Pumped engine fuel installation
 

Recently I was helping my friend to set up and tune his Thunder Tiger Pro 1.20 . This is a motor with integrated pump and if tuned on the bench or if the plane flies upright, the motor runs nicely (with tuned pipe). In negative G maneuvres, though, the motor quits - every single time.

Now, I suspect the problem is not in the motor itself, but rather in the fuel installation (when running, the motor does not know if it is inverted - or am I wrong?). The tank is moreless in the level of the carb. The only really non-symmetrical thing is the vent line - it goes to the top of the tank, the line exits under the plane.

Any thoughts?

Thanks,

David