The vent valve should be closed when the engine is running (key on actually), and open with the key off. More than likely the vent valve is bad, as the little rubber "hat" inside that forms the valve crumbles.
If the vent is open while the engine is running, this can affect your mixture. It is the difference in pressure between the area over the fuel bowl and the pressure drop in the venturi that make the fuel move. If the pressure over the bowl were to increase, then the mixture gets richer. The bowl should be vented to the inside of the aircleaner - venting it to the outside can mess it up.
If the valve is bad, then you're better off just blocking the vent at the carb. The only downside is that under really hot start conditions it may be hard to start due to build up of fuel vapors, but probably not a big deal.