This article is from the Chrysler-Plymouth-Dodge FAQ, by Dr. David Zatz with numerous contributions by others.
1). Are your coolant temperature sensor connection ok? If not, the
computer will see a cold engine and will run rich.
2). Are the oxygen sensor connections ok?
3). Is the heated air inlet operating correctly?
4). Vacuum leaks? Check all vacuum hoses with a religious fervor!
The leak's location many not even be obvious!
5). Carburetor problems: float low? valve seat damage? I doubt the
latter since it appears that the problem arose quite suddenly. The
following is something I've used on computer-controlled carbureted
engines many times: 1). Connect a high impedance dwell meter to the
mixture control solenoid, set the meter to the 6 cylinders scale, run
the engine around 2000 rpm until hot and see the dwell. If the a/f
mixture's ok, you'll see the dwell oscillating about 30 degrees. Low
dwell with oscillations => a/f mixture lean and running closed loop.
High dwell with oscillations => a/f mixture rich and running closed
loop. Dwell at or below 10 degrees => system stuck lean. Dwell at or
above 50 degrees => system stuck rich. The latter two extremes indicate
closed loop operation since open loop operation typical will show a
stable dwell reading between 20 and 30 degrees (usually, closer to 20).
Do not do this test at idle since some engines will be operated in open
loop at idle REGARDLESS of the coolant temperature sensor's output.
Incidently, I've just outlined the procedure for GM's "System
Performance Test" which is used on GM C3 carbureted engines.