Gig 601XL Builder writes:

I want you to do an easy experiment that will show you the problem with
a non gravity feed system that uses a "both" position.

Take a glass of water and two drinking straws. Put one of the straws in
the water and hold the other out side of the glass. Now put both straws
in your mouth and suck. Unless you are doing something to block the
straw that isn't in the water you will find it very hard to get a drink.
The same thing happens with a non gravity feed fuel system in the "both"
mode.

Add a fuel pump.