Here's why:
* Positive Crankcase Ventilation (PCV) System: The PCV system is designed to vent blow-by gases (gases that escape past the piston rings) from the crankcase. These gases contain oil vapor. A properly functioning system routes these vapors back into the intake manifold to be burned in the combustion chamber.
* Failure Modes: If the PCV valve is faulty (stuck closed or broken), or if there's a leak or blockage in the PCV system hoses, the blow-by gases can't escape properly. The pressure builds up in the crankcase, forcing oil vapor (and sometimes even liquid oil) past seals and into the intake manifold and eventually into the carburetor filter case.
* Other, Less Likely Possibilities: Though less probable, other possibilities include:
* A severely worn engine: Excessive blow-by due to extreme engine wear could overwhelm a functioning PCV system.
* A cracked or damaged valve cover: This could allow oil to leak into the PCV system.
* A failing intake manifold gasket: This could allow oil to seep into the intake.
In short: Oil in the carburetor filter case isn't a normal occurrence. It's a symptom of a problem within the engine's crankcase ventilation system, and ignoring it will likely lead to further engine damage. The PCV valve should be inspected and replaced, and the entire PCV system should be checked for leaks and blockages. A mechanic should diagnose the underlying cause before further damage occurs.
