Check your sprockets and make sure there almost as centred as they can. If there off centre they will go loose tight loose tight. If its a bit to off centre it can be really loose, really tight which would throw the chain off. Thats about all it can be. As for the smoke, white smoke is pretty normal on start up. It just means its cold. If its dripping oil it means ur running it with too much oil so run a lil less next time. It shouldnt blow to much smoke, white smoke should be coming out constantly with 2 strokes but not too much. If its enough to make u go dizzy when u revv the bike at stand still its to much haha. U shouldnt really be able to see the white smoke after its warm but on start up a bit comes out. Also ive never had one of these engines but if u can tune the carb then it might be running a bit to rich (to much fuel, not enough air) so maybe turn the high screw back a lil. It might be getting to much fuel.
Anyway its seems pretty good, my bike use to blow tuns of white smoke, enough to make me dizzy if i revved it when i was sitting at an intersection or something. If i revved it i had a cloud around me after 10 seconds. Running 20:1 in it fixed that. No 16:1 for me anymore. Also try cleaning ur airfilter before tuning it.
If its a 4 stroke then it can only be a dirty air filter and/or u need to lean it out a lil.
Hope that helps ya,
From Brendon.v