The bulb is a one way pump. It sucks when it goes from relaxed to full when the outlet valve is closed, pulling fuel from the tank via partial vacuum it creates. On compression, the input valve closes and the outlet valve opens allowing fuel to flow to the engine. The fuel flows through internal tubing and by the valving in the fuel pump and continues on to the carb inlet needle valve which is open because the float is at the bottom of the carb and forces fuel into the carb bowl.
As the bowl fills, the float starts it's upward journey and in the process the arm on the float pushes against the inlet needle valve forcing it to close. When the float is at the full up position which is approximately parallel to the mating flange of the carb and bowl, measured with the carb inverted, there is enough pressure on the needle valve to hold it shut against the pressure of the bulb. When this occurs the bulb fills the line from the carb back to it and it gets firm because flow has no where to go.
Under normal operation, fuel obviously flows through the bulb due to the partial vacuum caused by the fuel pump diaphragm movement caused by engine crankcase pressure pulses. In this situation the bulb will have a soft feeling as there is flow into the carbs and out into the venturi.
If you are underway and the fuel pump is not developing adequate partial vacuum for the amount of fuel required to operate the engine the bulb will move toward the collapsed position which can be corrected by hand manipulation of the bulb (like when priming) which will cause increased fuel to flow into the carbs and on to the engine resulting in increased rpms.
This help?
