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You don't need a solenoid for the fuel pump, it turns itself off when it senses the pressure increase from the closed carb float valve. Fuel flows into the engine because air pressure across the carb venturi is lower than that in the float chamber, to stop the fuel you just need to equalise the air pressure. SU did this themselves by connecting the float chamber vents to manifold vacuum and atmosphere. By closing off the atmospheric vent with a solenoid valve full manifold vacuum was communicated to the float chamber. Pulsing the valve stopped fuel being drawn up the vent pipes
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