MOT Day and running issues

[Darkvader]

Evening everyone, thought i’d give an update on my little GV.

Over the last few weeks I’ve redone:

fuel tap (I actually used one from a Yamaha, it’s the same footprint but without the sediment bowl that I found one for a tenner, it may leak in the future but was a bargain and is a mile better than the original on the tank),
fuel hoses,
Cleaned the fuel pump,
New plugs, oil and filter
Clutch lever (the old one had the pivot completely worn to an egg shape, weird for just 7000KM!)
New stock chain and sprocket (ordered ages ago)
shocks were derusted
front and rear brakes were cleaned as the old brake dust had turned to rust and was causing a squeak
rebuilt the rear brake light switch (I was curious why it would come and go, there was bands of rust. I wouldn’t reccomend rebuilding it, just get a new generic one and solder a new one in to the old wire)
wired in a new american gv250 left hand switch gear set (the one with a flash button that needed a switched 12v source, otherwise identical)
pulled all the chrome plastics off and gave everything a good clean.
I also attempted a repair with some epoxy putty to the broken bolt hole in the chain case and painted the sun damage on the front mudguard.

So i’ve booked it in for an MOT on wednesday 🙂 It’s not running brilliant but it’s rideable now.

What i’ve got left to do is the AIS system delete, new vaccum hoses and a thorough carb clean and rebuild.

Now the ifs and buts 🙂

So far I’ve noticed 2 things, firstly someone before me had completely messed around with the mixture screws on the carbs. At least, the rear one seems completely wrong. It never seemed to idle quite right and earlier I found when I counted the turns the rear one had 3 and a 1/4 turns out on it, while the front had 1 and 3/8th. that seems a bit extreme to me.

Because of the MOT on wednesday i’ve provisionally taken one turn out of the rear and the idle is much better but i really don’t like taking a stab in the dark. I also know the carbs will need to be spotless before i take any of the adjustments to heart.
When I rebuild the carbs and balance them I think i’m going to have to spend some time getting those screws right again. I have an idea on how to do it when balancing but I’m still thinking it over and i wouldn’t mind some extra brains on it, if possible. (would it be ok if I pm you what i’m thinking my method would be marcel? i’m making a guide for myself that i’ll post if it works out)

Secondly I think either 1 or both slides are gummed up or the diaphrams are hard, it seems to have rather poor throttle response. do they tend to go hard after sitting? I haven’t got any ordered as of yet but I will order them if i need to.

I’m thinking the best approach is to get it riding, get some proper heat in her, then start making changes gradually. I know with that mixture screw as far off as it seems to be i’m going to have to get it riding to read plugs (i’ve got about 6 plugs now specifically for this, halfords had to order a load more stock in for me)

Thanks everyone!

[♠️ M77]

Mixture with size 15 pilot jets – 2.5 full turns out front and 2.75 for the rear as its a 125cc  and leave this forever (no matter what mood she is in)
If you have 17.5 pilot jets  (sometimes Korea does this but not all GV’s have it, and tbh 15 is correct size as all GT125 Comets use Aquila engines too , with factory 15s every time i fix them they always have 15s in.)   ,  i am digressing, as i was saying if you have 17.5,  you will be hovering around 2 full turns to 2.5 turns max usually.  ,  less than 2 will overheat the bike even if it runs better lower. No true number of factory turns were revealed as korea sets them uniquely even so much as 1/10th of an turn less or more on each bike they test and ship out.

Trust me when i say if the slides look fishy, just replace them.  bad ones or worn down slides would cause fueling issues  and sometimes it won’t idle properly as a slide is stuck or not fully closed down. Another thing is to check the slide needle isn’t on fully lean setting or fully rich setting, you want to be in the middle if there is 5 grooves , then as the bike gets better, then enrichen it one step down towards pointy end.

