I´m pretty sure the line your talking about is a vacuum line that goes from the petcock to the vacuum port on the cylinder head at the intake to cylinder #1. What should normally be happening is the petcock won´t open, even if set to open, unless that line is maintaining a vacuum. If the petcock was leaking and you were getting gas down to the carbs, having the vacuum port on the intake open would probably give you a high idle and throw off your mixture.