To give some background. The last time I rode this bike it suffered a runaway engine.
This was probably because the intake from the airbox to the carb decided to slip off one side close to my destination. I was able to stop it after about 15 seconds by dumping the clutch on it. This was after the kill switch and ignition switch did not stop it screaming at 10k+ on a closed throttle. After letting it cool down and correcting the issue it started just fine and made it home.This bike was then brought in for maintenance to change the oil and sort a few other minor issues.
It ran fine after this maintenance was completed.

Two days later, I gave it a once over to check all was in order before returning it to the road.
It was at this point I found that it had lost spark while sitting untouched for two days.Starting with the most likely suspect, I checked the coil.
This was well out of spec and probably not the right one for the bike, measuring 3Ω.
This was replaced with the correct 1Ω coil (Negative, 5.9kΩ). The plug cap and plug were also replaced.
This did not fix the issue.

The stator coils were the next logical step.
They both checked out ok. They were measured from the CDI connection points.
Source = 276Ω (spec 300Ω, is down 8%)Pulse = 9.8Ω (spec 10Ω, is down 2%)This led me to believe that the CDI was at fault. Perhaps the runaway cooked something that took a while to fail completely.
So I obtained a new CDI from Rex's.
This did not fix the issue.

The last 3 days have been spent probing loom connections to see if there is a short.
The ignition switch was disassembled checked for damage and cleaned.
There appear to be no faults with any of it.

So, at this point it is looking like there is either an undetected short, somewhere.
Or there is an issue with the pulse coil not picking up.
Its all I have at this point. And would be typical given the price of a pulse coil or rotor...
It is also possible that the new CDI unit is also dead, which is less likely given that it was probably tested after assembly. (Or both are fine if its something else...)

Does anyone have any experience with something like this?

I appreciate any insight you may have.

Gosh, you're not having a good time.
I've never been unlucky enough to have a runaway motor without good reason like i caught the throttle cable on a something riding through trees etc & pulled it out off the carb & junction box.
So firstly a stupid question 'on my part' but is the kill swich on & ignition key on.
Ok i guess so--so, can you bypass the loom & just run the mag loom to the cdi box then to the coil--original coil, for a try.
If that doesn't work, can you check voltage at kick speed plug out from the source & pulser coils.
I'm guess although had a bit of experience--& failures lol--but, on an old fashioned ac volt meter i'd look for about 80 ac volts from the sourcy thing.
But not sure on the pulser but anything is herlpfull as only needs a few volts to trigger it i've found.
I use a peak meater which shows double like 160 volts & maybe 20 on pulser.
So maybe 8-10 on your ordinary meter from the pulser.
Your ohm resistances sound in the ball park.
Spray all the connection with something before pushing in.
I wild guess a bit but gotta start somewhere & in theory the stator sounds ok.
CDi boxes & coils hardly ever fail & seems odd that she reved her [his] nuts off then no spark after resting to recover.
Got compression--no hole in piston from the rev--should still have spark though unless alloy all over the plug.
Wonder if it poked the left crank seal out causing the rev.--should still spark though.
Flywheel key got damaged in the rev if not tight--but ok would still spark just not at the right time.
I'm lost--back in the box for me & good luck.--which reminds me if it's the 'right' seal-not left- it'd smoke bad enough to get arrested.
ps, just clicked on your name & wow your bike looks nice i must say.

It was all going on that day. Trying to dial in a needle change to cure a rough spluttery flat spot at 3-4k.
While also suffering a progressive clutch failure caused by the pushrod nut undoing itself.
This dragging grabbing clutch masked the surging on hard acceleration such pulling away as clutch issues. The stop start traffic for almost the entire journey also ensured it got nice and hot without going above 4k where it was surging when warm. It also decided halfway that It wouldn't idle properly in said traffic as well. probably due to the air leak of some sort developing around that time.

As I said, put the boot back on properly, let it cool down and it did 4 miles home at sub 4k revs just fine, even if it did need peddling to pull away or it would stall thanks to the extremely loose clutch.

The seals checked out fine. They were in place, intact. Held the 90Psi that my top end has which is not great, but it ran and pulled just fine for the last 150 miles.
Its on 4th over so I would need a whole new top end from, somewhere... to improve it.

