Blue smoke is oil, has nothing to do with carburetion. Rebuilt engines in my experience do not puff large amounts of blue every now and then. dIn fact, a rebuilt may not burn oil at all (I have seen that lots of times), or if it does, burns a little blue steadily until the rings fully seat. I think you have some oil leak down from a valve guide. Or they cracked a ring or ring land when they installed the pistons (that's easy to do if you are not careful). For my 2 cents, I would not do anything else and run it for about 1000 miles. If it has the same problem, start tearing down begining with the heads. When you pull the heads off, take a look for a vertical scratch or discoloration on a cylinder wall. Or take it back to the machine shop if they warranty their work.
