reason why I asked if sunk, as if was and dirt and crap was inside the motor, the longer you run it, the more damage that stuff does, and sooner or later a top end is gone and your smoking, a rather common issue in atv's today!
sellers SINK em, clean em up, and sell em, to guys that find out the hard way what they did!
about HOW Long have you ran motor since you got it, Hours wise that is?
and till you get a compression test, ??