GD250N – Hanging Revs and Stalling when Throttle opened quickly
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Bike Model = : GD250N
Hello Hyoriders!
I recently bought a used GD250N (9000km~) as my first A2 Bike
after a small bit of work (switching to direct lever from busted linkage, lubing cables for smooth and snappy throttle/clutch & having issues with the kickstand switch), it’s a very nice bike to ride!However, I noticed from the start that the bike takes quite long returning to idle when revving up (when revving up to 5k it’ll stay at 3k for probably around 5 seconds before going down to the normal 1.5k).
It also stalls when opening the throttle from idle too quickly or sputters then surges when in a high gear and giving it throttle (4th at 40kmh/h~ for example)
It runs without complaints at higher rpm though, very responsive and with full power from what I can tell.Looking online, it seems that the engine is running lean at low rpm, especially with the sputtering and sudden surging behaviour. These issues also seem mitigated somewhat when the engine is cold for the first 5min before it has warmed up somewhat, which could be explained by the ECU richening the mixture when cold.
I guessed it could be some vacuum leak. I checked today and it doesn’t seem to be the case. The engine dies, as you’d expect when you plug the airbox opening and the throttle assembly looked fine (no obvious cracks, oddities). Airfilter looks clean. Engine runs the same without it, so it doesn’t appear clogged)
My current guess is that there’s some issue with the ISC, but I’m not sure if it’s actually significant enough to cause the misfiring at low RPM.Thank you for any tips and suggestions!
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It may sound silly, but can you get carb cleaner ?
And do this outdoors due to vapors! ==> Spray the intake system in different spots to see if revs change
If idle takes long to return to normal after a high rev = air leak or ISCV unit possibly (but another air-related sensor may be involved that’s on the throttle body such as TPS)
If if you quick twist throttle and it dies = Injectors (clogged or underperforming (wanning) ) or else the fuel pump is wanning? or else again intake system (throttle body and its ancillaries attached there)
Air filter should be clean if its a young mileage bike (if under 10,000 miles , air filter should be fine unless there is some unexplained damage to it)
Let me know if the above ^ even helped make slight changes (then we are getting somewhere)
If possible, remove harness => clean every single connector you can find (resistance or voltage loss can happen along the lines to make certain sensors on the bike perform less than optimal , in thus the ECU may not always overcompensate for this)
Also , test your stator (importat) and your regulator (way too important, probably the #1 electric device every one discards for a better one)
Battery should be new (where possible), as it’s 2025 , give her a fresh one and make sure it’s a genuine YUASA one (USA brand) , not too keen on other brands (chinese crappy cells)
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Alright!
I’ve checked for a vacuum leak, seems tight! no changes when spraying with brake cleaner! (didn’t have carb cleaner on hand, but I think it oughtn’t make a difference)I’m thinking it might be the ISC malfunctioning somehow, it was running at somewhere between 1.5 – 3k revs depending on how it was feeling. I might try plugging the ISC air passage and using the idle adjuster screw to get it to idle and then checking again for hanging revs. If the ISC valve is permanently open, it would be equivalent to a vacuum leak (I think). I’ll let you know once I get around to doing that.
What I did notice today, is the first error code. I got a P0230 after running the engine for a while with it misfiring a couple times and checking the codes (got no codes before, even when checking). According to the service manual, P0230 indicates low voltage or open on the fuel pump relay circuit. I did switch out the kickstand switch relay and fuel pump relay and the issues persisted, so I’m not sure if the fuel pump isn’t getting enough power or something? I can hear it clearly when turning on the bike and it has never starved on me at high rpm or anything, so let me know if you have an idea.
Oh, and I did check just the battery, seems to be in decent enough shape (12.7V after several starts and stalls and without me ever putting it on a charger since getting the thing) I think it’s new, the owner probably replaced it before getting it inspected for road approval.
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When it runs , tell us the voltage of the battery at idle and also at 5,000 rpm. (Regulator test)
Fuel Pump Relays failing can happen but its not uncommon , the fuel pump draws a LOT of juice all day long. Replace with a new relay if in doubt and also make sure the contacts are clean (while you clean every plug on the harness.)
You will be amazed at even a rusty fuse can cause a 1 (one) volt drop enough to cause problems in the EFI system or a fuse that’s too old, replace fuses as precaution, the harness should have more stable juices flowing everywhere now.
Then recheck ECU for any codes later.
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Hello again!
A mean flu had me in bed for far too long, so I didn’t get around to the bike in a while. In that time however, I was able to order some wires and adapters which arrived in time for me to use Elmue’s HUD ECU Hacker to get an image of my ECU Flash and do some logging with the bike running today.
