When you took it apart and cleaned it, did you put a new carb kit into it? People talk lovingly about the older generation of cars, but the thing they skip over is all the wearable parts. As in parts that are fully expected to just wear out and stop working. There's tons of them too in old generation cars!

For your case a carb kit would replace all the gaskets, seals, and give you a new needle and seat. This became a kit because all these parts get old and brittle and wear out. Because the carb is a precise device that depends on the vacuum of the engine to operate, any tiny little gasket leak or poorly closing value causes all manor of performance problems.

I had a pinhole leak in the diaphragm of one of my two Mikuni carb, and my twin cyl bike would act really weird.

In that case, it wouldn't evenly provide power with throttle, but just as telling was when I got going and suddenly let the throttle go, the engine would die.

Point is, look at your rubber, old stuff gets destroyed by ethanol, mileage irrelevant.

Hows your anti-stall diaphragm (aka dashpot)? my buddy's old truck kept stalling coming to a stop, that was the issue with his.

I don't have experience with that carb, but two old vehicles: One was some slime goo had clogged one of the small carb holes. Looked like maybe a rubber gasket had reacted to something and liquified and been sucked into the hole. The other car having the issue you mention was actually a faulty EGR valve getting stuck.

I can't help with the problem since you've covered most of the things I would know about. Last carb I worked on was my beloved VW Beetle, and I understand how things can run fine and then suddenly just stop for no reason. But in the end, there IS a reason, somewhere.

I just commented to point out if it helps, a carburetor is just a mechanical computer. It changes the input of fuel and air based on other variables. So somewhere in the code something is now not calculating the same as it was, or the variables themselves are different. I know that doesn't answer the problem, but it's something I've always been in awe about since being told my my dad that a automatic transmission is just a fluid computer. Which is funny, since he could tear down one of those, but never could "get" how an electronic computer worked.