ZG222 already has a shutoff valve.
Behind R.R. tire buried back there. It also has a drain function built the valve so you can drain the bowl.
On the 222 and 227 both (same engine) the carb sits down in the engine valley. When you turn the engine off, it gets hotter. The heat radiated from the cylinder heads has been known to cause fuel boiling. Could potentially lead to fueling of the crankcase. It'd be odd. Typically the boiling issue results in no or hard starting when hot, have to let it cool in order for it to start. This condition is most often found with guys who use a small enclosed area to store their mower after use, i.e. enclosed trailer, small shed, etc.
Also if the float valve sticks it just normally fills up the intake manifold. The bowl doesn't hold much fuel. The tank is lower than the bowl, so it doesn't gravity feed (hence the need for an electric fuel pump behind the seat). Generally when the 222/227 float vavle leaks, it causes hard starting (flooded hard starting) when cold, before its' been used for the day. Only seen one that filled the engine, and it was on a mower that was stored in a shed that set on a hillside, such that the fuel would actually flow into the left cylinder's intake port. Weird one.
I still think that the evap system can be an issue with filling a crankcase on a 222 that is newer than about 2014. It's got a check valve and a pressure relief that is supposed to relieve tank pressure so that the fuel pressure doesn't rise so much to cause the tank to swell and hit the tire and enough to overcome the float valve. I've seen a stuck relief cause the tank to hit the tire, and owner will keep using it not knowing and next thing you know the tire eats a hole in the tank and creates big leak, but haven't seen a float valve leak because of it yet. "Yet".
Excessive idling WILL cause fuel vapors to build in the crankcase over time. Perfectly normal. Engine isn't designed to idle a lot. Full throttle/speed is what it's designed for.