Fuel flow issue

[Joker2]

Just got my new ( rebuilt ) 94' looper 150 have about 4 - 1/2 hrs on it running double oil not exceeding 4000 RPM ( as per mechanic ) I have a 74' v20 with aluminum saddle tanks (20 gal ea.) I ran all new fuel lines with a stainless fuel/water seperator, installed an in line fuel valve on each side ( ball valve style ) My problem is that I can't seem to get the fuel to be taken evenly from each tank. When I open both valves and run it only seems to take fuel from the starboard side tank which has a shorter line because the seperator is on that side. The port side line comes across the stern, into the seperator, then one line out of the seperator to engine. The engine runs fine if I shut the starboard valve and only run off the port. I don't know if it is because of this difference in length or what. Can anyone help ?