Flooding carb while running?

I have a 1985 Ford Bronco II with a Motorcraft 2bbl carburetor on it. We just rebuilt it and it starts fine but when it idles for about 5 to 10 seconds, fuel starts to pour out of the vent holes on top and flows through what looks like a vacuum line in the front of the carburetor. The float level is correct. Does anyone know what might be wrong with it?