Probly have one of those stinking rubber/plastic tipped float needles that didn't get packed out right when they were run. I toss the stupid things if they don't seal properly and put in a solid brass or steel needle that I have saved from other rebuilds.
I have heard that you can spin them in the seat to get them to seal. I have not tried it but if you didn't save an old solid metal needle then it would be worth a try. I guess!
I rebuilt a carb for my Oliver 70 last fall. I drove it into the shop to repair the seat Monday and walked away from it for an hour and it looked like a gallon of gas had run out on th floor when I came back. That rubber tipped piece of crap will hit the trash can as soon as I get a chance to get to it.
If you get a good one they are fine but I sure don't like the odds at this point.