Help. I have a 79 Chevy in line 6 cylinder in a G10 van, manual transmission. The exhaust stinks of rich at idle and on the road. Gas mileage is horrid on highway. Just replaced the carb, no better. When in gear and at slight gas pedal pressure, it surges badly. Give it more pedal pressure and it quits surging, but is still very rich. Exhaust pipe is black and sooty. Plugs good, wires good, new air filter, no inlet restrictions. Any ideas?