the bowl vent stays open at idle to feed it and closes with throttle opening, as fuel psi builds the vent closing keeps the fuel from possibly spilling out , let alone the psi is pushing and being drawn just fine rpm.at idle a siphoning effect
No. There is no siphon effect or liquid fuel involved with the external bowl vent. If there's liquid fuel coming out of it, there's something the matter with the carburetor -- stuck inlet needle, stuck or sunk float, improper float adjustment. The fuel PSI also does not build with increasing engine RPM; it is a function of the fuel pump mainspring, not engine speed.
It's there to prevent flooding under heat-soak conditions after the engine is shut off. Once there's no longer air flowing through and past the carburetor because the engine's been shut off, its heat rises (naturally), which heats up the fuel in the carburetor. If not for the external bowl vent, fuel vapours would enter the intake tract via the internal bowl vent and other bowl-to-throat passages. This would make the engine hard to restart. So an external vent is provided to allow the vapours to escape -- to atmosphere through '70 ('69 in California); to the crankcase in '71 (and '70 in California); to the charcoal canister starting in '72. If the bowl vent hangs open above idle, you have atmospheric pressure rather than carb throat pressure levels above the fuel in the bowl, which can in some cases upset the carb's metering and cause poor fuel economy.