I'm certain this has been asked before, but I'm asking again. I have rebuilt an NH. I put it on the stand for testing overnight and there are NO LEAKS! I put it on the T and drive it and there are NO LEAKS. I let it sit overnight and the next morning there is evidence of dripping from the throat. How do I stop those drips?
I wouldn't leave it overnight with the tank shut off open - too much of a risk with a gravity fuel system.
Dripping may indicate the float needle doesn't seal perfectly to the seat. If it's a repro needle it may need some finishing and a smack with a hammer into the seat to seal properly.
My Kingston L4 seals pretty well, but I have a shutoff valve in place of the OEM fuel inlet elbow and I close it when I'm done driving the car for the day. Then I drain the carb bowl and pour the bowl gas back into the gas tank.
Belt and suspenders, but I don't like gas dripping in the garage!
If the gasket under the seat leaks sometimes it takes a while before a drip shows up. A lot of times those gaskets are hard and need to be soaked in gas to soften them up. Since it's soaked in gas now from being on the car you might try tightening it a little more. Sometimes even new floats will have a tiny leak. Sometimes you might not catch a leak when testing it if it's really small and after sitting all night it could fill with a little gas. Or there might just be a tiny fiber from the felt packing got in the needle/ seat and you could drive around the block and it'll pass through. Still always a good idea to shut off the gas if parked overnight.