you might be getting oil left over in the exhaust from running if it is black.if the front seal bearing is leaking you would have vacuum problems,and usually see it coming out from the seal bearing after a flight.use some sort of after run oil when you are finished for the day unless you know you are going right back out next day.i like to clean my engines pretty good when I'm finished for the day.rubbing alcohol from the dollar store and they have both 70 and 90% usually.hope this helps some happy flying