Black Gas Cloud
1990 Grand Wagoneer. At startup, it sputters and jerks, and eminates a noxious, black cloud. It leaves a black stain on the snow. If I keep the throttle depressed, it runs very roughly until smoothing out somewhat. If left alone it will die. I live in Fairbanks, and it gets -50 degrees here. The Jeep is carburated, but the mechanic can "find nothing wrong". Any assistance, especially from a colder climate, would be greatly appreciated. Thank You.