There's a crankcase breather tube that runs from the crankcase (duh) to the airfilter housing. Your engine may be having more bypass than the system can handle or the bypass assembly may be getting old and leaking.

 

Is this repairable without pulling the engine?

 

I have the same mower and the only time it has ever leaked oil is the two times when the oil pan seal went bad. I'd call your dealer and ask them.

 

Is this repairable without pulling the engine?

More than likely, no. Too much bypass pressure is a result of too much cylinder bypass. It's a fancy way of saying you may need rings. Do a compression test of all 4 of your engines. Take the numbers from the 3 good engines and average it out. Take the compression from the fourth engine and compare it to the average. If it's more than 15% lower you have a problem.

 

Thanks guys, I ended up pulling the engine, it was the crankcase cover gasket, not a real hard job once you get going, I replaced the crank oil seal while it was apart. It's back together and running fine now. If anyone knows where I could get a shop manual for the FC420V please let me know. Thanks,
Leighj