Motorcycle Repair: help, yamaha bear tracker, wiesco piston

Question
i just recently purchased a 2000 yamaha bear tracker 250 the owner was a litle pissed off because it had started smoking so i bought it brought it home and tore it down had it bored .50 0ver over put a shindy piston an rings in it completely rebuilt the head new valves cam everything had it running good an it started smoking again so i figured probably that cheap ass piston so i bought a brand new cylinder had it bored .40 over put a wiesco piston an rings  in new rubbers on the valve stems tried cranking it the starter will not turn the motor over so i bought a new battery and still the same starer response so i got a  new starter which was not cheap an it still want turn over so i used the crank rope it ran great for about thirty minutes and started smoking again so wats the deal there my dad who is a great mechanic does not even know so anv advice would be helpful i know its not the piston or rings i mean there brand new and the diaphragm spring in the caurberator keeps breaking in half please help me out on this one thanks in advance

Answer
Wow nice work... HeH sorry but lets start and the 1st thing. The smoke is mostly and i would have to belive a crank seal leaking oil in the cylinder its self..  Two why put the rings on. there is only one way to put them back in. But 99.9% sure its a crank seal start there its a 15 doller part. Now who did the work and make sure the commpersion is not to high. the carb spring is nothing but a fluke are put in wrong. Start there man and good luck