Yamaha SR125 running with ignition off

[Svenington]

Hi guys,

First time posting here, but I have been lurking for quite some time 😂

I've been working on a 2002 Yamaha SR125 that I bought as a non-runner to turn into a bobber/rat bike project and I've finally managed to get it running again after a lot of new parts and a carb repair kit etc.

The issue that I have now is that once it is running, switching the Run/Stop switch to stop doesn't stop it running, neither does turning the ignition key off so I have to cover the air intake to stop it. However the engine will not start or turn over with the switch set to stop

After some googling I thought it might be dieseling so I adjusted the carb for a leaner mixture, changed the spark plug which had a fair bit of carbon on it, and adjusted the timing to be less retarded, however this didn't make much of a difference other than it making less smoke. After that I pulled the HT lead off and that stopped it instantly (though I did get a pretty good jolt from it which my girlfriend thought was funny at least 😂) so it looks like it is continuing to spark after the ignition has been turned off. I also tried disconnecting the battery earth strap as it was running and it didn't make any difference so it seems like the magneto and ignition coil/spark plug are making up their own circuit somehow, but I can't work out where to begin looking to try and resolve this.

Have any of you guys come across anything similar or have any insights?

Cheers,

Sven

[jaffa90]

Can you post a wiring diagram ????????????
WD forte on here may have the answer?

[jeffyjeff]

Hi Sven.
A quick Google search shows the 2002 SR125 has a capacitive discharge ignition (CDI) system. That squashes the magneto idea. My immediate reaction is that the bike must be miswired. I would start by looking for your CDI control module and determining how it is powered up. (Possibly through a relay). Somehow that module continues to be powered even after the ignition switch is turned off. The relay or its coil circuit are good places to investigate.
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History shows again and again how nature points out the folly of men - BOC

[Robby]

A CDI system can still run independently of the battery if it has one of the alternator coils dedicated to generating power for the coil. Common on lots of small bikes, means the reg/rec can crap itself but the engine keeps working.

If an engine is running with the ignition off, there are two main options. 1. It's still sparking because of a wiring problem (this is how you hotwire a vehicle). You seem to have ruled this out, because it won't start with the killswitch/ignition off. I'm assuming it has a kickstart, ignition and starter motor are totally different circuits. You could have a situation where all of the electrics are dead, but kick it over and the engine runs. Separate circuit, self-exciting ignition, relying on the ignition barrel and/or killswitch to stop it working.

2. Carbon deposits in the head are getting hot and igniting the petrol. This is very rare these days - petrol does a good job of cleaning your cylinder head. To have trouble like this, it needs to either spend a very long time running horribly rich, or be burning a lot of oil. It's a cylinder head removal job to diagnose and clean up, so check the wiring properly first.

Also, don't cock around with the mixture and timing. Set them to whatever it says in the Haynes manual and leave them alone.

[xX-Alex-Xx]

I’d rule out point two as the engine stops as soon as the HT lead is pulled.
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DILLIGAF

[WD Forte]

Why me?
iI dunno what an SR125 is

Never mind I'll chunter on anyway
My theory/guess is that if it's an AC CDI bike, the ignition and kill switches
usually short source coil output to ground to stop the sparks/engine
and the cable for this has been disconnected as part of the rat/bob/cafe 'improvement' process
This may well be a Black/White wire

If I saw a diagram or a decent pitcher I might be able to make a better guess
Pity no feckers got cameras nowadays or easy means to share such pitchers
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bikers smell of wee

[jeffyjeff]

[jaffa90]

Why me he says! (like a ybr man)

Also jeffyjeff you are thinking like a domestic sparky.

[jeffyjeff]

[Svenington]

Thanks for the responses guys, I've been having another go at it today and I still haven't quite solved it yet.

I had a look at the connectors and there were a couple that were corroded so I've cleaned those up. I also managed to find a wiring diagram and tested the ignition switch and run/stop switch against that and they both have continuity in all the right places. I also reconnected the sidestand switch (currently absent due to a bent bracket) and I found that the thing wouldn't turn over with the switch closed, but once it was running the switch made no difference at all.

I did notice that both switches are closed/connected for on/run then open/disconnected for off/stop so it seems like Yamaha have taken a different approach to having the switches open then closing them to ground out the circuit (unless I'm being twp and misreading what's going on).

The bike doesn't have a kickstart, but after reading what Robby posted, I had a go at starting the bike by bridging the terminals on the starter relay and it was able to run with the ignition and run/stop switch set to off/stop (I think there must be a circuit built in to kill the starter when the stop switch is open hence why I couldn't start it that way) - is this a possible sign that the voltage rectifier could be knackered?

In terms of wiring, I haven't done anything to modify any of the harness, just kept it stock and replaced a knackered neutral switch and replaced a few spade connectors for the lighting. I spent an afternoon a few weeks back with a wiring diagram and labelled all the components and connectors to try and get my head around it all. Is it possible that something could be grounding where it shouldn't causing this issue?