It is worth noting I had slight oil pump issues as well. It had (has?) a leaking check valve that was dumping a fair amount of oil into the case. An external check valve has now been fitted as part of that maintenance. It did make a lot of smoke on startup due to this oil. We also found a quantity of said oil overfilling the transmission. We are not sure how it made it out the right hand seal, unless its the oil pump shaft is also leaking, we are waiting to see if it continues to do it now the puddles of oil in the case should be solved before tearing the pump down, again.

The rotor was clean and dry, and when it was removed the left seal was ok.

The piston is ok, the bore is borderline but ok. It was checked when it was decoked from the thick carbon deposits left by quater liter of oil burning off through it each time I used it (2-3 weeks for that much to leak through). This carbon probably got warm enough to greatly help the runaway.

Yes the bike is nice, it just seems to be a bit tired. I think I am owner 27. According to the Swedish driving agency. It is also an extremely late production 1981 model made in October of 82. Probably one of the last MX models. It has things like an LC pattern steering lock on the frame.

How it did idle. youtube.com/shorts/aTusmFlWQrE?feature=share
What the oil valve leak did. youtube.com/shorts/pJ3kisFGahM?feature=share

Wall of tangent complete.
Now, back to the electrical issues. The ones stopping me from riding.

It did run absolutely fine for 20 minutes after we were done fixing everything.
Then two days under a cover later, nothing.

Yes the switches are both on. They both check out on the multimeter as doing what they are supposed to.

The original coil that I received it with I am not sure about as its 3x the resistance it should be. That had to have weakened the spark out a bit and put a strain on the CDI?

Its probably not the woodruff as there is no spark at all, not even an attempt. It worked and now its just completely dead.
Though the generator is the next thing to pull apart again.

Thanks for the voltage figures on how this should be operating, I suppose I should try putting a multimeter on the coil outputs. while trying to kick it over. Seeing as you indicate a meter should be able to handle the voltage and current involved.

This will not defeat me, I will get it running again whether it likes it or not.

 

Thanks for the extensive update.
Think i'll sit back & watch. I've run out of ideas anyway.
When it smoked 'the village' out in the video, apart from trying to get rid of the oil it sounded a bit like it wasn't going to rev at all--like muffler restriction.
But was probably just fact that i'd watched it couple of times before i realised it was restarting the video automaticaly & replaying.
When it ran & heading out for a ride--did it rev cleanly if you just give it a redline quick rev in neautral. After oil was burnt out that is.

That was only about 3-4k.
And right into the flat sputtering spot I was talking about. You can probably hear it.

Once you get past that it has good powerband from 4 to 6k.

As an example of what it was capable of. Once warm.
I am aware my shifting was sub standard, I had only done 40 miles on a geared machine.

Excellent video thanks.
I'm forgetting it's not actually running at the moment & i don't think the erratic running at about 1/4 throttle is related to the spark problem that now exists.
I'm also forgetting its only a 125 not 175 so has little torquee pulling power on road gearing unless revved a bit in a lower gear.
That said, the 1/4 throttle misfire sounds blubbery rich at that point but hard to be sure. I'll listen on sharper speakers later.
I'm wondering how you have the idle mixture screw set because if it's screwed fully in the richness could affect the running above idle slightly & show up more after closing then opening the throttle while riding along in a high gear.
Also i wonder how the reed petals are. Have they been replaced with some fancy fiber aftermarket options & fluttering at 'that' point in the revs.
How is the compression. Is it the correct piston. Is the piston the right way around--windows to the inlet side. Just has a bit of a hollow sound to it unless revving but might be the video plus it's only a 125.
I had a 125 [RX125 Yamy] that was a bit like that & had a rubber original noise baffle in the inlet between air cleaner box & the inlet hose. Was to stop inlet noise at open throttle i think. I removed it & ran so much better.
I don't know--got to get it sparking again i'm forgetting.
Could try running it minus the complete tail muffler as looks like you're out in the country.
Fit a 175 top end, that'll fix it.
So things to look at when running again. Pull the reeds & inspect. Check piston holes are visible. Pull exhaust completely off & inspect the rings in the exhaust port just to get an idea of good or bad.
Check idle mixture screw approximate setting is not screwed right in.
You say you've been altering the needle setting & understandably to help the blubbering.
So i wonder it the air bleed that goes to the needle jet emulsion tube is blocked as i had that once with a crumbling air filter.
Essay over for tonight lol. Keep at it & hope for the best.

