Check the oil level first as the other post said if over filled or if fuel leaks into the bottom end this can happen.
from my own experience with my kfx700 after I did a big bore kit hi performance cams and a rev box this problem all of a sudden showed up. This was due to excessive air movement inside the cylinder bottom end and there was not a breather box to catch any excess oil that will blow out with the moving air.
to fix this problem I got a breather box from a Yamaha raptor and installed this in line with the breather hose then it also has a drain and I had to use a extra unused port on the other cylinder for the oil to drain back to the lower end.
