exhaust smoke after idle period
 

(Re a 93 Protoge ,auto , 170 k mi.) When I idle the car , with the engine running, for 5 or more minutes, than take off, I blow a lot of smoke out of the exhaust (the longer the idle period , the more smoke). This smoke than clears up after a few minutes of driving and it is fine . There is no major smoke any other time (start-up , running, or even during the idle period itself, only after the idle period) . Outside of a rough idle, the car has no driveability problems. Any help would be appreciated. Thanks