[Trackrash]

Yeah, something about the needle valve.

I would use a piece of welding rod to poke down the vent tube and wiggle the float when it is idling to see if you can get it to seat with the motor idling.

With the top of the carb off blow into the fuel inlet and see if the needle will seat when gently pushing on it.

It could be that there is a problem with the part of the float where the needle rests. It is bent out of shape or there is a burr there causing the needle to not seat.

Also, make sure the float is installed correctly, not upside down and that it can move freely throughout it's range and not hanging up.