Float has gotten stuck carberetor filled with fuel continuous and filled the position chamber also it seeps down into the sump filling it with a oil petrol mix instead the carberetor.needs a,service or realigning with the float correctly a worn float needle can also cause this and debris inside the arbitrator holding the needle open so fuel don't shut off

Sounds like the float in the carb is ( was ) stuck and allowed fuel to fill up the carburetor and then from there found its way to the intake and down into the cyclinder and seeped passed the rings and into the crankcase. the first thing to do is to remove the carburetor and remove the bowl and spray all parts down with carb cleaner -- make sure the float moves freely and that the float pin moves freely and seats proper. clean out any rust or other debris and re-assemble carb -- re-install carb to motor and run the mower. all should be good at this point... now when storing the mower for a long period of time - it is a good idea to turn the fuel line valve off..

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