presuming your engine is still original and has never had a rebuild....look into the bore and about 10mm down you'll see a ring around the entire bore,if you can physically feel this ring with your finger nail then the chances are your bores are worn and therefore your piston ring gaps are too large,so they'll be allowing blow by and probably a bit of smoke

the only real way to know for sure is to remove the pistons,then take each ring off and place it into the bore,and measure the gap (should be between 0.4mm and 0.6mm i think but don't qoute me on that)

hope that helps