So with engine feed off, and the FFP handle down, what feeds the motor? What feeds the motor when the FFP shaft shears in flight? What feeds the motor when the A system hydraulic goes offline?
Okay so with access to the docs, open the 28GS and read the chapter on engine feed, clearly states "The primary method is by siphoning action". You don't need the FFP or the electric fuel pumps to supply fuel to the motor.
Curious how many years have you worked the Viper fuel systems?