360 won't idle smooth...can't figure why

The plugs appear to be burning clean and there is no soot in the exhaust so it doesn't seem to be running rich. It runs the same cold or hot. How would a temp or O2 sensor make just those cylinders not work? wouldn't it effect them all? I've added extra fuel and opened a vacuum hose to make it run leaner but neither will smooth out the idle....even for an instant.... before the computer compensates.