Why does my lawnmower leak gas into the oil?

While running? Look for a new lawnmower. The rings and cilender walls are shot. While setting in the garadge between jobs, Carborator, needle valve is wore. Rebuild the carb. Or cut into the fuil line and splice in a shut off valve. Shut the gas of after mowing. Cheap and works well. But will still probable need a carb job soon.