  (means float is gently resting on the needle) , 17mm lifted up…  But 8mm is a generally a max limit when the carb is sitting on its diaphragm covers.
• Tank Tap = Check OFF means OFF (Disconnect the hose safely and ensure the tap still doesn’t leak out fuel when its in OFF mode)
• Change Floats = If in doubt about them
• Change “Needle Seat” and “Float Needles”  , these 2 are literally the only items that will stop the fuel coming in after the floats have risen  (extra fuel will just overflow to cylinder overnight sometimes)  – Hence “float jamming” being a common thing on carb motorcycles , it just means we have to extra be serious with them   (unlike injectors lol)

 

140 psi via a flexi hose should be OK to make it “start” at minimum, the hard metal pipe is pushed down as we crank the engine or it shoots at you!😉  (Just make sure the metal bit uses a Spear Shape rubber tip.

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

[Darkvader]

♠️ MARCEL wrote:

Congrats on your test! Stay in the hyo family lol. Keep the 125 for winter or short runs as i own one myself for just that or as a work horse sometimes (if car isn’t being used haha) , but keep an eye for a GT650 or GV650 🙂

Yep, i’m keeping the GV125 :), i’ve been keeping an eye on that Japanese GV250DRA something about that tickles me. obviously if a 650 or st7 comes along i’ll seriously consider that too.

On your GV125C – Few pointers if it helps;

• Float height 7mm sitting down (means float is gently resting on the needle) , 17mm lifted up… But 8mm is a generally a max limit when the carb is sitting on its diaphragm covers.

this has confused me slightly, though I think it’s more about the terms used. Would you be able to walk me through how you measure things?
I have a little video here https://youtu.be/lwInsUauWfc
in this video i have the float set at 7mm with a soft set on the diaphram covers. before this I tried a hard set, with the float at 7mm and the spring fully compressed so maybe like 10mm with the spring.
I assume as well the 17mm is the maximum open drop on the float, i measured 14mm earlier but i may have this completely wrong.

Tank Tap = Check OFF means OFF (Disconnect the hose safely and ensure the tap still doesn’t leak out fuel when its in OFF mode)

Change Floats = If in doubt about them

Change “Needle Seat” and “Float Needles” , these 2 are literally the only items that will stop the fuel coming in after the floats have risen (extra fuel will just overflow to cylinder overnight sometimes) – Hence “float jamming” being a common thing on carb motorcycles , it just means we have to extra be serious with them (unlike injectors lol)

140 psi via a flexi hose should be OK to make it “start” at minimum, the hard metal pipe is pushed down as we crank the engine or it shoots at you!😉 (Just make sure the metal bit uses a Spear Shape rubber tip.

Quote

I tried the metal pipe one first, I made sure to get a tester with it, sadly I got worse numbers as i just couldn’t get a good seal even going full hulk. I think i must just have noodle arms though.
I actually reckon that the last time i adjusted the floats I may have set them too aggressively as I had a feeling the needle would bind up. it didn’t seem quite right

Sorry for all the potentially stupid questions, i’m just doubly making sure.
Thanks Marcel.

[♠️ MARCEL]

Thanks for the youtube clip;

I can see your confusion there…

• Did you already use a new rebuild kit ?
• Did you use new floats from us , as they are factory adjusted?

7mm = This is when the float is actually resting on the “needle” plunger (If you press down the float, it will drop to 4-5mm ish)

The spring is just there to make sure bumps , shocks , etc doesn’t cause the float to ram the needle against the golden seat (well)

17mm is when the float is lifted up to the ceiling  (as that would be how the carb is when it’s on the bike) , until the float bowls fill up with the float going down to 7mm to close the fuel delivery.   (it will hover around that number as you ride as not to completely drain the carb in full throttle)

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

[Darkvader]

♠️ MARCEL wrote:

Thanks for the youtube clip; I can see your confusion there…

• Did you already use a new rebuild kit ?
• Did you use new floats from us , as they are factory adjusted?

Oh that’s interesting, I wasn’t aware they were factory adjusted, yes i did use new needles and seats. I used the original floats at first, mostly to get a feel for how to adjust them (and this is more a learning project than a by the book thing anyhow) but I will swap in the new floats.

