It is normal for the fuel pressure gauge to bounce at low rpms if you are using a vapor return line. If you do not have a vapor return line a pressure regulator me be required. If the carb your using requires a pressure lower than the pressure supplied by the pump, a non-return regulator such as RobbMc PN 1046 or 1050 can be used between the pump and the carb(s). Also, using a vapor return line with the pump will reduce idle pressure about 1 psi which may eliminate the need for a pressure regulator.