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Discussion Starter · #1 ·
Hey,
I have a prefl model and recently i’ve been having this problem with my bike where it idles at 1000rpms and sometimes shuts off. Also when I start it and start to drive instantly i can only stay at around 3000rpm, I pull the throttle down all the way down, but the rpms just dont go up. But after a minute or so of running, it just goes back to running normal, but it still stays at 1000rpms on idle at a red light. I also noticed that when i start it and try to keep my revs at for example 3000rpms it stays there for a couple of seconds then drops down to 2500ish and starts popping, then it goes back to 3000rpms and so on. My first suspicions were the spark plug and fuel-injection, but I am really not sure. It might also be the distributor or something in between that. I ordered a new spark plug and while I wait I appreciate anyone helping.
P.S - I’ve tried messing with the idle rpm screw but that doesn’t seem to do anything at all.
 

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2008 YZF-R125 (180cc)
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what is a "prefl" model?

> P.S - I’ve tried messing with the idle rpm screw but that doesn’t seem to do anything at all.
do you mean the small brass screw set into the throttle body, or the little bolt on the outside of the throttle-body next to the throttle cable which adjusts how closed the butterfly gets?
 

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Discussion Starter · #3 ·
what is a "prefl" model?
A pre-facelift model is before 2016 if i’m not mistaken. I don’t know if that matters here but i still mentioned it since a lot of things changed that year.
do you mean the small brass screw set into the throttle body, or the little bolt on the outside of the throttle-body next to the throttle cable which adjusts how closed the butterfly gets?
The small brass screw which is supposed to change the idle rpm, I wasn’t aware of a little bolt next to the throttle cable but should I try to adjust that?
 

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A pre-facelift model is before 2016 if i’m not mistaken. I don’t know if that matters here but i still mentioned it since a lot of things changed that year.
ah right, that's useful, thank you, I think I know which bike you mean now. Does that still have the instrument panel wihth the triangle segments etc?

The small brass screw which is supposed to change the idle rpm, I wasn’t aware of a little bolt next to the throttle cable but should I try to adjust that?
I ask because if it ran ok before then there should be no reason to adjust that screw now. Personally I wouldn't touch the little idle adjuster screw. Hopefully you counted the turns adjustment you made so it can be set back to stock.

I raised the idle on mine too as I kept stalling it when pulling away but did so using the little bolt on the side of the throttle body which just props the butterfly open a little more. I don't have a tb to hand or would send a pic. You can see it when you look into the throttle body from the right-hand side. You just slacken off the little locknut and then can adjust the headless bolt either by hand or i think you can poke a tiny allen key in the bottom. I replaced that with a longer bolt so can adjust easier.


Have you got a multimeter? If not, you have a motorbike, buy one. Cheapo ones are fine. Then could you please take a voltage reading across the battery terminals with the bike switched off, and then again with it running? Switched off it should be around 11-12.5v and when running the voltage should be more like 13.6v. I'm asking as if there's a problem with your charging circuit that might cause intermittent issues. If we know the voltages are good then spotty power is not the problem.

So if I understand correctly the bike runs ok for a while.. then randomly it drops to ~1k and then even if you give it lots of throttle the rpms don't come back up.. but then after a bit it jsut works again?

If so it means something is intermittent or faulty.. things which would affect the running like that might be o2 sensor, intermittent fuel pump (this happens quite often), or maybe TPS sensor. Any of those would give you these symptoms

Pops and bangs normally happen due to too much or little fuel, or a sudden change in fuel - e.g. if you're at 5krpm but then the injector isn't delivering the right amount of fuel for 5k, then it will pop and may sound kinda horrible.

If it's too much fuel it would "bog", which is more a splot/farting type direction for the sound.

I'm not sure what the wiring looks like on the o2 sensor so am not sure how to advise taking a voltage reading from it to check so maybe let's start with the other two.

Next time it starts misbehaving, please see if you can feel/hear the fuel pump still. This will def be easier off the bike but I realise you may not be able to make it do the thing on command.

Also next time it happens - rather than giving it all the throttle, can you try just gently rolling on the throttle? and if there's time also try slamming the throttle fully open and closed a few times and note what it does and how it sounds. My hope is that if it's the TPS at fault that would show with engine response whcih isn't consistent with your throttle movements - jerky/smooth may behave differently. if the throttle has no effect then I guess we might be looking more at fuel pump or o2 sensor.

I realise you may not be able to make it do the thing but if you're able to record what it does please do it might be helpful.

I also noticed that when i start it and try to keep my revs at for example 3000rpms it stays there for a couple of seconds then drops down to 2500ish and starts popping
this kinda sounds like fuel pump to me.. which is probably also the cheapest/easiest to replace (~£20 iirc). If you've had the bike a while it may not be the worst thing to replace the pump just to rule it out, though understand everyone has budget constraints.


It might also be the distributor or something in between that. I ordered a new spark plug and while I wait I appreciate anyone helping.
as this is a single cylinder engine there is no distributor as such, just a coil which gets a signal from the ecu. A new spark plug is not a bad place to start, good idea. Please post a photo of the old one, photo looking kinda down at the electrode in daylight so we can see the end and a little bit into the hole. The overall colour and state of the plug may tell us something useful about what's happening.

jump in the discord server if you want to talk it through in realtime. There's an invite on the contact menu of my website.

NC
 

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Discussion Starter · #5 ·
Next time it starts misbehaving, please see if you can feel/hear the fuel pump still. This will def be easier off the bike but I realise you may not be able to make it do the thing on command
So today when I tried to start it won’t start at all and i noticed a couple of things.
1. When I turn on the ignition i can hear the fuel pump priming so I think its good in that regard, but I will make sure to replace it after the spark plug since it is in fact not that expensive and I’ve already had it changed when I bought the bike 2 years ago and it was an offbrand fuel pump so it could’ve broken down by now.
2. I can hear the injector pumping fuel into the cylinder on the same intervals so I dont think it has something to do with the injection system being off or not getting fuel, I may be wrong though.
3. The spark plug tries to ignite the fuel sometimes, i can hear the spark while pushing the starter every once in a while, and sometimes it starts and does one revolution and I think the spark plug doesn’t follow up on the ignition so it just shuts off instantly.
So im pretty sure its the spark plug or the ignition system but while I wait for my new spark plug to arrive, I will try to take out the spark plug and push the starter then to see if it is giving spark consistently to maybe see if it may not be the problem.

Have you got a multimeter? If not, you have a motorbike, buy one. Cheapo ones are fine. Then could you please take a voltage reading across the battery terminals with the bike switched off, and then again with it running? Switched off it should be around 11-12.5v and when running the voltage should be more like 13.6v. I'm asking as if there's a problem with your charging circuit that might cause intermittent issues. If we know the voltages are good then spotty power is not the problem.
Good idea, I’ll try to check later today or tommorow morning.

If so it means something is intermittent or faulty.. things which would affect the running like that might be o2 sensor, intermittent fuel pump (this happens quite often), or maybe TPS sensor. Any of those would give you these symptoms
I don’t really know about the various sensors in the bike so this will probably be the last thing I’ll check, but I will keep it in mind.

I will also be sure to check out the discord and maybe provide the photos and videos there.

Thanks,
David
 
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