I have the standard fuel valves in my boat (shut offs for each tank then a union that allows either tank to feed both engines) but it seems the boat will draw more from starboard until it empties the tank. Occasionally when I've shut off starboard to try and control the behavior I get a fuel starve on that side and the engine runs dry.

Any ideas why it draws more from one side ? By the way I just replaced the check valves yesterday on starboard (there is a dip tube for the generator and one for the engine) and the check valve had been replaced with a straight barbed fitting. I realize now this is dangerous and it's resolved. However could a simple check valve missing allow this type of thing ?