Thinking about it...  diode might be on main harness side on connector and not stator side.  Should be a plastic plug if I remember correctly.  Diode is near the plug and it's like a solid stiff section rather than a bendy wire like rest of wires.  It's been too many years since I worked on my twin shock Dt175...  and it's not accessible at the moment so I can't go look at it. 

Anyway, try bypassing diode. 

Ok, seeing as it won't now start. Try plug out & spin it with a reverse drill on the crank as fast as you like & go looking in the flywheel & around. Is the plug coil earthed well. Mount screws would need to be loose seeing it was running ok--then it wasn't. This is turning into an interesting one for sure. I'll put the headphones on for video listen again.

I could suggest dropping the complete exhaust for a quick try, but i won't say that as don't believe it. But i said it if it is that lol. Ok thinking outside the square--has it cracked a magnet in the flywheel, cracked across. I saw a similar thing at work where the magnets were removable & the owner had been playing. & he'd put one in back to front which upset what magnets do push pull whatever & the foreman spent some time sorting it but 'he got it' & was a happy camper!!. Not sure i would've ever worked it out but sure was one to never forget.

Thanks for all the suggestions. During my rebuild I replaced all three stator coils and the wiring harness including the diode with NOS items, so the diode "should" be good, but it will easy to bypass it and test it out.
Also, thanks for the link to the info on relocating the condenser. I will probably do that as well.

I will try bypassing the diode. Thanks,

Thanks for the reply. So many possible causes. I will have to keep checking and trying different things until I can find the problem.

Hi, looks very much like my journey with my dt175c. I tried everything and suspected everything. Took me weeks, and tried most things other members suggested.
Turned out, spark plugs can foul incredibly fast and will throw you off track with diagnosis. And I've never worked on a two stroke that is so sensitive as this one.
Have your timing a little bit off and you would swear you have blown the crank seal. Or it will produce a tiny intermitting spark.
In the end I will always use a timing strobe and by now I know the sweet spot for the spark.
I also isolated the points with an bypass wire for diagnosis, just to check the stability of the points and the resistance. Good looking points can still go all over the place with their resistance. Too much wobble in the advancer cam is also not helping.
Persevere, you will become a happy rider at some stage.

Finally got the bike running again. Thanks for all the suggestions, I followed up with all of them. It wasn't a plugged exhaust, a bad spark plug, timing, carburation, diode, short in a wire, etc. Once I got a new condenser (Ford part) and remote mounted it under the tank, it fired right up. This is the fourth condenser I have had on the bike. Seems like the ones available online that are supposedly a direct replacement are junk.
Thanks again!

That’s always been a mystery to me… which condenser can go with which engine… A company specifies one… but is it really THAT specific?
Sneezles61

That’s always been a mystery to me… which condenser can go with which engine… A company specifies one… but is it really THAT specific?
Sneezles61

No.