Piston port engines sometimes spit back fuel,depends on how it's ported..Reed valve engines do not...The 3W is a reed valve engine..When the piston comes down the reeds close off the intake...If the reeds don't seal well there will be fuel spit back..Otherwise it can't happen...
Leaky reeds also cause hard starting...Part of the crankcase pulse leaking past the reeds won't let the pump in the carb work well....