For a 1 cylinder rich problem that DIDN'T OCCUR before you need to focus on what changed for that cylinder.......for a 4 CYLINDER problem that DIDN'T OCCUR BEFORE, you need to keep in mind common denominator type causes......unless all of your carbs are now flooding AND your petcock is leaking to feed them all, then OR all your pods are plugged OR mechanical timing jumped OR timing advance is stuck, then it might be otherwise ignition related. You asked about fuel in the crankcase.....excess raw fuel into the cylinders, especially if mis or not firing will tend to wash the cyl walls causing loss of compression until corrected. Draining the oil, you'll notice a distinct fuel smell associated from the contents (you can confirm this often by lighting a small sample with a match etc.; whereas straight oil will seldom ignite readily by itself and substantial fuel will cause an easy flame to start).
Another common possibility, if running the original type carbs, is that the enrichment valve shaft (choke) is stuck lifting the valves to the enrich position (or partially so).
Is your history accurate as originally posted?
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