Old age is the enemy of rubber slides but sometimes its just last keepers using E10 then letting the bike sit for months rotting, so it degrades things further

[Darkvader]

Thanks Marcel,
The MOT went sucessfully and she is now back on the road!
I have adjusted them to 2.25 front and 2.5 rear, I don’t know what jets it has at the minute, so i guessed it’s better to be somewhere between the two until i’m able to pull the carbs off.

In the last week i’ve done 180 miles just to see what she needs before I start making more repairs. firstly, that slow throttle response has cleared up nearly completely now. It revs like it should and the only issue is when it’s cold, it stumbles when it’s not quite up to temp.
at the moment there is a lack of top end power, past 9k RPM it seems to not want to rev much more. I have managed 70mph once but generally 60 is the max i can get and the other day i was struggling to get more than 50-55. I can only guess the main jet is ever so slightly blocked, so between 8-10k it’s running too lean. Round town, 30-40 or 3k to 7k though is ok.

When i get paid on the 21st i’ll be getting some 90 and 92.5 jets, I think that would probably help.
thanks everyone for the help in getting her back on the road.

[♠️ M77]

Change the slides and do the “slide mod” tutorial on this forum to help with the higher rev range, try using LASER plugs too  (if not already but get a 2nd set just incase) , usually a good carb tune goes through a few plugs first 😉 ,  you got this.

Congrats on the MOT too! Happy days to have another hyo stay on the road. It upsets me when they got sorned off and rot in gardens

[Darkvader]

Thanks Marcel I will be doing that at the end of the month so i’ve ordered some :), i’m booking a big day to do a laundry list of little jobs and a first running service.

I’ve also got some ignition stuff in the order as well, including a regrec, coils and some new boots because I had a little bit of a terrifying experiance.

I was coming down the dual carrigeway doing 60 and the bike cut out earlier this week. Complete loss of power. The dash and speedo, lights etc were all working and the bike was taking in plenty of air (lots of induction noise) but it just seemed like it had been ‘switched off’, it was bizzare. for 10 seconds I was completely unable to accelerate, until i downshifted and gradually applied throttle. I can only imagine that I had an issue with spark because it was so sudden and lasted so long, i’m pretty sure if it was fuel she’d be popping and coughing but it was just gone completely.

After it went and came back i was then again able to accelerate up to 60. One thing that tipped me off about electrical was a really sparodic tacho needle too, at and after 9k it’s not constant like the engine note seems to suggest, it bounces up and down about 500RPM but at the minute will only do that at 9k+. plus when i was stuck doing 50, the needle was also bouncing all over the place while the bike was fairly smooth. It seemed a few other people had issues like that too caused by regrec/coils. So going to see if that fixes things. What would you suggest? I’m only really able to make an estimated guess. obviously the carbs are coming off and getting done.
Thanks Again!

[♠️ M77]

Hmm, can i just ask , how many tries did it take to start it back up again?

Also if you managed to get home that’s good, but on the 2nd ride, if you tried to exceed 60mph, what does the rev counter do ? does the engine still feel smooth or just it start jerking forth and back ?

[Darkvader]

Of course, At the time I didn’t have to restart because i was trying my best to keep rolling with the bike in gear. it was completely not doing what i was asking though until it came back at about 30mph, then was fine the good 5 miles back home even at 60mph, like it had to cool off for a couple of seconds.
It’s always had little starting gremlins though, when it’s cold it’ll fire up, then come and go for a few seconds and i’ll have to rev the bike for a second for it to start being happy to run, even on choke, i put that down to carbs originally but i’m not sure that’s the full picture.

To be honest it’s never been jerky or juddery, yesterday it was running fine the only thing i can see directly that’s a bit weird is the tacho being jittery at 8-9k. I’ve not had a flat battery or anything because i use a battery maintainer though the thing that made me think electrics is that the original owner had put a new battery in after a year and i’ve had another new battery (though i think that was more frost, we had a bad winter that year).

