A trick I showed my Brother to find a vac leak was to take a piece of hose (like garden hose) about 3 feet long.
Put one end up to your ear and sniff all around the intake with the other end.
You'll find it real quick.
Found the "hiss" to be coming from the "bowl vent valve" on top of the carb float bOwl. Is this normal? Is it supposed to vent out air constantly like that when hot? It does not affect idling quality that much now, just the annoying hiss...