black flluid coming out of passenger tail pipe

[yorle]

THIS JUST STARTED A FEW DAYS AGO AT IDLE THE CAR IDLES SEMI ROUGH AT TIMES THEN IT RUNS SMOOTH.
WHEN I GET ON THE GAS TO TAKE OFF I CAN ONLY GET UP ABOUT 30 - 40 MPH THEN THE CAR STUMBLES. I HAVE HAD A COMPLETE TUNE UP 6-8 MONTHS AGO INCLUDING CHANGING THE WATER PUMP AS WELL..I PULLED OUT THE LEFT SIDE SPARK PLUGS AND THEY WERE BLACK

[chevymec]

Sounds like the car is flooding. If they are all black on one side you may have a dead O2 sensor. The black fluid is just carbon that is mixing with condensation.