once I was just helping a mechanic out in the shop, he kept taking the bike out for a test ride and coming back as it would misfire / stall but all the static tests kept showing positive. We sat down and had a chat with a good old cuppa, which is the part I always enjoyed. Then suddenly he saw the problem - 'is this a fuse I see before me?'. The glass fuse would read positive in a continuity test, but under vibration it would read negative as one end of the fuse had worked loose and would only cause trouble when the bike was moving / vibrating. All that to say I wouldn't be half surprised if your fault was electrical.

That's a very interesting fault [on a battery ignition bike] & probably something you'd never come across again. Bikes are funny like that--lots of one off faults. Good one to remember though.

If you buy a different Mikuni, a modern "28" will have a larger body and won't fit.  The "small body" 28's were mostly OEM as far as I'm aware... they certainly are not available from Mikuni new anymore.  To match up, you need to check the OD of both ends and the length. 

Pilot jet is tiny on the 2N5...  going to at least a 25 is helpful. An obstructed starter jet in the float bowl makes that model nearly impossible to start and run cold.  All it takes is a spec of crud getting into the starter jet or the pilot jet.

Now that I know it’s a 79’ thanks to member Gr8uncleal I was able to confirm the OEM carb I have for the bike is correct. There’s doesn’t seem to be a manual for a DT175 for the 79’ year on here.

The main manual is the 1978. There was a very small supplement manual for the 1979 US bikes that you use with the 1978 manual The 79 supplement is not in Tech library here. What did you need to know? Carb specs might be in the online parts diagrams, if not, I'm sure I have them.

The exploded parts list I found for this carb does not show an oring on the idle/throttle adjust screw but there is definitely a spot for it on the shaft. Wonder if this is a miss on the diagram??
I did order a new O-ring for the main jet holder and this oring fits nicely. Wondering if anyone knows or can check if they have this print on theirs?? I still need to order the Oring for the air screw as I melted it with carb cleaner. Would be nice to order this other if it is indeed needed.

www.yamahapartshouse.com/oemparts/a/yam/...9bc78606f/carburetor

The choke pickup in the bowl has a small hole at the bottom. It does not look like this connects to the tube where the choke pickup goes into. I can’t say for sure but looks like fuel will not be able to get into the tube so won’t work? Or maybe it just flows over the top.  I can’t figure out the purpose of the small hole at the bottom of the tube other than allowing fuel to flow in.

I hope you can figure out how to post pictures because it seems you might have a "different" perspective on a few things you seem to be trying to describe...

The 79 had a "anti tamper" cap over the idle air screw...  I don't recall any of my 175's having the air screw made of anything but brass?  (It's never a good idea to put a carb into carb cleaner without completely disassembling...  I learned that the hard way..  plus crud will hide and the carb won't be clean)

O-ring on the idle speed (slide stop) screw won't hurt anything either way.

Yes, there is a well in the float bowl with the starter jet at the bottom restricting flow into the well.  The brass tube up to the starter plunger that opens or closes the fuel passage (aka "choke") simply draws fuel from the well.  Nothing "connects".  The small jet in the bottom of the bowl must be clean. 

Ignore my woffling on, i'll read & catch up later.
Different carbs for different countries but i think i read where you mentiond there's nothing to stop fuel spilling down the choke pickup cavity & without going looking i think your right so.
I think--again without looking-- that there's a small hole in the tube end that must be equal to the fixed jet so why have the jet.
It works, & if the jet gets blocked, it doesn't work, so they must have a plan.
Oring up above the main jet holder i've never seen but i'm still a century behind with my carbs but what does 'that' seal, or is it just a safety thing just incase it can draw fuel & bypas the main jet.
Is there a step to trap the Oring on the screw in piece. How did ya melt a air mixture screw if 'i understood that correctly' what were you soaking it in. Or is something plastic that disolved. Hope you get it sorted.
You obviously understand its workings.
Look forward to pics when you can.
ps, not normaly an Oring on an air screw 'i think', but if it's a fuel screw then different story.
Once i had a suzuki 185 carb that had a fuel screw plus an air screw at oposite ends of the carb.
Fuel screws are alway forward of the slide & air screws at the back. Interesting things carburettors lol.
Got mine running with No carb--who needs them!!.
Revved that high i ran away & hid. I was about 11 years old with a 1940's 125 excelsior & poured some terpentine i think it was--straight down the inlet as i had the carb off overnight. Couple of kicks & she revved its nuts off. Shoulda put my thumb over the manifold.

O ring on the mixture screw is a thing. Mine was hard and chewed up so I just deleted it and greased the threads instead.

With or without o ring made no difference.