I'm only running the fuselage tank which means that the all of the circuit to the pumps is gravity feed from about 14" above. The tank outlet is in the rear, which means it's the lowest point in the tank also in a climb.
Circuit is: tank, fuel selector valve, fuel flow sensor, fuel filter, fuel strainer, T'd to two pumps.
Fuel flow sensor is FloScan and if I remember correctly the inlet hole is only about 1/8" in that unit. Fuel filter is a cheap G3 auto paper filter.

One plausible theory is that in level flight the vent is pointed straight into the wind, presurizing the tank and pump feed circuit. In climb attitude the vent may be obscured by the lower part of the engine cowling.

Finn

Chris Sargent wrote:

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)!