Can i also say--that small Oring on the jet setter thing where it compresses into the bowl looks way too fat. Is it the correct one?, genuine. Might be just a trick pic. Was only thinking if 'that' is 'way' fat then it won't compress fully & let the bowl come right up to the gasket. I'd guess any slight gap would be obvious though, & bowl would rock without screws as won't be up to the gasket sitting flat.

I wondered about that O ring.  It's fatter than the original but the original was pretty wiped out (the fuel filled the bowl and came out the overflow on the bottom) and was the main reason for the teardown.  That O ring is not the original part number but a substitute number on parts lists for Partzilla and Stadium Yamaha - same substitute part number on both lists, and appears to be a little bigger than the original.  Unfortunately it looks like the original part number isn't available, so I have to see if I can get that one to work.

Each time you put the carb back together, put just a touch on grease on the O-ring & the seat in the float bowl where it fits to help it seat without damaging itself.

SUCCCESS!  The little beast is running happily again.  It turns out I didn't get the slide in the carb properly so the needle wasn't down at all (duh!).  Once I fixed that, It started right up and idles great.  I still have a little weeping at the float bowl gasket, but I will deal with that turning the gas off  and running it out of gas.  Still lots to do, but when it's running it makes everything easier. 

Good news! 

Leave the gasket for a while - the weeping might sort itself out.

Not sure if this applies at all, but it's worth mentioning, maybe only for someone else later on...
I have a 74 DT250A that when I got it, was weeping through the carb float bowl gasket, same thing. Turned out the PO (my older brother) had cut a gasket for it out of a piece of cereal box. Though the rest of the gasket was working nicely, the (I wanna say) right rear of the upper carb body had a small slot cut into it in the casting process about the size of a flat screwdriver blade. The cereal box was thin enough that it touched the top and bottom edges of the carb bodies but apparently the resultant squeeze was not hard enough to seal it. When I replaced it with a new gasket that was factory thickness, it stopped the leak.
Not sure why that slot is cast into the carb body; to pry apart the two body parts if they are stuck together? But the factory gasket thickness accounts for that slot. I'd have to look/see if other Mikuni carbs have that slot molded into the joint; as I say, this might apply, and it might not...

One very important consideration is when you put the slide in the carburetor you want to look inside at the front. Is the slide almost all the way to the bottom where it's just showing a small fingernail of air? I hope this helps. Then bolt the carburetor in.

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Yeah, I didn't check to see that the slide was fully seated down in the carb the first time; this time I put the cap on with the carb off the mounting screws so that I could verify the position of the slide and the throttle was working properly. Stupid mistake that made me take it apart and put it back together but now I know. The weeping was in the back of the carb where that slot you are talking about is, but is has stopped now. The old gasket looked to be about .05" thick, and the new one was around .03" or so; I had two new ones, so I put those together and they appear to be working, and no gas leaking from the bottom drain, so the O ring is sealing so far.

Good to hear!

I guess I need to add that it doesn't leak as long as I turn the gas off at the petcock - then it's bone dry. But I forgot to turn it off yesterday and the shed floor was pretty wet with fuel. It doesn't leak when running, just when I space out on the petcock.