Thanks again guys

[Robby]

Someone has cocked around with the wiring to leave it permanently hotwired. Possibly to get around the dodgy side stand switch.

Hard to read the lettering on the wiring diagram, but it looks like you need to trace the blue and yellow wire from the CDI and see where it goes. I think I could reproduce your problems by connecting that to a permanent positive feed (so just wiring it straight to the battery positive terminal).

[Svenington]

Thanks Robby, I'll add that to the list of strange things previously done to this bike, along with random parts missing from the carb and holes drilled into the exhaust 😂

I'll have a go at that tomorrow, I've got the full PDF of the manual with that diagram which does let you zoom in properly but I couldn't work out how to share the full diagram on here.

I think I've been staring at it too long because it just looks like a confusing mass of black lines and numbers now 😂

[McHattrick]

This is wiring for '99 SR125. Is it same?

https://www.manualslib.com/manual/842634/Yamaha-Sr125-99-3mw-Ae3.html?page=25#manual
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If it aint broke... fix 'til it is.

[WD Forte]

Had a quick look at the ( crap ) diagram
I'd say its deffo an AC CDI bike but it uses a more complex system
to stop sparks than just dumping source coil power to ground.
I get how it functions and how the side stand, neutral and clutch switches
work for go /no go conditions but cant read the colours.
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bikers smell of wee

[Svenington]

I've attached a pdf of the wiring diagram, so hopefully you should be able to zoom in on it tidy - I'm just crap at reading diagrams

I've got my multimeter out today, so I'm going to have a go at tracing what's going where, just need to work out which bits are supposed to have continuity, and which bits aren't

Edit - it also makes no difference to starting whether the clutch switch is open or closed, but the neutral switch does stop the starter motor from turning

[yen_powell]

https://www.youtube.com/watch?v=guSdIlmO30A
Have a look at that.
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Blackmail is a nasty word........but not as nasty as phlegm!
XT1200Z and a DR350 in bits

[Robby]

See if you get 12 volts between the yellow/blue wire at the reg/rec and the battery negative terminal, with ignition and/or killswitch off. If you do, they have been bypassed.

This would allow all of the starter motor stuff to still work/not work as expected, it just wouldn't kill the engine. It looks like the engine will run as long as that wire sees 12v.

[sickpup]

There is likely either a corroded, disconnected or broken earth wire so the ignition cannot route to earth when its switched off.

[Svenington]

I've just had a check with my meter and there's no voltage between the yellow/blue wire going to the CDI and the battery, but I do have voltage going between the red and black wires going to the Voltage Regulator/Rectifier and the battery with the ignition off, so I'm guessing that means that it has been bypassed somewhere.

I think my task this weekend will be to open up the harness on my bench and look for any dodgy wiring mods. Do you think this is likely to be around the red wire going to the Reg/Rec or could it be somewhere else too?

It is starting to feel like I'm starting to zone in on the issue now though - it can only hide for so long

[WD Forte]

The thing is, how do we know the diagram matches
the actual wiring of the bike?
Who knows what's been done since 2002?
If I had the bike in front of me id probably sort it quicker
then it takes me to explain it

In an idle moment I made the diagram ( hopefully) simpler to
understand
(I still reckon it works as I said earlier)


https://imgur.com/riCL2vw.jpg
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bikers smell of wee

[Owais]

Hi everybody I have a yamaha sr 125 and I needed to know whether it was conceivable to have a rundown of the articles that you with purchased for instance I need an earthy colored seat and earthy colored handles however I am worried about the possibility that that either not a similar earthy colored thank you for any data you could give me
snaptube vidmate word to pdf
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owais

[hellkat]

[Svenington]

Hey guys,

Sorry it's taken a while to reply, been busy with work and all that fun stuff that pays for my food and parts.

After spending hours going through the wiring harness and comparing it the diagram and checking all the connections, I still wasn't able to find what was causing my issue. As Forte said, there could have been anything done to it in the past 20 years or so.

As I wasn't getting any other issues, I decided to kind of bypass the issue, so I desoldered the leads running to the run/stop switch and permanently bridged them together as they weren't doing anything to stop the engine, and I inserted a extra set of cables between the CDI and the ignition coil and ran these through the switch instead. So now the switch can stop the engine by opening the circuit and stopping power running to the coil.

It still doesn't stop on the key, but the switch now works every time and I can turn the engine over without a spark, which I guess can be useful when I inevitably flood it trying to start it in the cold with too much choke (or am I just too used to 2 stroke boat engines 🤔). I've given it a load of test runs and I haven't come across any other issues from this workaround, so I think I'm all set.

My current job is repurposing an old ammunition tin to use as a battery box, then I just need to sort out a number plate, brake light and indicators then I'm all ready for my CBT 👍

I can't put the picture on here because it's too big, but I have got a picture of it here; https://www.instagram.com/p/CA56R40At8_/?igshid=1ucvpkj4dchdd

[WD Forte]

Too big my arse
here it is in all its glory

https://imgur.com/1CMMpxr.jpg
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bikers smell of wee

[MCN]