7mm = This is when the float is actually resting on the “needle” plunger (If you press down the float, it will drop to 4-5mm ish) The spring is just there to make sure bumps , shocks , etc doesn’t cause the float to ram the needle against the golden seat (well) 17mm is when the float is lifted up to the ceiling (as that would be how the carb is when it’s on the bike) , until the float bowls fill up with the float going down to 7mm to close the fuel delivery. (it will hover around that number as you ride as not to completely drain the carb in full throttle)

Quote

This is what I was thinking when I started but i didn’t adjust the floats so they may have been set wrong (and i was using the originals) when I first tried.
Then I asked earlier for some clarification and ended up gooseing it up completely with the hard touch thing that someone else said. I’m sure there are carbs that are set that way but obviously not the bds26.

In the video i set the height to 7mm soft touch and with the original floats. I reassembled it and managed to get it started but the rear cylinder was sending fuel out of the breather. I quickly changed the float and it’s now happier, with the original on the front cylinder and the new one on the rear. it actually idles rather nicely now, revs don’t hang and quite honestly i’d say it’s about where it needs to be but it still seems quite rich at idle. I’ll change the front float, it’s probably that which is still causing my grief.

Thanks for being so patient Marcel (and everyone too).

[♠️ MARCEL]

That’s great to hear you got it running now! 👍❤

Now on to the “fine-tuning” , and we shall leave her alone unless you couldn’t resist changing her looks for whatever reason but i think once the carbs are done ,  you can lend your hand on other areas anyways to make sure its all good like  Oil , Clutch ,  Clearances and that’s it 🙂

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

[Darkvader]

Honestly, i really like the stock look 🙂 i’m not one for the bobber style.
I was out earlier today and while it doesn’t feel any quicker (and still seems to get stuck at 60mph) the plugs are nice and tan. there is still a weird idle issue, revs will climb a bit to 1800, then a blip of throttle will take that down to 1200 but I think that’s more on the mixture screws than anything as the idle isn’t unsteady it just seems to want to climb a bit for some reason.
Here’s a video of it running https://youtu.be/u4JYxObujJs
This is the front plug:

This is the rear

Next job will be a full service, oil change, new plugs etc. 🙂

[♠️ MARCEL]

I saw the youtube clip, hmmm i can try break this down for ya , follow me, i think i am on to something but will need you to also verify next time you have your hands on it.

• Spray  the airbox rubbers and intake rubbers = see if revs change or engine grumbles or screams = air leak.
• Rev up to 5000 rpm then let go, do you see any kind of smoke, do you smell any oil stench on the exhaust ? = too much engine oil , or concern for oil in engine (replace it, if the bike ever had fuel leak out of the carbs when it was OFF)
• I see you have a full tank, buy WYNNS FORMULA GOLD PETROL (this bottle only) pour all in the tank, ride a bit and let us know if the bike starts to behave a little better as the fuel bars drop low.
  I trust the carbs are clean already but the WYNNS bottle might also make its way through the fuel lines, spark plugs , pistons ,to get some gunk off and take away some carbon (it will try!)

Plug Analysis

• Front center cone = main jet is OK, full throttle seems OK,   but idling a bit rich due to dark black sooty ring but…..is the black ring oily or ashy-dusty ? – The plug’s threaded body looks dark at the base and almost silver rest of the way towards its white insulator,  it means front cylinder isn’t overheating like crazy  (thanks to winter and the nakedness of the GV!)
• Rear Center Cone + Rear RING = OK.  Nothing to add here, it’s good! – This is the “aim” or sweet spot if you will.
• However, rear body of the plug suggests its a bit hotter than the front (threads are darker = hotter cylinder head but …. not concerning yet)

Throttle dipping / climbing means either these but we will narrow it down as we progress on servicing her…

