The only other place it has to vent on that airhorn is past the gasket where the secondary metering rods go thru it. There isn't a vent in the airhorn behind the choke flap and POE wells vent internally back to the float bowl.
I've never seen one that would stall the engine instantly when you covered the large vent in the front but I've never really tried that move very often.
In any case it wouldn't be "vacuum" in the bowl it should be at standard pressure and pressure would only build up if you blocked off all the venting so the incoming fuel would raise the pressure some or quit flowing into the bowl as venting is required for fuel to be able to come in under low pressure.
Anyhow, "rough" idle can be caused by many things.
Big cam with tight LSA is one. Not enough compression for the cam being used is another.
Not enough initial timing.
Not enough fuel available from the carburetors idle system.
Vacuum leaks like sucked in intake gasket.
Bad spark plug(s), plug wires, broken valve spring(s), worn cam lobe(s), etc, etc, etc.
I would try several things here.
First try advancing the timing. I'd also gently "tip-in" the choke flap to see if it smooths out some.
Eliminate any vacuum leaks.
If it's missing on one or more cylinders try to figure out why?
Compression test, pull one plug wire off the cap at a time and see which ones are firing well and which ones aren't...etc....etc.....
Also no mention of engine specs, what is the engine and what has been done to it?
More information is better than less..........Cliff
