Jul 25, 2022 at 10:13 PM #8183
As the title says, after riding for about 45 mins in traffic and up to 50 mph i then got on to the dual carriageway and after about 10 mins riding at around 65mph the bike just died as it did once before at nearly the exact same spot. I stopped the bike, turned ignition off then back on and it restarted after 2 attempts, was a bit sluggish but was then fine for the rest of the 45 min ride.
On the way back it happened again on the same dual carriageway after a stint of doing 78mph (yes 78) i slowed down to 60mph then 50 mph as the speed of the road drops then the same thing happens. Its as though its ran out of fuel, i stopped restarted the bike, it felt sluggish to begin with then revved fine and off i went and all was fine.
No idea why it does this. I filled up with super unleaded at BP 2 weeks ago and the bike seemed to perform its best with no issues when the weather was hotter (maybe coincidental) but thats how it seems.
If anyone has had similar happen to them and sorted it id be interested to hear what the problem is or might be.
Jul 26, 2022 at 6:58 PM #8191
You have the situation called the infamous high speed fuel starvation, this is how it happens:
– Carb bowls are full
– Bike is running normally (under 20-30mph)
– The road opens up to split lanes (dualways etc, 70mph section)
– The revs rise as the carb slides rise up
– It starts to drink the fuel inside the float bowls faster than the fuel pump and tap can keep up.
– Once bike feels like its running out of puff, it coasts to a dead stop
– The engine is hot and fumming (no pun intended!)
– It waits for the carb bowls to fill again whilst cranking and cranking to start it.
Fortunately there is a remedy for this, so try these and every last thing you tick off below the bike will be grateful.
– Replace the lines (OEM Carb lines rot) and a monster filter is big enough to fill the carb bowls fast, or acts as little reserve for 1 mile to petrol station if you ride slowly and steady to maintain high MPG
See this (link here) and (this tutorial here)
– Better lines can hold more air in for the vacuum system to hold open the tap and pump under high revs and don’t swell near the hot rear engine.
So see this (link here) and the tutorial here
– Get “JIS” screwdriver (it is a must) before even thinking about stripping the carbs to upgrade their jets or just to check the previous owner hasn’t messed around.
Then see ( this link here )
– Do a “SLIDE MOD” while still at the carbs, look here -> (this link) , this will enrich the mid-range a little bit (around the 4-7k mark), so the bike is less flat.
– Get carb cleaner at halfords = Buy 4x cans (Yes you must go through 4 cans it is a must) then do the following
- Blow the jets clean
- Blow every hole inside the carb clean (every last of it)
- Clean the exterior hard
- Get a fat stubby screwdriver (JIS type!) and then remove the fuel pump, do not open it, as there is no need. Blow carb cleaner inside the “GOLD PORTS” only! (DO NOT SPRAY the silver port of the fuel pump)
Consider replace the fuel tap from the tank (it is too common to digress as Korea taps haven’t been great on the hyosung 125cc motors) , plus you probably want something that flows a little better (like this one)
Last not least, always (i mean always use “E5” fuel)
Let us know how you get on. I hope i wrote everything correct 🙂
Only if you do every last word above ^ with the fuel system and bike still persists , then we will discuss the air system and electric system but i hope it doesn’t come to that , as we still have some few warm days left of summer to enjoy
// Meditation doesn't mean you have to sit still....
Jul 28, 2022 at 11:06 PM #8207
Thank you for the very informative list of checks to do and possibilities.
As soon as i get a free hour or two i shall remove the carbs and start with those because i noticed now its cooler sometimes the bike will start with no choke at all and sometimes it needs 3/4 choke but will never start with full choke. Also sometimes the idle can be 2k rpm then after a blast it sits at 2.5k so very irregular. When i first got the bike it was set to 2k rpm idle so i dropped it 1.5k but it didn’t like that so put it back to 2k.
Feels flat sometimes, like its under-fueling. First start snappy throttle but after 20 secs feels restricted and flat.
One more question. Do these bikes have some sort of EGR system and/or a restrictor of some sort (oxi cat?) in the exhaust. Reason i ask is because it feels as though it’s not breathing properly, the end can is actually bronze in colour now like its getting too hot. The bike goes but you have to wind it up.
Jul 29, 2022 at 9:55 PM #8214
Keep us posted on the above comments i mentioned on what to do, as i believe it is also fueling woes. trust me it will show itself to be in a better mood.
Also because revs change , it may be due to crappy orings leaking under the inlets (even if the bolts are tight)
See this link here
Very very crucial items.
Also get your self “PUTOLINE RACING GREASE” and soak the orings in them.
Hide the intake holes of your engine please, then use cotton wool ball or very fine fine wet sandpaper and scrape off any debris where the orings sit on the cylinder heads.