• Mixtures  , try to ride at 2.5 full turns out each cylinder and see how it generally rides. This will be our base as going beyond 3.5 turns is totally pointless , it will have less effect once you in full throttle (main jets and “slides” take over) so full throttle issues should be jets and slides (sticky?) …)  or floats or fuel pump wanning or tap wanning , or the fuel parts have debris hence Wynns Formula in petrol to collect em and engine to burn them off out of the exhaust
• Pilot Jet clogged again  (revs flactuate) is a sign its “hunting” for fuel to prevent the bike stalling abruptly
• Starting = Permanent Jet + Pilot Jet = If you get access to air compressor , blow them hard
• Fuel Levels = float heights , pilots “barely” reach the fuel but i think you know how to adjust them to a sweet spot by now 🙂 or i understand you’re experimenting with fatter aftermarket floats vs OEM ones but they still have max resting height of 7mm-8mm max .  7mm is the OEM sweet spot, i understand float needles and what not can affect the height a little bit , too much here or there can overflow fuel out of the carb so adjust and check accordingly.
• Richness = revs up, then dips below 1500  (see what i said prior to this sentence)
• Needle of the Slide = If it has 5 steps to go down to the pointy tip, try step #5 , if its worse, go up to step 3 which is its default middle setting but never go beyond step 2 and 1 as those will lean out the bike.   5 step or 3 step needles Korea uses are always at the middle setting from factory.   Going to the middle lets you see if full throttle makes it hit 70mph or not.  More speed more fuel or too much fuel can bog it if rest of the bike isn’t optimized   (air vs spark (all electrics from coils, plugs etc,) vs fuel  (carb, tap, pump)

 

Full service is not a bad call! – You want to get to it before it gets too cold and Xmas is around the corner where families rob our time until new years lol

I hope this reply made any sense but if at one point i confused you, just let me know , ill explain better next time

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

[Darkvader]

♠️ MARCEL wrote:

I saw the youtube clip, hmmm i can try break this down for ya , follow me, i think i am on to something but will need you to also verify next time you have your hands on it.

• Spray the airbox rubbers and intake rubbers = see if revs change or engine grumbles or screams = air leak. [\quote] I did this earlier and no change (this also applied earlier too, i think my rubbers are good just the carb doing something funky) Also the biggest change to speed happened when putting the bike on it’s side stand, i believe that’s pointing me towards floats (argh lol)[quote]
• Rev up to 5000 rpm then let go, do you see any kind of smoke, do you smell any oil stench on the exhaust ? = too much engine oil , or concern for oil in engine (replace it, if the bike ever had fuel leak out of the carbs when it was OFF)

What seems to happen is the opposite, it will smoke a bit during idle and revving the bike causes it to clear up. I suspect it’s just so rich that the little smoke is actually from being super rich. it doesn’t smell oily to me

I see you have a full tank, buy WYNNS FORMULA GOLD PETROL (this bottle only) pour all in the tank, ride a bit and let us know if the bike starts to behave a little better as the fuel bars drop low. I trust the carbs are clean already but the WYNNS bottle might also make its way through the fuel lines, spark plugs , pistons ,to get some gunk off and take away some carbon (it will try!)

When i next fill up i’ll get some, currently there’s some redex in the tank :).

Plug Analysis

• Front center cone = main jet is OK, full throttle seems OK, but idling a bit rich due to dark black sooty ring but…..is the black ring oily or ashy-dusty ? – The plug’s threaded body looks dark at the base and almost silver rest of the way towards its white insulator, it means front cylinder isn’t overheating like crazy (thanks to winter and the nakedness of the GV!)

as for being black, it’s like ash, rather than oily.

Rear Center Cone + Rear RING = OK. Nothing to add here, it’s good! – This is the “aim” or sweet spot if you will.

However, rear body of the plug suggests its a bit hotter than the front (threads are darker = hotter cylinder head but …. not concerning yet)

awesome 🙂 I’m not paying a huge amount of attention to the colour of the threads at the minute, they’re old plugs and were like that anyhow but i’m glad they not concerning 🙂

Throttle dipping / climbing means either these but we will narrow it down as we progress on servicing her…

• Mixtures , try to ride at 2.5 full turns out each cylinder and see how it generally rides. This will be our base as going beyond 3.5 turns is totally pointless , it will have less effect once you in full throttle (main jets and “slides” take over) so full throttle issues should be jets and slides (sticky?) …) or floats or fuel pump wanning or tap wanning , or the fuel parts have debris hence Wynns Formula in petrol to collect em and engine to burn them off out of the exhaust

My GV has 17.5 sized pilots, currently they’re both at 2 front and back. I wonder if the rear cylinder is a touch too lean at idle and needs a quarter turn more, and the front a quarter turn less.

