1.check not over filled with oil.vacuum line could be sucking oil. if dirty drain and replace quick solution could go thicker eg 20w-50 bike oil
2. check filter
3. spark plug what color?
4. compression if on the low side ~120psi its rings or valve. if high could be sticky or broken oil ring( if they are sticky, try SEAFOAM, and or intake cleaner, warm upto temp and add little and rev a little and then add more until it stalls. let sit for 30min and run it until warm and exhaust should stop smoking.
5. take head off check valve guilds.
6. check rings and bore size and roundness