I did not read your #11 post correctly about the plane having 15 previous flights...sorry about that.

The difference between holding the nose up while checking for proper needle adjustment AND the plane actually taking off and rotating it's nose up naturally is...a force created by the momentum of the forward traveling aircraft that wants to push the fuel to the back of the tank.

Check the fuel lines inside the tank, especially the main feed/pickup line near the brass tubing for a pinhole or slight nick or cut.