Worse case scenario. Float needle stuck in the carb with piston on the intake stroke. Rings may have washed out allowing fuel to pass into the crankcase. You didn't state how much gas though. Your yz uses the oil in the gas to lubricate the crankcase so a little is normal.
Typically this is caused by a dirty carburetor causing gas to leak through and into the crankcase.
Crankcase oil, gas needs to be premixed
If engine has fuel pump mounted on the engine then it is leaking gas into crankcase.
chop it up with an axe then use duct-tape
Exhaust gas recirculation and crankcase ventilation are two different things, and not related. EGR allows some exhaust gas to enter the combustion chamber to reduce combustion temperature, crankcase ventilation usec a pcv to remove oil vapor from the crankcase.
no, the oil is mixed with the gas before it goes in the carb, so there should never be oil sitting in the crankcase.
Drain the oil and gas out of the crankcase and put oil back it should be OK
Because the carburetor on these motors are no good after a few years. The carburetor will spew large amounts of gas into the crankcase and you do need to change the oil about 3 times after this happens to protect your engine and get all the gas out of your crankcase. The replacement carbs are supposed to be much better than the ones supplied with your engine. A new carb is about $125.00 excluding labor.
Too much crankcase pressure or the breather pipe is split/dosconnected. The crankcase breather goes into the rear of the air filter and the crankcase gasses are drawn in to the cylinder with the gas/air mixture and burned
Positive Crankcase Ventalation and Exhaust Gas Recirculation
Float valve or float are bad , letting carb overflow into crankcase .