Cleared all Errors codes while I was at it, no new codes so far (even after encountering issues with frequent stalling and severe misfiring at the end)For the Voltages:
Ignition OFF: 12.5V
At Idle: 13.8V
At 5k RPM: 14.4VSadly the longer logs, especially the fault testing ones at the end are incomplete, probably because of vibration (I’m a dunce and put the laptop on top of the bike while logging, because I didn’t want to sit on the floor and plugged it into the loosest USB port on the device) I’ll try again tomorrow and share them if they’re complete and I think they could be useful.
What I also tried today, is plugging the IAC passage and disconnecting the IACV and seeing if rev hang persisted.
I honestly could not tell if it was better. I was considering doing an IACV delete and just using the idle adjuster screw, but without tuning adjustments, even at consistent throttle, it was not holding any sort of steady rpm, so I don’t think that’s the way to go.At the very end of testing it started misfiring more and more at higher rpm as well until it could barely keep igniting consistently. Then it stalled and had a hard time starting for a while.
I did try to get the spark plug out, but because it’s sunk really deep and the opening is very small, I’ll need to find a thin and long 16mm spark plug socket to check what the plug looks like. I’ll probably want to replace it.
Cheers
(Posting for what looks like the 3rd time, as it’s not showing up on the site)
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Get the spark plug out , when you can and show us the colours, but try again with a fresh CR8E plug only , check if the injectors work properly
You may possibly misfire due to poorly injector? (Something similar in this topic, but it was the injector in the end)
Voltage regulator seems to be working OK for now , so keep an eye on it. Idling at 13.8V is not bad , it’s good. 14.5V should be the max , if it spikes beyond 14.8V i would replace it as EFi ECU doesn’t last very well running too high due to delicate electronics inside. A carb bike will handle 15V but it won’t like it long term.
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After finding an appropriate socket, the spark gap was out of spec by a quite a bit (I think) at about 1mm, corrected it down to 0.7mm and cleaned it off a bit.
didn’t look obviously rich or lean to me, let me know what you think:Ran basically the same after the adjustment though, no misfiring at above 4k but misfiring below (misfiring then surging when rolling in gear), worsening with temperature. I also did some data slewing (modifying ECU parameters at runtime) and offset the IAC valve to close a little bit, idled with the same stability but at 1400~ instead of 2000~, still stalling when opening the throttle too quickly. (Haven’t yet checked if rev-hang improves with this modification while riding)
Additionally, I noticed the engine speed to be unsteady when the throttle is held half open in neutral, going up and down (not misfiring) which might mean a fuel pressure issue? I’ll see if I can find a way to measure fuel pressure somehow,
I currently think that it’s either
– Fuel pump (weak and or overheating)
– Ignition coils (heat related?)
– Clogged Injector? (Could I clean it?)I can’t really gauge which is most likely given symptoms though, would be happy to hear your thoughts!
sorry for another long post! -
Fuel Pump could be #1 suspect and also its Relay. But the pump for that bike needs to maintain some good presure as the injectors could only spray as quick as they can based on ECU timings . The ECU won’t tell you if the pump is faulty (if its still running), it can only trigger it on or off.
It brings us to the injector being the next domino to fall. I’d try to clean the injector but be ready to replace it.
Ignition coil = Remove the HT Plug Cap (unscrew it, don’t pull it off) ==> Test only the ignition coil for PRIMARY & SECONDARY resistance. Tell us the values. Make sure the coil is cold.
HT Cap on its own = Measure the wire and the spark plug tip inside ==> Tell us if its 5,000 , 20,000 or 50,000 ohms.
(All OEM caps can sometimes be the reason there is sparking issues , but rarer on a GD as it’s not spoken about as much) -
Hello again!
Finally decided to take off all of the fairings and troubleshoot again, after the bike recently started to idle at about 4000rpm and rev hang got worse. (Can’t comfortably ride, if you need to slip the clutch on upshifts to get the revs down)
Went to measure the resistances for the coils and for the cap itself
Cap/Cable -> 10.3 kOhm
Primary -> 0.6 Ohm
Secondary -> 6.7 kOhmI should get around to checking the injector, but I currently have no idea on how to clean it and it’s a bit challenging to get to.
– IAC Experiments
I’ve been playing around with the IAC some more because it seems most plausible to me.