Pilot Jet clogged again (revs flactuate) is a sign its “hunting” for fuel to prevent the bike stalling abruptly

Starting = Permanent Jet + Pilot Jet = If you get access to air compressor , blow them hard

When cold the bike starts super easily, it’s actually harder when it’s warm

Fuel Levels = float heights , pilots “barely” reach the fuel but i think you know how to adjust them to a sweet spot by now 🙂 or i understand you’re experimenting with fatter aftermarket floats vs OEM ones but they still have max resting height of 7mm-8mm max . 7mm is the OEM sweet spot, i understand float needles and what not can affect the height a little bit , too much here or there can overflow fuel out of the carb so adjust and check accordingly.

Richness = revs up, then dips below 1500 (see what i said prior to this sentence)

i’ll double check the floats, maybe my calliper is more a callipo than a mesuring tool lol

Needle of the Slide = If it has 5 steps to go down to the pointy tip, try step #5 , if its worse, go up to step 3 which is its default middle setting but never go beyond step 2 and 1 as those will lean out the bike. 5 step or 3 step needles Korea uses are always at the middle setting from factory. Going to the middle lets you see if full throttle makes it hit 70mph or not. More speed more fuel or too much fuel can bog it if rest of the bike isn’t optimized (air vs spark (all electrics from coils, plugs etc,) vs fuel (carb, tap, pump)

the front cylinder from stock is 4 and the rear being 3. I’ve left them at stock but maybe with the new jet it needs to be dialed back a touch on the front.

Full service is not a bad call! – You want to get to it before it gets too cold and Xmas is around the corner where families rob our time until new years lol I hope this reply made any sense but if at one point i confused you, just let me know , ill explain better next time

Quote

I’ve made a start on a service today, I’ve changed the Oil and filter. I also used the time to fit a 250 oil cooler 🙂 my oil lines were getting a bit frayed and looked quite rusty so it made sense to change them and do an OEM plus upgrade 🙂
I have yet to do the plugs or look at the mixtures. but I have a few jobs under the tank as well that when i have more light i’ll lump together.

[Cobra2]

Just to throw in some confusion? https://hyoriders.club/forums/topic/gv125c-float-question/

Old and grumpy,(?), have fairly wide engine & tuning experience, "restoring/resto-mod" on cars & mc, worked as electronic tech. (retired).

GT125R - sold. New (used): GT250R-efi

[Darkvader]

Yes! It’s all a bit confusing because i’m muddling through learning and trying things. Your advice in that post doesn’t seem to apply as hard touch doesn’t seem to work for my carb though thank you for trying to help :). I didn’t realise there were different ways that it could be done and the potential minefield with how things are measured. I’ll make a post on that thread to come to this post. being that they’re kind of the same thread.

I’m certainly closer now than I have been before, either i misunderstood an instruction or I’m just a donut in that previous post and didn’t check that the float wasn’t getting bound up (which is what happened for me to have to change that rear one). It also didn’t help that I didn’t have a huge amount of time to make sure before life whisked me away. This last week i’ve managed a lot more than i’ve been able to and I’ve basically had to start from scratch.

I’d also rather document all of my misadventures so people don’t make the same mistakes I do.
What I’m considering doing is making a carb clean and float setting guide once i’ve figured this all out. it’s obvious there’s a lot of difference in how different mikuni carbs do things.

[Cobra2]

Any float-setting could/should be checked “wet”, with a bit of clear hose from bottom, open tap-screw just a half-turn, and see the inside level….

(engine may be cranked a few rounds just to fill the bowls)

Old and grumpy,(?), have fairly wide engine & tuning experience, "restoring/resto-mod" on cars & mc, worked as electronic tech. (retired).

GT125R - sold. New (used): GT250R-efi

[Author]

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