It sounds like the float needle valve is leaking.. Fuel continues to enter the bowl although the float rises and attempts to shut it off.. the level gets way too high and overflows.
I'm not familiar with the PC50 carb but generally the float needle has a plastic Viton tip .. if the tip is damaged (look close with a magnifying glass) a new valve is needed. Or the valve's seat could be damaged and can usually be smoothed out or made round again with the proper little tool.
Or some bit of dirt (rust and crap from the missing fuel filter or screen?) is preventing the valve from making good contact with the valve seat and cleaning will fix it..
in any case, having fuel spilling out of the carb's vent is a bad idea .. could be dangerous like you say.. So i'd do whatever needs to be done to fix the leak and to be sure that any overflow is properly channeled down to the drain tube..