Yes, having the fuel run back from the carb is sometimes normal. If there are bubbles in the line after the engine has been running a few seconds, then there is a problem. if there's one bubble that stays put, then don't worry about it. If you can see bubbles moving up the line, then the fuel is foaming or there is a leak.

Make sure the tank is surrounded by foam rubber that is not compressed too tightly. Also make sure the propeller is balanced (spinner too).