Feel around with your hand and see if you can pinpoint any leaks, check the gasket seal under the gear plate. Judging from the picture that looks like a likely place to look. Also, check the gasket at the intake manifold/intake connection as well as the carb/intake connections and check at the exhaust gasket too. You might have to replace one or more of the gaskets if they are leaking. Also, sometimes if you are using a particularly high oil/gas ratio it's possible that oil can shoot out the exhaust and the blow back while you are riding it too, is any on your pants or shoes?