On any new carb I recommend removing the bowls, inspecting the needle and seats, and double check the float level.  The reason for this is the float level can be knocked out of adjustment due to bouncing around in shipping.

A lot of times that "problem" can be solved by whacking it with a screwdriver handle.

That tube is a vent for the floatbowls, you cannot plug it without the carb no longer functioning. What you can do though, is take a piece of 1/4 inch hose, put each end to the float bowl tube vents. Make sure you then cut a hole in the middle on the bottom so the vent tubes are still vented to the air, but any excess fuel bouncing out (or from a stuck float) will then go into the the carb rather than onto the outside of the carb and manifold.

If you aren't worried about that, then you can shorten the tubes, and many times you want it at an angle so the air filter / spark arrester doesn't block the tube.