check your head and cylinder gaskets. make sure your spark plug is in tight (but dont go crazy on it, strip the threads and your screwed) make sure your head nuts are on tight (to spec ft/lbs, dont strip those either). i dont know specifically about your bike (i just bought one and havent torn it appart yet) but check and make sure your cylinder posts are screwed in to the case tightly. they may be loose and allowing for a leak.
basically make sure all your gaskets are good and all your nuts and bolts are tight (but not too tight).