If air vents are stopped up the fuel will not easily siphon from the fuel bowl. The air comes in and displaces the fuel leaving. Bowl vents are important.
Air bleeds if stopped up will cause it to run excessively rich because you will not get the air needed to emulsify the mixture and give a proper air/fuel balance. That will not cause it to pull fuel from the fuel bowls upwards and out while sitting still or going down the track. It will cause an excessive rich mixture that defies tuning.
You could have an internal vacuum leak somewhere. For fuel to get up to the bossters.
Ed