rx7locost wrote:
Carbie pumps do not use a return line.
Actually, some do. Lots of late-60s and up cars did; it prevented vapor-locking with the higher underhood temperatures from emission controls and standard AC. Both mechanical and electric pumps had returns from the engine to the tank.
My 1984 Mazda B2000 had a particularly evil variant; it had an electric pump near the fuel tank. After the truck got to where it wouldn't maintain speed at 55mph any more I started serious diagnostics. It was running way lean, but I had good fuel pressure.
In the mess of hoses there was a tee in the fuel line, routing back to the tank. Pretty normal stuff. That's where hooked up the fuel pressure gauge. But - jumping past days of WTF? - the cheap plastic tee was *also* a fuel pressure regulator; it had a ball and spring inside, and the spring had gotten weak over the years, until it was easier for fuel to go back to the tank than to the needle valve... I discovered that entirely by accident; it wasn't mentioned in either the Mazda or Haynes service manuals.