A fuel pressure regulator sits between a high-flow fuel pump and a sensitive carburettor float chamber, reducing the pump's output pressure to the carburettor's design specification. SU and Stromberg carburettors are designed to operate with petrol delivered at around 1.5 to 3 psi at the float chamber, and any higher and the floats cannot maintain the correct petrol level, with the result that the carburettor floods, runs rich, overflows from the carburettor overflow pipes, or fails to seal properly when the engine is shut off, producing a strong fuel smell, sooty plugs, and a fire hazard. Performance fuel pumps typically deliver 4 to 7 psi at the outlet, so a regulator is essential to bring this back to the carburettor's design specification.
Regulator Options
Two established regulator patterns are stocked. The Filter King combines adjustable pressure regulation with fine fuel filtration in a single body, fuel passing through a replaceable filter element housed in a visible bowl, available with either a glass bowl that allows visual inspection of fuel colour and any sediment collecting in the chamber, or a metal bowl paired with a pressure gauge showing the regulated output directly, both adjustable through the full 1.5 to 5 psi range with replacement filter elements available as a service item. The Petrol King is a regulator-only unit without an integrated filter, using an internal diaphragm to sense and maintain the output pressure and deliver a consistent supply regardless of fluctuations in the inlet pressure from the pump, adjustable across the same 1.5 to 5 psi range and supplied with a mounting bracket, the cleaner, smaller installation where a separate fuel filter is already part of the system.
Setting & Mounting
The regulator is fitted in the pressure side of the fuel line, downstream of the pump and upstream of the carburettors, in a location accessible enough for adjustment and gauge connection, orientated so the inlet faces the pump and the outlet faces the carburettors. The standard setting for SU and Stromberg carburettors is typically 2 to 3 psi, the adjustment made with the engine running and a fuel pressure gauge connected, with the regulator set to deliver the appropriate pressure and the setting checked periodically as part of normal service work, as the regulator occasionally needs a small re-set to compensate for spring relaxation over time. Setting the pressure too high causes float chamber overflow and rich running, while setting it too low causes fuel starvation under high demand, typically a hesitation or flat spot during hard acceleration, so the correct setting is a balance between these extremes and should be verified at both idle and under load. Some regulators use a return line, returning petrol that exceeds demand to the tank through a separate hose, which suits fuel-injection conversions and certain bypass-style regulators.
Application Notes
Fuel regulators are needed alongside any performance fuel pump installation, the exception being for owners running standard-pressure pumps, whether the original SU pump or a direct replacement of the original specification, where the pump's output pressure is already within the carburettor's design range. A regulator may also be considered when the original float valves are worn to the point that they no longer reliably hold fuel level under standard pump pressure, in which case a regulator set slightly below standard allows the valves to continue in service while float-chamber repairs are scheduled. The regulator acts on fuel pressure rather than carburettor type, so a single regulator serves a twin-carburettor installation through the common supply line, and separate per-carburettor regulation is not required.