Its called a "pump sizer". It is used if the pump is too big for the system. It allows fuel to be sent back to the tank and regulate how much goes to the barrel valve. You don't see too many of these because its more of a bandaid fix for a system that was piece mealed together. The best thing you could do is to send your system out to someone like Koehler injection and have the system flowed for your motor. Makes a world of difference and is worth every penny(Costs around $450.00)