I checked one more time to see if it’s a vacuum leak, which it doesn’t seem to be. The airbox pulls hard when you cover the opening, the idle normalizes, rev hang is improved and if you cover it more, it stalls.I did some adjustments to the throttle follower setting (how quickly the IAC closes when you close throttle) as well as its parking position, which works pretty well until it somehow goes back to lagging and idling high
which I think is due to it becoming miscalibrated (adressed further below)
see video (this was directly after a flash re-calibrating the IAC position and settings):
https://www.youtube.com/shorts/0WL2oA8j36M (didn’t mean to upload it as a short, mb)– IAC Calibration
I decided to remove the IAC to inspect it, it looked fine, I cleaned off minor grime.
After some messing around and measuring, I discovered, that the valve would not fit into the throttle body, when first calibrated to the pintle seat and then moved out all the way. It was off by about 24 steps.
I assume that, basically when telling the IAC to fully close (position 0) it’s physically stopped from moving at position 24 while thinking its position to be 0, so it would be miscalibrated by +24 (more open).
For some reason the IAC also seems to get quite hot when the bike is on (50°) not sure if this is normal considering it’s a stepper motorI’m still unsure at the moment, I haven’t found a tune that gets the behaviour I want to stay for longer than a few minutes. (did get one very nice ride where revs dropped nice and fast for quick shifting, which lasted about 5 minutes)
and I’d still be puzzled as to why it started doing this in the first place. I’ll see if I can find a replacement IAC, in case this one is broken or damaged.Could a dirty fuel injector cause this behaviour? it seems to be running lean, but I don’t understand how it would be idling at 4k
Thanks for any ideas and tips!
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I’m back again with some data after a test today!
While thinking about the possibility of an IAC delete and reading another thread in the forum, where someone put a setscrew where the IAC would be. I finally got the idea to just set the IAC to a fixed position in software and ride, and lo and behold! It worked quite well! Revs behaving as expected, mostly stable idle (still occasionally misfiring and dying) I did notice a slight performance decrease to before, which I assume to be because the decreased intake airflow (I suppose it isn’t trivial, as I initially assumed, but I guess if it can idle at 4-6k revs on just the IAC passage it isn’t).
by the end of the ride it also started misfiring/sputtering at low rpm, which I attribute to it running rich once warmed up (like riding with choke on)I’ll look into ordering a replacement IAC, I’ll also try changing out the little spring on the stepper motor in the meantime to see if that changes anything, (I felt like the motor was struggling to push out the needle when manually setting it)
Again, if you have any thoughts or ideas, let me know!
I couldn’t find a spec for the spark plug cap resistance, so I don’t know if 10.3 kOhm is fine, seems to ignite fine at high rpm as it has from the beginning -
Cap/Cable -> 10.3 kOhm
Primary -> 0.6 Ohm
Secondary -> 6.7 kOhmCoil is OK
Cap is at the limit (10K is about the limit for these OEM caps) , the NGK ones you see at the shop are recommended!Idle Speed Delete = I’d exercise caution because it does engage slightly when rider leaves throttle to change gears to keep the engine “momentum” going, but if it rides OK, then keep an eye and make sure the ECU doesn’t get annoyed later on (putting more tax on it to regulate the EFi system properly)
Buy a spare ECU just incase!
Getting a fresh idle speed controller is a good idea.
Because it occasionally dies = Does it die when riding ? or just stalls when you come to the traffic lights? Does it fire up right away ?
I may suspect something else is failing when it gets up to too much “heat”
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Great Success!
(Grinning ear to ear right now)
Finally replaced the IAC, had to rewire the plug for it (didn’t get an OEM part). Didn’t run right from the get go, in fact, it behaved even worse. Then I did some testing with locking the IAC position at certain positions, found a stable value (idle around 1800-2000) and set the base value in the ECU to that value and disabled PID/Closed loop adjustment for the IAC, with slight adjustments to the throttle follower.
Runs without flaw now. Returns right to idle when revving up, never sputtered, hot or cold, I can (even if it sounds horrible and isn’t good for the engine at all) lug the engine like crazy (30kph in 5th) without it cutting out at all. No sputtering/misfiring on acceleration from a stop like before. Full throttle doesn’t feel weak anymore like my experiments with the IAC locked in one position. Decel enlean is working, where it wasn’t before. I’ll monitor the spark plug to see if it’s running properly in terms of mixture, but it has been running great in all my testing today.Oh, this isn’t necessarily relevant anymore, but while waiting for the part, I did get hold of some feeler gauges and checked my valve clearances, all in spec! (Intake L,R: 0.15mm ,0.13mm Exhaust L,R: 0.25mm, 0.23mm)
A fun exercise XD and gives peace of mind on a machine that seems like it wasn’t cared for that well.Thanks for the support!
