Originally posted by CafeenMan
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).
What if there are no bubbles while the engine is running (i.e. there is a steady stream of fuel while the motor is running?)