if you are hearing a hissing at the mixer when you turn the key on even while the engine is not running this should not be happening. the diaphragm in the mixer is what regulates the flow of vapor into the engine but only when it is running, it opens and closes using the engine vacum from the engine via a vacum hose coming from the intake manifold and should only open the mixer under vacum while the engine is running. If you are getting a hissing sound just by turning on the system i would make sure the diaphragm is not damaged. Pull the cover off the mixer and remove that diaphragm and check it for any holes, cracks and pay close attention to the center button on the diaphragm itself and also inside the mixer where it seats, this is what actually seals the vapor. A dirty or bad seat on the diaphragm button can allow vapor to pass when it is actually supposed to be closed and can make the engine run badly or not at all.