blue smoke but only on overrun, not oil

[ubertourer]

t reg 650 curvy, with 13 thou on the clock. changed rings, valve guide oil seals, and still puffed blue smoke out when I shut the throttle down. bought a 19 thou engine today. installed, started first prod, runs a dream,(so does the original engine) also smokes exactly the same as the original engine. leads me to think that both engines are fine, and that its something to do with the carbs.
any ideas.....

[Sid Squid]

Blue smoke does suggest oil, are you sure of the colour? If black rather than blue then rich mixture? Check the obvious first; air filter, floats, needle valves, jets are fitted correctly and that the pump diaphragm is sound.