Your hit list is exactly what I do on almost every bike I purchase. This one was recently done when I bought it at Mid Ohio. So I simply rolled with it. Lesson Learned it seems. Mechanical tasks I handle well. Electrical issues kick my ass.

Lean off idle SPEED not throttle.
Hunting until pilot takes it richer and normal idle.
Cut-Away too lean.

Try to cover that up with a richer pilot jet.

cliff

Try valve grinding compound to get a grip if you want to re-use the screws.
if not chisel edges to twist.

cliff

Sorry, I just read through the whole thread and it was messed with by the PO.
Complete tear on the supposed best seal outer lip? Assuming the better 175's have double lips.

Engine builder is not, and a crook.

There is your off off IDLE RPM hanging, obviously.

Forget about the past comments I have made to fix.

cliff
ps
This crap I make $$ on other's' messes.
I fix or I buy.

Ok. I now have spark again. I fixed it with my typical troubleshooting method of throwing parts at it until I fix it….

Changed the ignition switch with a known good one that I had on hand because the original was acting up and would only run in the second key position for some reason. So I assumed it may have finally failed for good. But that didn’t fix it….

I swapped the ignition coil with the a known good one from my DT100B. But that didn’t fix it.

I bought new Yamaha points and condenser and Bingo! I have spark again. I had forgotten how much fun it is to try and solder those three extremely short wires to the condenser.

I really like that $60 dial indicator kit I bought off eBay for timing these old Yamahas. I don’t have a buzz box, so I simply used a strip of paper between the contacts and set the points so that the strip just started to go loose at 1.8 BTDC. Hope that’s close enough. Surprisingly the flywheel mark aligned perfectly with the metal tab, so I gotta be pretty close.

Weather permitting I will give it a test ride tomorrow and see if all is well now. What a saga.  It started with a question about an o-ring on an idle speed screw…….

If its a pain to solder in to the stator--Honda had the right idea.
Condenser can mount anywhere to earth & connect in at the plug coil wire in, " black from the mag in Yamaha speak but yamaha coils often orange i think as it goes into the plug coil. Only one wire in so cant go wrong.
DT1 250's had a female connector on the black off the mag loom 'for points checking' so easy to plug in there if you have a condenser with bracket & an inbuilt wire as i think honda do with a sort of sealed top around the wire--as you can see in that pic anyway.
That's just a single wire AC magneto typical plug coil, not DC which would be two wires anyway. Sorry, raining here so just woffling on about nothing useful lol.
www.cmsnl.com/honda-ct125-trail-1977-usa...yac-ign_30500950405/
Umm, blimey not at that price though lol. Mind you i'm looking at our NZ useless dollar so about double=$210 NZ

Shu.  Consider the lot of vintage bikes.

They are made.

They are loved.

They are abandoned when they are no longer fashionable.

They slowly die and rot away in the back of a shed. As nature befits.

Then they are a barn find.  And are forced from their slumber....

I once saw a movie called Chiller.  A most disturbing thing to watch.  A treasured son died, was frozen and brought back to life in the future.  He had no soul.

That is what we are dealing with.  A bike that is forced back into life.

Any wonder they get a bit of a strop on?

Ok. So here’s a boring 3 minute video of it idling, if that’s what you want to call it. You will hear it stumble around the 40-50 second mark. I just don’t know what to make of this particular 175. I have four others that idle just fine. This one doesn’t and it never has. I wonder if it’s time for me to do my first ever pressure test to see if this thing has an air leak. I will ride it tomorrow and see if things clear up. But to recap, both of my cranks seals were completely shot. They have been replaced and I was hoping for a nice smooth idle.

That stumble at 41 & 47 seconds is electrical.
But didn't do it again for the rest of it.
Can only suggest running it in total darkness with mag cover off & look for random sparks while running it at 'that' same revs if that's the only time it plays up.
I'm surprised it didn't do it when you back it off to idle. Is the plug gap excessively wide, or the points gap.

Thanks Morley. I will verify proper plug and points gap today. And I will do the random spark check this evening when it will be good and dark in my garage. I think you are correct because just before I took the video I had a couple occurrences where it just shutdown as if I turned it off. It didn’t stumble and stall. It just abruptly shutoff. That is what I expected to capture on my video, but it hasn’t shutdown like that since the initial startup.

Stay Tuned for further developments……..