[Gaz64]

If your gauge maxs out at 15psi, then your pump internal bypass is jammed shut (pump is internally regulated at 14psi) AND the new regulator has some debris not allowing the ball to seat.
The 12-803 adjustment range (dictated by its' spring) is 4-9psi.
You should not be able to see above 9psi at the carb with this regulator.
Did you flush the lines after fitting the regulator?