Finn,

It sounds from your post below as if the connection leading into your fuel pump might be leaking.  If it was, it would produce bubbles on the high pressure side and would be worse in a climb with the tanks farther below the pump, creating a higher suction.

Chris Sargent

>Did you solve the problem of "bubbles" in the fuel line between the tank and the boost pump? 
>
Actually the air bubbles are in the line between the pump and the carbs (after the pump)!