Picked up a used 2013 Artic Cat 450 XC. Only has 1,000 miles on it. Knew it was smoking when I bough it. Pulled the head and base off. Oil clearly leaking on exhaust valve side, intake side looked clean. So I replaced the all four valve seals. Cleaned the exhaust side ports and valves of residual oil; and carbon, replaced head and base gasket. Piston looked clean. Put all back together and it is still smoking bad. Have not pulled back apart yet, but just wondering what else it possibly could be.