My mower typically starts on the first or second pull . . . Recently, after 6-7 pulls, it fired up, ran a bit rough with sputtering, black smoke emitting from the exhaust. As it idled, I noticed the RPMs slowly decreasing. I shut off the mower, opened the air filter housing and observed what appeared to be oil on the filter, as well as inside the housing, itself. I DO NOT use fuel with ethanol ("rec' gas only). Any suggestions . . . past experiences . . . remedies? Thanks for your help.