Two notes here:
1) Although KenW is correct there no physical valves in most 2 cycles like 4 cycles, there are some that do have reed valves. These reeds do sometimes fail to close when they should.
2) Tyler you are basically incorrect about the tank venting outward. They only vent inwards on most 2 cycle except for very old equipment where they use slivered bronze vent. This is an EPA requirement here that gas vapors must not be expelled un-burned. This why you find that when you loosen the fuel cap you will find it pressurized most times. The inward venting is so the tank does pull a vacuum causing lost of fuel flow. Most 2 cycle cube carbs are design to hold back a pressure 5-10 psi depending on the design. Now needles and seats do wear; therefore, they can get to where they will not this pressure level and need replacing. As note a stiffen diaphragm will also cause this leakage as well as carburetor with either primer as part of the carburetor or as an external primer for their check valves can go bad and leak backwards.
Now some engine do have a slight blow back during operation due how a 2 cycle moves the fuel mix. As the crankcase might be slightly pressurize by ring blow-by just the piston passes the intake port. Some of the old saws that I have actually have quite a bit of spit back from the carburetors.
There is other note here but it related to the handheld 4 cycles that uses a separate oil tank. Many are design in such a way that PNC fails from lack of proper oiling and oil is blown into the air cleaner.
