From what I have observed, overflow fuel vents from the bowl to the top-front of the carb at the intake and on into the intake when it overflows with some of these CV carbs. When the bike is on the jiffy stand, it is leaned over and fuel can, and will, flow directly into one of the combustion chambers if the intake valve just happens to be in the open cycle when the engine stops. Nothing to stop the fuel left in the fuel line and bowl from migrating into the cylinder, not even turning off the petcock. BTDT.

I do not know why he has fuel leaking around the manifold unless he has bad gaskets already or other issues he is unaware of...if he had gaskets that bad, it seems that the vacuum leak would cause the engine to run like crap.

Now, if the petcock is vacuum opened and there is a bad fuel line or leaky petcock, then it is very possible that fuel would leak only when the engine was running if it is the petcock, and until the fuel level was below the damage if it is a damaged fuel line.

Of course, a damaged accelerator pump diaphragm can weep fuel which will drip off of the bottom of the carb...BTDT also.