The crankcase ventilation on these units is down in the valley under the air cleaner base. There’re a rubber elbow that connects the air cleaner to the crankcase breather (that silver thing under the carb). It sounds like your motor might have developed enough crankcase pressure to force oil past the piston rings which would result in the condition you described. Check your plugs to see if they’re oily and clean/replace. Have you redone the top end in yours or set the valves anytime recently? It’s super easy to do.
Also, it’s also super easy to replace the original 2001 motor with a brand new one for less than $1000 if needed. My worn out 18 has been pulled and will be rebuilt for another project.
Located in Western Montana
2003 Argo Bigfoot 31hp Spray Rig