Engine Engine rough and smokes

I just acquired an MTD riding mower Model 13AN678G022 with a Briggs 313777 OHV engine. It smokes and runs rough. It also does not seem to come up to full speed. The throttle is adjusted properly. The oil was overfilled, so I drained it (seemed very thin) and replaced with fresh 30 W and also changed the oil filter. I still have the smoking and rough running problem. Tomorrow I will check and see if the choke is stuck closed , but I don't think this is the problem, as this doesn't account for the smoke. The dipstick is sealing properly. Any suggestions for diagnosing this problem?

Thanks Robert