The engine feels completely smooth at the speed i was doing, it was holding well but the needle will go from about 9k down to 8k in like a uneven flicker over half a second or so. plus when i was stuck at 50 the needle was doing the same thing, even though normally it wouldn’t at that speed. I’m puzzled.
I’m not so much blaming the boots or coils as such but I’ve ordered them as well being that i would like to run the laser iridiums properly and I like having spares.
What i’ll do is i’ll hook up a camera to the bike and record how it’s running, try and get the speedo, tacho and the engine note for you, perhaps your trained ear will help.
Thanks Marcel.

[♠️ M77]

Well explained , now i fully get the picture, so if you want a tequila shot answers , ill shoot em below:

• Fuel Pump wanning (happens, carb cleaner sorts it when you take it apart CAREFULLY) or as pre-caution, replace with a new one if you want us to get it.
• Fuel Filter Clogging or not flowing fast enough  (if its OEM)
• Fuel Float Level  low  (repeated in service manual too)  (check floats are on a good height, as fuel goes low, then the jets won’t reach , due to pump & tap not replacing the fuel fast enough = high revs . high speed , more demand)
• Carb Slides wanning.
• Carb Slide needle may be on the lean setting  (sent 1x needle for free, just incase one of yours is bent too)
• TANK TAP , wanning = Common.

 

Since its naked, after a ride , check carefully that both cylinder heads are hot or get an oven thermometer against each head,  mark their temps,  then check plug colours.

Plug checks will be a very good way to tell me how your “spark” “air” and “fuel” system is doing  , not a full picture but enough hints to show me.

Side view of the plug threads  (heat check)
Top down view of plugs  (to see the white cones inthe middle)

I could advise further after your next comments.

Normally , when a hyosung that is all OEM and negelected  (either by garage or last keeper) , the carby 125 ones at least ,  they usually spend their lives in the city with a max of 40mph runs ,  then owners use crap service parts on them to keep em going,  but never touch the carbs , or do engine clearances  (valve clearances do eventually get too tight , being a hot engine doesnt help)   , never changing air filter….

Fast forward , the next owner buys it 100 miles away from his house.

He decides to pay the neglectful seller, goes on A1 even to ride home 100 miles,  40 miles are okay but last 60miles involve dual carriage ways of 70mph , he breaks down as soon as he goes from 40mph to 70mph and races it for 2 mins straight, the bike stops sparking  , but all the owner can do is coast it to the side of road , electrics are still on!

What happened?

Negelected Fuel System!!!   (& some times heat damage to coils,  stator & the reg rec = also common)

He starts the bike again after letting it sit for 6mins ….

How?

The cranking & cranking was sending fuel back to the carbs so that the floats can raise and the jets can finally reach the fuel level needed to start.

Now he must ride 40mph max so that the “wanning” fuel system (all of it) can still keep “trickling” down more fuel to maitain fuel level to get him home

Or

His electrics are kinda calm under 5-7k revs …..but the bike rides better at 8-10k revs  (catch 22 for him!)   UNLESS he is in fifth gear above 60mph, then lower revs is fine but his fuel system already forced him off road if he goes beyond 40mph on dual carriage ways.

He is now forced to ride away from big roads until then.

Seen it.  Witnessed it myself.

[♠️ M77]

Sorry if i digress or not making any sense at 10pm but ask away if missed something

 

[Darkvader]

No worries about things not making sense thank you for replying so late. I used to work in IT and you’ve given me heaps to go off, making sense of ramblings I got quite good at :).

• Fuel Pump wanning (happens, carb cleaner sorts it when you take it apart CAREFULLY) or as pre-caution, replace with a new one if you want us to get it.
• Fuel Filter Clogging or not flowing fast enough  (if its OEM)
• Fuel Float Level  low  (repeated in service manual too)  (check floats are on a good height, as fuel goes low, then the jets won’t reach , due to pump & tap not replacing the fuel fast enough = high revs . high speed , more demand)
• Carb Slides wanning.
• Carb Slide needle may be on the lean setting  (sent 1x needle for free, just incase one of yours is bent too)
• TANK TAP , wanning = Common.

