My carb came back looking good.
all put together.
it leaks gas on the bottom little screw. Like a sieve. See pics. What could this be.

I can't tell where the leak is from the picture...  little screw? 

What model and year is it?  Came back from where?

I recall that you had "a local guy" rebuild it.    yamaha-enduros.com/index.php/forums/gene...rb-rebuilds?start=10

Was the carb leaking prior to the rebuild?

Not to my knowledge. But it's been sitting for years. Cleaned by my son in shop class in 16. Sitting since. It's the tiny little screw.

Stolen picture
area leaking is circled  

That appears to be an overflow vent, usually having a piece of clear tubing draining it behind the engine. Usually a leak there means either the float level has been set too high, or the inlet valve from the gas line is leaking or sticking open.

And it's not a screw; it's an open tube.

What Jim said. 

Aha thank you. My eyes aren't what they used to be.
I bet the doggone petcock is leaking internally. Could it be

Parts are available to rebuild the petcock as well. But even if the petcock were leaking, the fuel shut off valve I referenced above should stop most, if not all of that, if the float level is set right.
I don't think your carb is right yet internally. Read the manual on setting the float level and see where it is. Then while you are in there, check the float valve, too. If those are correct, only then worry about the petcock.

Has nothing to do with the petcock. Adjust the flot level on correct hight, but towards the upper limit of the tolerance. Clean or replace the needle valve with origin Mikuni parts an it should work.