Hello Dave, my name is XXXXX XXXXX I can help you with this problem. There are two common causes for gas leaking into the engine oil. The most common cause is a faulty carburetor float or needle valve allowing gas to leak into the intake and then into the crankcase. To check for this problem follow these steps. 1  Shut off the gas to the carburetor, if your mower doesn't have a shut off valve simple pinch the fuel line with a pair of vice grips. 2  Using a wrench remove the carburetor bowl retainer, this may look like a bolt head or a nut with an adjustable valve through it.