I searched with no luck.
My DT175C has always had idle issues and quite often it will stall if you don’t keep it running. My throttle screw can be threaded in fully with no effect.  My throttle screw has an o-ring on it. My manual does now show an o-ring, and the Yamaha Parts diagrams do not show one. The only parts diagram I found that shows an o-ring on a throttle screw is the DT250B. So I am wondering if I have an incorrect throttle screw in my DT175C carb. And maybe that is why it can be fully screwed in with no effect. Just looking for suggestions. 

imgur.com/a/nRJnFYz

Edit:
Just compared the part numbers between 175B and 175C and they are identical. So I will go pull the one out of my B and see how it compares.

The Oring on the separate screw to the right of the picture which I guess is off a different carb would make no difference to running with or without it as that's the idle speed screw not mixture screw. Regardless--an Oring on the mixture air screw wouldn't make any difference anyway. Sorry, on a borrowed phone so might not be seeing it properly. OK, on reading again I see you say screwing right in makes no difference so I guess you mean the small angled mixture screw, but if it has no affect then that points to an idle mixture jet blockage possibly. Maybe I'm misunderstanding your post. Let's see what others say.

Here are a couple photos that show the witness marks of the throttle screw making contact on the ramp portion of the slide.

imgur.com/a/q4931Px

Ok, if i'm on the right planet, were definitely talking idle speed screw not mixture screw. So if it's lifting the slide by screw but still wont idle it now has me thinking its overly rich & drowning out. Not easy without a video but is my guess for now.
Not choke accidentaly left open?

Of course i could be way wrong & could be lean. Does flicking the choke on then off make an instant difference for a couple of seconds of running.

It actually runs really good throughout, but it just doesn’t want to idle. I’ve adjusted the throttle cable to ensure that the slide bottoms out, and then I eliminated any excess play at the grip. Then I adjusted the throttle speed screw until it touches the slide ramp, which leaves me about 2 full turns before the screw stops because it is run out of threads.

I’m pretty sure it has the wrong throttle speed screw in it. I’ll try the one out of my 175B tomorrow and report back.

In my opinion, two full turns after touching should have it reving way beyond idle--just saying.
If you want to test another way & have enough cable adjustment just wind the adjuster out at the carb or might have one at the throttle control too.
Just dont overdo it & don't turn the steering or it'll change.
Hve the kill switch/ignition switch working.

The throttle speed screws are the same on my 175B and C, except someone must’ve added an o-ring to the screw on my C. So I guess I need to mess with it a little more to see if I can set the idle speed. The bike runs good, but the idle is way too low and the speed screw wasn’t doing its job. I’ll give you an update once I mess with it some more. Hopefully I can sort it out because it’s one of my nicer bikes!

imgur.com/a/OjfrCaR

Edit:
If you zoom in real close on the throttle grip you will see that the cable adjustment is fully screwed onto the threaded tube. None of my other bikes are like that. They all have some exposed threads. So I think my problem lies within the two cable adjustments.

Sorry yeah didn't mean to set the idle by cable only. Just as a test to lift it beyond the screw if screw doesn't lift it far enough to see if it'll idle.
Gotta have cable clearance when the screw is sorted though.
Maybe ignore my earlier post.
So starting again, do you think someone's fitted an Oring where its not wanted in that groove. & is the Oring coming up against the end of the hole stopping turning in any more to create an idle.
Best remove the Oring & try again, see if it'll go right in further.

Not sure if this will help or confuse you more, I mainly deal with the 1975-76 DT250-400’s most recently 1975-76 DT250’s, I  have not had the issues you are having, although I will say the ramp that the screw rides on of your slide is definitely scarred up, looks like the idle screw is digging into it instead of lifting or lowering it, maybe the slide is interchangeable from your 1975 and 1976 to see if that helps any or clean that ramp up with some sandpaper, I do know on some parts manuals the 1976 DT250C shows an oring on the idle screw while the 1975 DT250B does not show the oring on the idle screw, I have rebuilt probably a dozen 1975 & 1976 DT250 carbs and have found both years with & without the oring when the should have had or not had an oring, I usually always install the idle screw with the oring, possibly it was a 1976 forward Yamaha update?

I am going to try and dress out those indentations on the slide ramp this evening. There are two definite marks from exactly what you describe. The screw dimpled the ramp instead of raising the slide. I always twist the throttle to slightly raise the slide before I turn the idle screw. Then I let it settle onto the screw. But the PO may not have used that technique. Probably won’t test drive anything for a few days though. Rain rain rain.