Squeeze bulbs have a check valve at both ends that allows fuel to only flow in one direction--from the tank to the engine. When the outboard is running normally, the engine’s fuel pumps create a slight vacuum in the fuel hose. Atmospheric pressure in the tank through the vent pushes the fuel from the tank to the engine. In this case, the check valves are both slightly open and allowing fuel flow at the rate that the engine is using it. When the day’s boating is over, most boaters disconnect the hose at the tank, and allow the engine to run itself out of fuel. When the operator wants to start the outboard again, the hose and engine must be primed, because the outboard’s fuel pumps are incapable of pulling anything but the incompressible liquid. Therefore, the bulb must be squeezed repeatedly until the entire fuel hose is full of liquid fuel again.