If indeed, the rebuild was done properly, you might still be in the "break-in" stage of things. Piston rings need to have some load put on them to burnish their fit with the cylinder walls properly.
Perfect Circle recommended seating the rings by doing 10 full throttle accelerations from 30 MPH to 60 MPH after a rebuild.
Jerry
That's the way I was taught at Oregon Tech back in the dark ages when they had classes that got your hands dirty.
