The list of possible problems is a short one..You absolutely must have breathers on both covers. PCV valves don't do Shiat on a big lift cam (no vacuum). The oil level must be low enough to reduce centrifugal windage...
If all these things are OK, then you may have an engine problem internally. Marine engines biggest killer is rust, it gets on the cylinder walls when they sit for prolonged time periods then you run it, the rings wipe the walls of the rust and all of the oil....
Two stupid things to try.....Even if the leakdown test shows issues.
Get some decarbonizing chemicals and run it through the engine per the directions....I recommend Amsoil Power Foam, but there are a lot of different ones on the market (Yamaha makes one called Carbon Guard, GM makes one called Top Engine Cleaner) they all do the same thing, but I sell Amsoil so I'm a bit partial...

The second thing to try is to get the engine really warm and right before you shut it off, pour some Marvel Mystery oil down the carb and let it sit overnite...Then run the test again...
Works about one in five times for me...

Good luck!
GT