I guess crank seals have been talked about but be good to replace then just because!!
Sorry i'll go back & read. I skipped over it & see piston is ok.
You say crank seals are ok then say it holds 90psi.
Do you mean 90psi compression pressure measured in the plug hole. If you mean you pressurized the motor to 90psi with the air compressor checking for leaks it'd likely blow the crank seals out of the case. Maybe i'm misunderstanding so we'll go with compression pressure. In which case 90 is a bit low & thinking 120 or so but guessing. Certainly over 100.

i see in this link #14 & #13. What's the story there. Does 13 fit over 14 . Does 14 have any inlet restriction in it that's not drawn in the pic.

www.partzilla.com/catalog/yamaha/motorcycle/1981/dt125h/intake

This carb is a bit of a hack.
It is a mix of an old 24mm 175 carb (U5100) and two generic knockoffs, one of which came with it.
The needle was changed to a 4G2 (Fatter and less taper), which is the one that should be in there, by the book.
It did fix the low speed blubber at the expense of top end power, and the whole surging when warm leading to a lean runaway thing. The needle in the video is an unmarked Chinese one. Unfortunately Mikuni VM 4 series needles are hard to find.
The idle screws are all set to factory specification when the needle was changed, and set to factory again when the original needle went back in. The main is currently 150. We are currently trying a smaller pilot jet down to 22.5 from 25. to see if that helps, But its not run since, so we don't know what its done yet.
It was all taken apart and spent a few hours in an ultrasonic cleaner when it went on, so is about as clean as its going to get.

The piston seems genuine, probably Wiseco judging by the W on it, Arrow is towards exhaust port as its supposed to be. I believe its 4th over as it was apparently rebored just before I got it. Its still not looking great in there but it has enough compression to run. The 90 PSI was a compression test at the plug hole. I am aware it is low to the point its borderline. It usually runs in 2-3 kicks.
The rebore was possibly done poorly, or its already been scraping the exhaust port. Only one scratch is deep enough to just about feel, caused by a metal shaving that I assume was from the rebore, that they must have missed. I did possibly go a bit hard on it for a rebore, but as you know the 125 doesn't move unless you rev it.
See attachments, you can see the excess oil tidemark, carbon deposits and the exhaust port polishing, with scratches. This had 100 miles on it from "new" at the time.

The hollow sound I put down to the round end on the 78 silencer I put on there to replace one that had been cut and shut rather poorly minus a third of its length.
But it might be something in the engine? I know the barrel dampers are out of place and perished solid.

I am getting the parts for a 175 top end together but it seems the supply of cylinders has dried up recently.

Most of the fuel intake system has been replaced with new and used parts. The only part original to the bike would be the reed cage. It was all cleaned well.
Yes 13 goes inside the filter to give it strength and then its slots over 14 that is screwed down with a knurled plastic thing in place of 15/16.
14 looks exactly the same as the picture, its open. 13 often gets thrown out with the old filter, I had to find one. I actually had a square DTX filter duct taped over 14 when I got it... Which explains the need for the rebore probably. 17 is missing as they all seem to disintegrate into little bits, not much to be done about that.

And yes the reeds have been replaced with "carbon fiber". They are new as the carbon fiber ones that came out of it were worn.

But, I need to get it sparking again before I can adjust fueling. And its being stubborn about it.

Sounds like the carb is a bit of a mixture although i don't think it's affecting the overall torque--or lack of--.
If i said 'hollow sounding' i just meant where it doesn't make power, the blubbery bit, it just goes hollow--no power--then gets back into it.
Talking reeds & fiber reeds i had a customers KTM 125 motocross bike which blubbered just like yours & he'd changed the reeds for carbon fiber looking reeds. Think they'd been in a little while at that stage. I couldn't carburate it to go clean until someone smarter than me haha said change reeds back to the std type. Fixed it straight away so lesson learnt 'by me'. Mind you his was up in the revs about twice where yours is at when it went through blubber.

Considering your carb issues & nothing to lose--I'm running a chinese cheap carb on my 175 [was a 125--oldy 1970 ]. I'll link the page for a laugh if i can find it just for the carb fitting/ I know chinese carbs are rubbished on here--but--if ya desperate!!. Cost about $16 & free freight but a few years ago now.
Ok i found the post to link. Brings back memories if changing jet threads etc. Goes good & strong motor--until it blows lol.
yamaha-enduros.com/index.php/forums/1968...-chinese-carb-on-175
Back to the spark--no spark.