Welcome to the site !!
Just because it is a "new" carb , does not mean it might not need the float adjusted or checked , sometimes carbs sit for a long time and can stick or even rot the rubber parts .... What I would do , is give the new carb a thorough going over ...... also check the vent line to the tank/cap , it may be clogged and that is causing a vacuum draw which would keep the gas from flowing out thru the valve/line ... if you remove the cap and it sucks or makes a noise like a jar of jam when you first open it , that would also indicate it not venting correctly
That is where I would start ....