The orifice the needle goes up and down always has fuel in it...  the float system keeps the fuel from rising to that level.

When the engine is running, the "venturi effect" causes the air pressure in the carb throat where the orifice is located to drop...  float bowl is vented to outside air so the pressure differential now allows fuel to be drawn up into the carb throat.  The jets regulate the flow.

It's kind of self-regulating...  more air flow through the carb lowers the pressure in the carb throat more which allows more fuel to be drawn up and into the engine.  But when the float system stops regulating the level of fuel in the bowl?  Then it just overflows. 

The standpipe tube AlNarv pointed out is the "safety".  If level in the bowl gets too high, it should escape via that standpipe tube to help prevent the engine from filling with fuel like you're experiencing. 

If no fuel is coming out the overflow fitting that when you see fuel overflowing from the slide needle orifice,  the overflow tube is plugged up...  possibly purposely by a previous owner... 

I've seen that dozens of times...  the float system is messed up, fuel comes out overflow like it should... and someone decides the "fix" is to block off the overflow pipe which makes things worse.

Somehow either the float system isn't working or there is a hole in the carb body that lets fuel in someplace without going through the float valve.

fuel does come out of the overflow, I just think the level is so high and flow so steady that the little overflow pipe can't empty it fast enough

OMG! 
This is a good video IF you can tolerate the awful background music. What were they thinking? 

Absolutely on the overflow pipe...  it won't pass enough fuel if the leak is major. 

Schu, do you know the name of that song?  I'd like to add it to my playlist. 



Many people find things like carbs and airplanes and electricity kind of "magical".  For good reason...  it turns out much of what I "thought" I knew turns out to be questionable... 

For example the same airflow principle that makes a carb work is allegedly the same thing that gives an airplane wing lift and allows it to fly...  Top of wing is curved so air has to flow faster over top of wing than bottom of wing.  Faster air flow = less pressure so air flowing over wing creates lift... Sounds great and while "true", it's not a complete explanation.  There are airplane wings made with top and bottom surfaces parallel.  They fly just fine.  (So does a paper airplane).  Airplanes can fly upside down...  even the ones with the curved top wing surface.  It sounds simple like "angle of attack" or something probably explains the anomalies away...  but there is still more that can't be fully explained and probably crosses into quantum physics. 

Sit the carb upsidedown somehow with no float but needle in place & with fuel supply hung above just see howmuch finger pressure is required to stop the flow. Just use water if you want. Could sit float in place i guess & see how much pressure is needed to stop it. Be almost zero pressure i'm thinking but not tried that for a long time. Then if that's ok then fill the bowl & look, although i think you said its coming from the high vent so maybe not bowl related. I'll go back & read.

I like how to simply start eliminating a problem RT! Makes way too much sense.
Thank you

I broke down and bought a new Mikuni from a place called Racing Planet. They sell scooter parts, not motorcycle parts. The 2 bolt flange mount that attaches to the reed valve manifold is kind of an oddity, and I am not sure if Mikuni even still makes them, but this place had one that fit and worked perfectly. It is not identical to the OEM carb, but it works and I was able to get the bike back up and running. Their description of the carb says the main jet can be changed without removing the carb, but there is no main jet plug in the bowl like the stock carb.
I'm still baffled as to why the carb I built could not stop flooding (literal flood). I looked at the carb body with a magnifying glass and couldn't see any defects that would make it do that.
I want to still send my trouble carb to Mark, and see what he can find, and hopefully have a backup carb for the future

do some pix newb