Before replacing the pump, while not try removing the line from the tank at the pump; and running a line to the pump from a different source (like a portable marine gas tank).
You may have a stopage at the supply tube in the tank.
You may have multiple issues. A backfire at the carburetor occurs when there is fire in the cylinder and an intake valve open. Possible causes:
(1) burned or sticky intake valve
(2) incorrect valve timing
(3) incorrect ignition timing
A compression test will confirm/eliminate burned or sticky valve.
Jon.
