Could be carb problems. The cylinder is getting to much fuel and it is going past the ring into the crank case. Try rebuilding your carb, could be out of time : check to make sure the engine is timed...
I Think you have other problems. Change the fuel filter and if that doesn't do it take it to a garage.You purchase a gasoline can and a funnel. Fill the gasoline can with fuel from your local...
The fuel in the combustion chamber is not burning completely and seeping down the cylinder wall(s) into the crankcase. The spark plugs may be fouled or possibly one (or more) is not firing. You might...