I had the same problem with my Cyclone which uses a HD engine. When the previous owner had replaced the factory air cleaner, he hadn't reconnected the breather system. I found some advice on a Buell forum that showed me how to fix it, and I got a small "puke tank" from ebay that cured it 100%. I don't know if that is your problem, but it sounds similar to my issue.
In my experience, they HATE to be even slightly overfilled with oil. Again, that may not be your issue, but it's an issue I had myself.
Good luck.