24B-323 Tillotson Inlet Tension Spring

Inlet Tension or Pop-Off Springs is one component that controls the rate that fuel flows to the high and low side fuel circuits of a diaphragm carburetor especially at low-speed operation. 

Pop-off pressure, the way we check and measure pop-off, should not be confused with fuel pressure. It can't not and does not control fuel pressure. The correct fuel pump will not make much more than 1psi of fuel pressure to the carburetor. Therefore, 10psi of pop-off pressure does not mean 10psi of fuel pressure nor does it mean the fuel pressure must overcome 10psi to open the needle valve. 

Pop-off pressure is based on the type of inlet tension spring and its install height.