"A 1968 Cessna Skyhawk, Continental O-300-D, in flight, doesn't feed from port wing tank, with fuel selector on Both. "
How long did you run the test for? My old 172 used to feed from one side for around 45 min then switch itself to the other. I decided never to use "both" so I could keep track of how much was in each tank. Engineering opinion (M3) at the time was that this was normal.
Rod1