...where's it going to go if it can't vent properly and it's under pressure?

 

The tank should be vented through the cap or caps, the petcock has nothing to do with venting the tank. If the tank were pressurizing and pushing fuel past the needle and seat that would mean the tank was sealed and that during the time when the motor was running it would also start to draw a vacuum in the tank as fuel was removed and would soon starve the engine to the point that it would no longer run. It still looks like its falling back to the needle and seat or like was said above the tanks are not venting between the two.

 

what rebuild kit did you use to seal the sucker back up? it may be a bad needle valve all together.. let me know on brand i may be able to help you!

 

Do you have the stock petcock or an aftermarket one like Pingel? If aftermarket, make sure you plug the little tube on the top back of the carb. Not sure if it has anything to do with your problem but just incase.

 

Did you replace the seat too, or just the needle? They should be replaced as a set. If you replaced the seat, maybe it's not tightened all the way and leaking past the threads?

It almost has to be either the float level or or leaking needle valve. It would take a lot of pressure to force fuel past a properly seated needle valve.