Fuel pressure on Carb motors

[Bobcat]

Let`s go back to basic theory . A fixed displacement pump ( fuel , hydraulic or whatever ) delivers volume , or flow , in gpm , lpm or whatever . A restriction in the circuit creates the pressure , or more precisely , pressure drop across the restriction . In this case , the restriction/orifice is/are the openings in the float needle valve . The pump will deliver a flow of so many gpm across the orifice at a certain pressure drop directly related to the pump flow . Assuming the pump flow is constant and you could vary the orifice , you would see the pressure drop increase or decrease as the orifice size was decreased or increased . The float provides in effect , a back pressure check that varies somewhat the orifice size ... although it is fairly crude .
Why would volume drop as pressure increases ? Pumps are not 100 % efficient and always have some internal leakage ... either through the internal valves ( diaphragm pump ) or past the gears or pistons . The needle vale also isn`t a 100% bubble tight seal when closed .
Also , most fuel pumps have an internal relief valve that limits the max pressure ... as do some piston pumps .


Bob