More Pictures
One thing I would verify on your old carburetor are the floats themselves in good shape, maybe fill a small container with fuel and verify that your floats actually “float” and stay floating over a period of a few hours, I have seen floats get porous and randomly float and then sink.

I appreciate those pictures, Lizeec... and there's certainly no need to apologize. It's my fault for just assuming that my '74 would be identical. Live and learn!
To those asking about the brass parts in my carb. The main jet is a genuine Mikuni with the square stamped in it:


But EVERY other brass part is aftermarket! Are they really that sketchy?
I have another long post (now several years old) on this forum when we were troubleshooting what turned out to be the wrong stator on my bike. But during that whole process of trying to pinpoint the issue, ruling out carb problems was one of the steps. Thus, I got "all new parts" for my carb (likely when I didn't need them). Worse yet, I tossed out my old OEM parts when I did that. I think I threw the baby out with the bath water.
Here's the aftermarket float needle:


As I look at my DT400 carb, the float valve looks different. There is a retainer clip that holds the needle in place if the floats drop all the way down. My aftermarket brass float needle assembly doesn't have that. Here are the two side-by-side:


Is there a reputable source for OEM float needle assemblies for these old carbs? Perhaps this is the source of my problem.

Finally, I noticed that my DT400 carb has these rubber caps that sit on top of the slider posts that the floats go up and down on. My 250 doesn't have those. It has a machined ridge where it looks like they should be, but they aren't there. Are those important? Perhaps they let the floats go too high in the bowl and bind/stick?

Yes, the float valve could be the problem. Most of the time the aftermarket valves are "okay" but not always.

Your DT400 valve might be the same and work... 250 might have had a smaller size of passageway but threads and stuff should be the same..

Here's a recent post that links to a catalog of genuine float valves. yamaha-enduros.com/index.php/forums/74-7...oat-needle-set#92579

Caps are not needed on float pins. New carbs with that design no longer come with the caps and I've run many without them with no issues other than the floats can fall out when the bowl is removed.

If I remember correctly on another project awhile back where I rebuilt the original carburetor on a 1975 DT250 it did not have the retaining clip on the needle valve, nor did it have the little plastic caps on the float guides even though it looks as if the guides are made to accept them, I checked partzilla and the parts diagram doesn’t show them for the 1975 DT250C, but they do show them for the 1975 DT400B, I was going to order them till I saw the price, I can’t remember off the top of my head but they were expensive for what they are.

I did the same as you and checked my 1975 DT400 and it did have the little plastic caps on the float guide pins. I don’t think they are necessary as the float can only go so high. I read somewhere the plastic float guide pin caps are for ease of assembly if you have the carb upside down, for me it was just as easy to assemble the carb right side up. I’m sure you know this but the little tangs on the floats themselves go towards the bottom of the bowl, I’ve made that mistake before and it just kept overflowing from the get go. Did you verify your floats themselves are in good shape and float? For what it’s worth I’ve used aftermarket needles and seats with no issues, but I agree as everyone else says try to use genuine Mikuni parts when possible.