If fuel is flowing, there is no problem. If all carbs are sealed no problem there either.

As mentioned, you may have other than factory set-up.

Perhaps you have a vacuum activated petcock that someone modified to make the fuel flow.

I never had mine apart but would suspect that there is a spring pushing a diaphragm to close the flow off. The vacuum would work against the spring and open the fuel path. Maybe someone put the spring on the backside of the diaphragm to keep the path open. I'm just guessing......