I have seen this before. I had a strange one last week. Pouring rain I revved a Raga to the moon before a large rock. (30 ft high, 50 ft long) Cleaned the section, just as I started to ride the next section I realized I blew out all the oil. None came out the over flow. I had blown the gasket at the top of the clutch cover completely out. Lazy me has not looked yet as I have all the parts on hand. Vent was clear. (First thing that confused me) Now I`m wondering if the hole through the clutch bolt had plugged up?