While I do not have direct experience with this specific carb. The one glaring statement in your post is that the needle is not Mikuni. If it were mine i would start by locating and installing a NOS Needle Valve Seat Assembly. It appears that there are several on eBay which claim to be the correct ‘74 DT250A part. Correct part # should be 168-14190-25-00. In addition, even many ultrasonic cycles will not in most cases improve a gummy needle spring which can also cause an issue like this.

I totally agree with Gersh.
You need to solve this mystery with the inlet needle set first.
A few extra dollars for O.E. parts will make the bike & you happier.

EDIT:
To add to your search, I have attached a pic of the inlet needle set I am currently running in my wifes DT250A. It is the yamaha replacement for the original set 168-14190-25-00. I got this from Partzilla. It is working perfectly in her bike. Shown along side the original you can compare the two sets, for what it's worth. I will remind you it is Yamaha's replacement set NOT the aftermarket stuff you are dealing with. Also, have you reinstalled the old set to see if it makes a diff ?

As always, excellent ideas. And once again, Steve F, your pictures are SO helpful. Now that it appears an OEM/Yamaha part IS available, I have no problem getting that and ruling that out. However, my intuition tells me that SnglSmkr and MarkT are hitting the real clues.
If fuel flow is 100% shutting off PRECISELY when the float arms are parallel to the base of the carb, that's pretty good evidence that my non-OEM float needle is not the culprit.
The buoyancy of the floats... now THAT makes sense, and it is something I hadn't considered. It also 100% aligns with the trial and error testing I've done.
The brass float arm is parallel AND shuts off fuel at 17.3mm (which is spec). I've always thought this was the end-all, be-all proof that it should just work. But that logic is predicated by the ASSUMPTION that the brass pegs on the floats are also at precisely 17.3mm when the carb bowl is "full"! Since my floats would float, I thought they were good. But do they float HIGH enough? Or has the density of the material increased so that they are no longer as buoyant? If they're less buoyant, then Yamaha "spec" goes out the window.
I honestly think this makes the most sense. My floats don't float at high as they should (OR they're non-spec for this bike and perhaps the brass pegs are at a different level than they should be). But through trial and error, I have found the level that keeps the float bowl full without overflowing. I'm solidly in the camp of not messing with something that isn't broken right now.
Last night, my daughter wanted me to take her for a ride on it, so we went on probably a 30-minute ride. 2-up, going up decent hills and 50 mph roads. I'm telling you, it runs PERFECT. I was half expecting some type of issue to arise on the ride (such as fuel starvation going up a long steep hill) but it never happened.
Right now, I'm happy with it. I also want to say how thankful I am for this community. All of the various perspectives, experiences, and extra trouble of taking/posting pictures... that stuff takes time. All to hopefully help out a stranger. You guys are the best.

Speed and Sport in Bloomsberg PA has NOS floats for you carb if this is the way you are going. Be prepared though as they are $$$.There are others on eBay also.

Here's a pair on ebay pretty cheap... if the floats have become porous or something, they might get worse?

Anyway, for this price it might be worth having them on hand?

I had same issue, my floats & pin where stuck, cleaned everything got leak to stop but bike will only run with choke on. Where did you get your rebuild kit from or is that a new carburetor? Mine needs to be completely rebuilt or a new one would be better, my bike isn't in the best shape, but I gave up on it a year ago when it started leaking, now it's getting cooler out I think I'm going to start working on it again, I can't find much for it for parts on line. Someone painted my carb more reason why I would like a new one. Any help would be appreciated

Sounds like pilot jet is dirty. Likely carb just needs a good cleaning.

Yamaha painted carb from the factory.