Using the bolt you took out, go to halfords and get the stainless version inlet manifold bolt you just took out. The Hyosung bolts for inlet bolts rust very easily.
So go careful with them as they are known to round off!
For EGR = See the vacuum diagram linked above , which talks about how to delete the EGR
or see this page for info on what it does etc, there is no need to order it now,
// Meditation doesn't mean you have to sit still....
Aug 3, 2022 at 9:07 PM #8240
Thanks again for being very informative as usual.
I think before i start buying any parts i shall strip the bike first just to see what if anything has already been done because i checked today and the EGR/AiS has a blanking plate already installed so now im thinking this bike may have an inherent problem.
The other thing i was thinking about was to remove the vacuum petcock valve and replace it with a simple on/off tap? Has anyone tried this before?
My mates YZF 125 has much better low down torque than my twin which is disappointing tbh because i thought the twin would of had more grunt.
One other thing, i read a post somewhere a few weeks ago (from memory so bare with me) where they cut a green wire on the cdi unit on a 250GTR to give the plugs more power. Think it was something to do with stopping the power dropping once started or something. If that is the case that would explain why my bike has a snappy throttle from start for about 10 or 20 seconds then it just goes into lethargic mode.
Aug 7, 2022 at 7:03 PM #8277
The green wire from the 250 CDI only works if the 250cc (not 125cc) has 2-wire neutral sensor.
All 125cc have a single wire neutral sensor. So their mod was just wasted cut wire and a placebo effect. Better coils and ignition system like caps / plugs will do a far better job than a CDI cheat (which is really not worth the trouble)
The green wire on the CDI goes straight to the Neutral Sensor Plug
That’s why your 125cc has 2 wires on the neutral sensor plug from the harness , but the ENGINE SENSOR itself has 1 wire coming out of it.
So don’t worry about it.
if the EGR/AIS has been blanked on the engine side, then check under the airbox has been blanked too incase last owner forgot.
Hyosung did away with the ON/OFF Taps on the Comet 125 because owners always left the taps on , and their engines eventually seized to constantly having flooded engines or their carb floats eventually got jammed open as the big tank of fuel is awalys trying to rush through it (incase the pump failed too).
Later Naked 125GT had vacuum taps , and GTR Carby always had it its too awkward to turn the tap off under the fairings , so vacuum tap was installed from the get go. Honestly, the Korea taps work as they should but commonly fail a lot, that’s why i never stock them on the shop.
YZF vs GT = Light frame, it is able to run away quick. A tuned hyo will race past it beyond 88mph indicated (which is the true old gen grey engines did factory) this assumes bike is factory mint and under 1000 miles run in. Old age, neglect of previous owners, bad fuel (eg. E10) , etc will make it run poor. But discussion for another day haha. For now let’s revamp your entire bike to is last nook and cranny down to the engine tappets etc (clearances and compression) will be the last major thing to do (can’t be avoided) , 8 valves it has (most 125 only have 2 or 4 and a single piston)
// Meditation doesn't mean you have to sit still....
Aug 8, 2022 at 11:07 PM #8303
Thanks again Marcel very informative as usual.
I am going to make the most of the good weather this as im using it daily but time permitting i may whip the carbs off and have a look inside and give them a clean at least.
Will update this post when i have more info.
Sep 23, 2022 at 10:46 PM #8708
Not had much time to do much with the bike and that includes riding it but what i have done so far is, temp removed the pipes from the egr/ais. just disconnected them and plugged the hoses with bolts for now just to see what difference it made.
The result is deeper exhaust tone and slightly better throttle response but a little flat spot mid range but nowhere near as bad.
Fitted a race NRP straight through race can fully open. Wound out the co screws 3.5 turns rear and 3 turns for the front.
Cutting out problem now happens after 1 mile at full throttle. Seems the float bowls cannot fill up quick enough with fuel used over 60 mph.
50 mph and below the bike performs well with no cutting out.
Weather is getting colder so il strip the carbs off completely (re-jet), blank the egr and fit bigger fuel hoses and filter, might even fit a low pressure electric fuel pump if needs be?
I like the bike but if these issues persist il have to revive my 1994 Fireblade 😀
Nov 8, 2022 at 10:54 PM #9048Cobra2Member
- Topics: 4
- Replies: 68
I have a somewhat different solution to the problem… -nobody seems to think about; icing. I have no definitive proof yet, ( need a remote camera & light-source inside the filter-box), but I believe the carbs freezes over…after a minute or 2, they thaw, and you can drive again. Several other bikes have heating-elements built-in the carbs, to prevent this…
Old and grumpy, have fairly wide engine & tuning experience, "restoring" cars & mc, worked as electronic tech.
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