You have given me food for thought. Thanks. It has two source coils, one normal type and one small one. I think I once heard someone call them the high and low speed coils, but I never heard of that before.  I only ever came across a lighting coil and single source coil setup before.

If the " high speed" coil is failing when it gets hot, that looks like it could be the culprit.

The CDI must have a means of switching between the two, but the manual is as clear as mud.  It looks like the second coil produces half wave AC and the other a full wave DC via a bridge rectifier somewhere.  However electronics are not really my forte.  I muddle through somehow at best.

I am judging that the smaller coil is the " high speed" one?

I know when I made up a new stator harness to replace the butchered one, all the coils were testing in spec at the terminal block, but that was at room temperature and before the misfire.

Can anyone throw any light on this?

I will put up a pic again of the stator with the butchered harness. The charge coils are at the top with a small one piggybacked on top of a normal looking larger one.

That stator has, source, trigger, and lighting coils. There are no high and low speed coils. Some TZ race ignitions had that feature, but its not something I have ever seen on a road bike ignition.

Source coil failure is very rare on those systems, if the wiring is all in good order, the trigger coils occasionally fail, but in most cases will mean the bike wont run at all

I would suggest stripping and rebuilding the bike properly, as its very easy to waste a good deal of time, chasing endless problems, with something like this.

Not entirely relevant or helpful but my "83 YZ490K had a faulty high speed coil exactly like yours.
So being 'tight' when parting with money, i just removed It & used a cdi box of a scooter with a black/red wire to which i ran the brown from the fat source coil. Was still going [stretching arms] 10 years later. Just saying!!.

Its not a high speed coil. Its the pulse coil, which triggers the CDI unit. Some CDI systems are triggered by the flywheel magnets, passing the poles of the source coil. I would guess that's why your YZ ignition worked, after the pulse coil had been disconnected, and you fitted a different CDI?

Sorry yamfan. You are wrong. What you are looking at in the pic is two source coils, a lighting coil and a trigger coil.

Don't be too hard on yourself though, I never saw one before either.

RT did your YZ display a high end misfire, extremely harsh one like mine, yet smooth at the low end?

This setup is beyond my experience.

Coil - source 2


 

RT.  The later six wire CDI is far easier and cheaper to source than the seven wire one. Do you think it is feasible to fit the six wire and delete the red wire?  I happen to have a brand new source coil one (the fat one) in my spares box.

That is of course if the source coil two is the "high speed" coil.  It may be that the fat one  charges the battery and the "high speed" energises the CDI.  I really have no idea.

 I said in the trials of the restorer thread that I am not the luckiest person in the world.  Trust me to pick a bike with a completely alien ignition system that few people seem to know much about.

Of course, I do not really want to alter the system, I am chasing originality.  I seem to have read somewhere that a kawasaki motocross bike had exactly the same coil as source two, although it appears CMSNL have source coil 2 back in stock.
at a heartstopping price.
 

I can see we're gonna have to agree to disagree to a point YAMFAN.
I'm likely backing myself into a corner though bringing up stuff from years ago TINKICKER
Was too long ago now to give a true answer. Actualy in hindsite i might've suspected the cdi box, even though i recon they hardly ever fail, & the two coil box will have a brown & a red combining the two stator coils within the cdi box 'i think'?
Trying to find a readable wiring diagram.
From what i remember the stators that have two source coils combining a smaller piggy back coil 'i think' use it to boost the spark at revs. So that has me wondering why don't all models run 'two' if two is better, so am i wrong. Must be somethig written about it somehere.
The cdi box designed for the extra coil will have a red wire [Brown plus a Red from the two sources] & if that coil is faulty to be honest i don't know if would fail to run or depending how faulty perhaps missfire up in the revs. So the question must be asked, if the main source was failing, would that do a similar thing, like not put out enough peak volts to assist the little coil.
Little one is probaly only 100 ohms or so to help boost the spark.
I know YAMFAN you're mentioning the trigger coil but that's the smaller one further around to the left on the stator in the pic in post #81 & for sure if that's faulty it could do the same thing i guess. So i don't know.
Hell i don't know anything anymore haha. I've had source coils show up faulty by just running the paint stripper gun on low heat sitting there stationary untill it went way low on resistance quite instantly after a while then came back up when cooled. Sort of trying to simulate running conditions. Could do that test with the trigger coil too.
It's impossible really to diagnose a misfire without being there & can only guess going by description.
Found unlimited A to Z manuals in link below just googling away. Not sure if any use to anyone. I didn't have much luck but did find a "78 DT175 although not in english.
Ok back to the top end misfire--dare i say the word exhaust again [think i already said it again somewhere]--9 pages & climbing. Is that some sort of a record. Bike looks too stuningly nice to have issues.

www.carlsalter.com/download.asp?p=2912
www.carlsalter.com/all-motorcycle-manuals.asp
www.rickystator.com/product/yamaha/yamah...tion-coils-1982-1990
Link above showing two coils on the YZ490.

Thanks RT. The main source coil is specced at a shade over 420 ohms, mine if I recall, came within the 10 percent tolerance, but slightly lower.

The smaller coil is specced at just 13.6 ohms and again that was within the tolerance but lower.

Perhaps I shoukd dig out my old Yamaha pocket tester from my toolbox at work and use that. I am sure the pics of it will bring back memories.

Ok 13.6 i was miles out gussing 100. I'm learning lol.
I edited the wording slightly plus added a link direct to the DT or MX 175 link.
When i start typing & then add brackets in the first line or maybe further on too then post it, the brackets with words disappear.
Crazy, but think they stay put later in the text if it's a long text. Never mind.

Give Steve at Bradford Ignition Services a call.

Some TZs also use dual coil CDI systems. The reason for dual coil, is to provide more uniform voltage, over a wider range of engine speeds, and in the case of the TZ to ensure the timing retard point, can be determined more accurately.

Hitachi made the TZ systems, and your 175 one, well be Hitachi as well? Only downside is that the CDI unit determines the timing, and your running problems may be related to that?

HPI in Belguim might be able to assist with a new CDI unit, or maybe even a replacement ignition, if CDI proves to be faulty?