I am suspect of the fuel pump, there was green thick petrol residue inside it (which i noticed when i did the fuel hoses before I ran the bike) and I have already had it apart to clean it. I didn’t exactly think the pump was bad but the rubber did seem fairly firm. The fuel lines and filter were the ones you supplied so I can comfortably rule those out. I am questioning the vacumm hoses, that is the next job on my list though, they are the original ones and if there’s a restriction or leak in those hoses that could stop the pump working at high demand too i’d imagine.

I will check floats and slides carefully when I have the carbs off, I keep reading 7mm for the floats on here so that’s well in my head now. Thank you for the needle 🙂

I am wondering about the fuel tap flow rate, I fitted a new one on my tank that was a cheap chinese one and it may be more restrictive than OEM. though it does flow freely when opened and it’s the same design as on the yamaha i have (which is a 650). I’ll drain the fuel and see if the tap is giving a bit more of a restriction than it was when I fitted it. No big loss needing another tap if so though, it was a fun gamble.

One thing that comes to my mind, how does the vent in the tank work? is there a check valve or so? I’ve seen fuel issues before (in my car) where a restriction in the venting of the tank will stop fuel flow as there is a vaccum being built in the tank stopping the pump working.

I will go for a little ride tomorrow and read plugs and head temps when i get back.

Normally , when a hyosung that is all OEM and negelected (either by garage or last keeper) , the carby 125 ones at least , they usually spend their lives in the city with a max of 40mph runs , then owners use crap service parts on them to keep em going, but never touch the carbs , or do engine clearances (valve clearances do eventually get too tight , being a hot engine doesnt help) , never changing air filter….

Clearances are next on my list too, i know when things get too hot/tight it can bump a valve open slightly and cause a loss in compression/power. I don’t think I have the skill to do them myself though so i’ll have to book in somewhere.

I’ll send a reply tomorrow with photos of the plugs after a 2 or 3 mile run 🙂

Thanks again Marcel.

[Darkvader]

I’ve sent another reply but i believe the anti-spam/system has caught my message because I also added some pictures and video links for how it runs for any noises and the fuel pump.
Just in case that was a problem both cylinders are hot, they’re both obviously firing but the front cylinder is obviously leaner than the rear, it’s squeaky clean white. the rear has just a touch of tan. Obviously the carbs haven’t been off yet but it’s interesting to see how lean it’s actually still running with.
the threads on the plug don’t look very hot at all, there’s a band of black around the first 2 threads, gradually tapering down to normal metal colour over the next 3 or 4 threads.
thanks again Marcel

[Darkvader]

Hi Marcel, I went for a little ride earlier, nothing particularly taxing and these are some photos of the plugs in question. sorry for not getting completely top down on it, I just couldn’t get a good focus. I’ll have another stab at it when I fit the new coils and stuff (which i got today btw! super quick)

1. Rear Electrode 2. Rear Thread 3. Front Electrode 4. front thread

From this I can easily tell the front cylinder is running leaner than the rear and the rear is still a touch leaner than i’d have thought it should be.
I didn’t get any measurements of the head temps, sadly. they were both equally hot to touch when i stopped. I couldn’t find my thermometer before the bike started to cool down for any numbers though. I’ll take them tomorrow.
I do have some videos though, One of the fuel pump and two for timing chain.

This is the idle noise video. I do hear a slightly metallic sound somewhere.
https://www.youtube.com/watch?v=vkNrRK57OgM

This is a quick rev

https://www.youtube.com/shorts/k9R1RwjsWSg

and this is the fuel pump

https://www.youtube.com/watch?v=B8JATX56ld8

Hopefully this helps you narrow whatever weird gremlin seems to be posessing my bike at the minute.
Thanks Marcel

[♠️ M77]

So my original comments still apply before.your video replies.

