Leave the rubber reducer plugs in place or too much oil mist will come out. Yamaha made the outlets too big and this is how they "fixed" it for a time. Eventually, they redesigned the outlet with only one tube with a very small hole in it. On earlier models, before they came up with the reducer plug idea, they actually capped one tube off completely and only used the other one. There were complaints from owners about too much oil coming out the breathers when the bikes first came out. During the production run, these were some of the steps Yamaha took to reduce it. Another was lowering the oil level in the cases by reducing the amount of oil used.