***"sitting idle"***
As in "mower is turned off and stored between uses" or as in "sitting there running at idle RPM"?
If you mean "while the machine is stored between uses, the carb overflows and drowns the engine, filling the crankcase with oil". This is a very common problem and is a consequence of a gravity fed fuel tank not having a manual shutoff valve to positively block fuel from flowing past a leaky float needle valve in the carb.
If this is your problem, you can opt to install an inline fuel cutoff valve in the fuel hose between the tank and the fuel filter (and do nothing further). But then you must learn to ALWAYS shut this valve any time the engine is not running (even if it's just so you can take a break).
In most of the cases, the leak past the needle valve is so "small volume" that the normal operation of the engine during use when running will not be affected. Only when the engine is shut off and stored does the small leak turn into a big problem given enough time to leak.

Now, if you mean that at idle RPM you see fuel leaking to the open, then you have a defect in the carb float and/or needle valve.

And if fuel has been getting into the crankcase lube oil, change the oil and filter (if present) before operating the engine again.