Could be a 'couple of things, First go thru the carbs CLEAN them real good paying close attention to the pilot jets. Could be the over popular leaky needle & seat, causing excess fuel to travel down into crankcase-BAD- washes all oil from crank bearings & Piston skirts. Could be leaky carb boots also check them real good in lots of light @ room temp. Good luck.
I'm sure someone else will chime in with other possibility's.