 

I will add a few notes though thanks to your Youtube Shorts & Plug pics.

• Both Plugs = Slightly overheating… (((With oily deposits on threads????)))
• Front = leaning out at high throttle
• Rear = leaning out too but seems to do better than front.
• Both Plugs = idling OK. (Assumes mixture is 2.5 front and 2.75 rear using size 15 Pilot Jets) but see next bullet point….
• The carb needs to have its jet needle at a richer setting as the white cones need to be brown after full throttle runs.
• Mixture screw will have less effect once the carb slides & main jets take over at past 4k revs. BUT it does have an effect during gear changes as bike momentarily lowers revs or coasting while rider carefully disengages/engages clutch to change gears. The carb slides then lift off again.

Fuel Pump = It looks young.  I think you could check your fuel tank.  Take care not to let fuel eat paint. And clean the tap of any debris as switching between ON/OFF/RES seems to have an effect now.

If a bike cuts out… Tap is usually the 1st thing i get rid off & replace.  Work my way to the pump & filter.  Then carb (dreaded airbox removal but needs must!)

 

Im sure you will follow my tutorial on airbox removal.  If you get pics and do better than me ; people will love you for it!

 

I do a lot but never seem to get round to taking pictures to make new tutorials.

I need a kick up the arse for it!  Ill get it done lol.

 

If the cam chain is making noise……does it go after its hot or get worse?

Does holding clutch reduce it?

Does high revs make it worse or quieter?

 

Same questions again for the Top Engine Heads (Valve Heads = valves chitty chatting)

You will want to find a way to have youe ear behind each engine cam tensioner behind their heads.

[Darkvader]

• Both Plugs = Slightly overheating… (((With oily deposits on threads????)))
• Front = leaning out at high throttle
• Rear = leaning out too but seems to do better than front.
• Both Plugs = idling OK. (Assumes mixture is 2.5 front and 2.75 rear using size 15 Pilot Jets) but see next bullet point….
• The carb needs to have its jet needle at a richer setting as the white cones need to be brown after full throttle runs.
• Mixture screw will have less effect once the carb slides & main jets take over at past 4k revs. BUT it does have an effect during gear changes as bike momentarily lowers revs or coasting while rider carefully disengages/engages clutch to change gears. The carb slides then lift off again.

That oily stuff is copper grease thanks to my uncle. I know it’s not always needed but Me and my uncle had a bit of a fight getting the old plugs out so when i started on the sprocket he plopped them in. It may actually be overheating worse, there’s only 160-180 miles on those plugs.

I’ll be pulling the carbs off when I get my cleaning liquid for the ultrasonic. I’ll leave the settings as what they were, do a couple of full throttle runs and see how much of an improvement. I fully expect i’ll have to clean the carbs two or three times to get them right, they were sat a LONG time.

I’ll drain the tank and have a look over the tank and tap again. When I fitted it (it is new) flow seemed OK but it was cheap and i’m sure that the most expensive stuff starts cheap lol.

I’m going to do a guide for a few things, I like making documentation. I will take some photos of the airbox when it comes off. I’m really trying to mentally prepare myself for it, i’m not quite ready yet to allocate a day to it.

As for cam chain noise it doesn’t actually get worse or better, it’s consistent. really I was just wondering if it sounded ok.
I do have a suspicion the clutch is on it’s way out.  I believe it’s the original and the biting point seems to like to wander. it also seems to stuggle to disengage some times. Maybe the oil i put in before I started work is just rubbish (Motul 5100 semi 10/40 i think).

The heads get a little more clattery when hot but it doesn’t seem to get worse under revs. I’ll use the screwdriver trick to listen to the tensioners. The video was the bike when it was hot/hot ish.

Thanks again.

[Author]

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