The reserve position alows fuel from the bottom of the tank to flow. It sounds like you have crud/water/blockage in that fuel path. Turn the fuel off. Remove the fuel line at the tank. Place a long fuel line on the petcock to drain the fuel in a can. Place the fuel on the reserve position and watch how much fuel flow you have. Now blow back through the line while on reserve to clear the crud and again look at flow. This will let you know if you neeg to remove petcock and clean filter screen
inside tank.
Good Luck