I’ll be back if I do encounter any issues
I’ll also post some pictures if I get around to fixing & customizing the bike visually (tank cover is cracked, others scratched) -
What’s up again Hyoriders!
Got an update and asking for thoughts
Been quite happy with the bike so far!
Still handles very nice in the twisties we’ve got here! Nice torque to pull you up through hairpin turns!The idle did trend upwards over the course of weeks. Diagnosed that to be some hidden parameter pushing up the value I manually set. So finally I decided to make a manual IACV adjuster using an idle adjuster cable and the carcass of the old broken IAC. This worked really well, and I’ve been able to adjust the air amount when necessary, getting the bike to idle really well at about 1800 when cold in most weather.
I did however notice that the bike started having issues when it got hot (before and after I replaced the IAC)
The fault is misfiring, this time even at higher rpms, but especially noticeable at low throttle in the transition between idle and gas. The bike jerks a little bit when it misses once, and the frequency of the misses roughly increases with temperature. Riding fast (60-120kph) It has never happened unless the engine was really hot right before. I was able to ride without problems for an hour on country roads and then around a half hour on the highway at quarter to full throttle at high to relatively low rpm (4000).In the city, the bike still idled fine for a few minutes in slower traffic, slowly starting to stutter until it ultimately refused to run at all. It would crank, fire a few times and die or die as I slowly brought it up with throttle. Had to pull over, wait for it to cool down (the temp gauge didn’t show overheat or high temp) and set the IAC to let in more air, after which it ran begrudgingly and at a high idle with some stuttering but very slowly getting better once I got on a more open road again. This has happened a few times before the IAC switch when I was doing maneuvers on a parking lot as well.
The starting difficulties sound like it’s running rich to me, but really only happens when the bike is really hot
I assume that the airbox gets really warm when standing or riding really slow and the bike isn’t compensating? (IAT? problems?)
I’ve heard of vapour lock, but was also told that it’s really rare on Injected systemsThe misfiring when warmer is something I can’t explain to myself properly. The bike runs without misfiring for longer when it’s cold out or at night and not at all at higher speeds.
Could the ignition system also struggle with higher temps? (It is quite close to the radiator and engine)
I did report pretty high resistance on my spark plug cap (when cold) so it could be more grossly out of spec at temperature? is that something that would cause misfires every now and then?Lastly, I did recently check my spark plug and it was matte grey looking like a more lean condition. Not sure why it would be lean and then run super rich other times.
Thanks again for any ideas!
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Something i’ve learned about high temps is the spark plug cap , i can’t remember if you have an NGK one fitted but the stock ones just aint good. Another thing to look out for is the ignition coil, it does happen too but less often than the plug cap.
the plug should be mud brown / beach seaside brown all the way , if unsure , post a picture of it.
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Went and measured resistances yesterday to get a clearer picture and solid values.
Also took snaps of the spark plug. Looks odd to me, A friend suggested that the spark plug is running too hot and that I should get a colder type (like a cr9e)Primary (between the prongs of the coil): 0.60 Ohms
Secondary (With Cable and Cap): 17.12 kOhms (this is how measuring is noted in the manual which, if accurate would mean I’d be around 10kohms out of spec)
Secondary (Without Cap): 6.72 kOhms
Plug cap on its own: 10.40 kOhmsI’m a little confused about the manual, if it’s accurate, my coil would be totally out of spec even with a normal 5kOhm resistor cap
Might order a coil as well, they’re not as expensive as I expected.
Already ordered an HT lead and cap from a local store, will be updating once I get that installed and tested. -
Update:
Installed the plug cap yesterday, had to use all the old rubber pieces (I got an NGK SY11 cap).
Rode around a few hours today to challenge the bike, stood in traffic idling for minutes until it was really toasty.
Didn’t misfire once.
Was also able to set the idle down way lower without it stalling, throttle response good.I’ll keep an eye on it (especially the spark plug) in case something comes up, but it looks promising.
Thank you again!Will also post images once I’m done with the custom tach cover I’m making (the stock one looks kind of silly to me)
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Keep an eye on the spark plug , see if it turns brown because “pure white” isn’t good long term, it suggests overheating soon due to low fuel at high revs / high loads or the sparking system stops firing every now and then when it gets hot (but then if it stopped firing, plugs would be very sooty wet with unburnt fuel)
Keep an eye on the injector and fuel pump!
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