Why is it blowing black smoke out of the exhaust?

Make sure that at full throttle the chock is not being used. Black smoke coming out of the exhaust of gasoline engine is a sign of a rich fuel mixture (too much fuel, not enough air). A brief puff of black smoke may be visible during hard acceleration when the fuel mixture goes momentarily rich, but under normal running conditions and idling the exhaust should be transparent. If black smoke is visible all the time and the inside of the muffler is coated with heavy black carbon deposits, it would tell you the engine has a rich fuel condition.
On engine, a rich fuel condition and black smoke can be caused by a stuck or maladjusted choke, a leaky metal or hollow plastic float inside the carburetor fuel bowl, or a fuel saturated foam plastic float, or an incorrectly adjusted float (set too low). A trestricted fuel filter may also contribute to a rich fuel mixture.

no. (but the isc valves loves to stick but will not cause black smoke)
the 4.0L is one of 5 million made, it's well known,
the black smoke don't lie. it's rich and the cat seems burned up too.
as with black smoke the cat will try to catalyze all that and glow red hot. is your cat missing?>
so why not believe the code 171, its real and is catastrophic failure. there are 2 engines here, FED and California.(exhaust)
(2 different exhausts and sensors there.
as they say, fix what is OBVIOUSLY bad, 171
(find why using tools or inspections)
Fuel System Lean (Bank 1)in most cases the exhaust is cracked near B1S1 0xy sensor
this causes it to stick at 0volts, (lies) and the PCM sees that obvious lie then goes rich. (limphome rich)
if the pcm cant measure oxygen, it must now guess at fuel trim.(bad that.)
it goes rich, so you dont melt pistons running lean.
(by design)

bad front oxygen sensor (aka , up stream bank 1)
in calif cars, it can have a split exhaust on the I6 and have 3 sensors. (hope not yours, do look..)
most have 2 sensor, front and the useless behind cat sensor(B1s2)
S2 is useless for running mix, the rear is for smog test the cat.

so it rich, and now misfires. and the spark tips are black.
and it misfires to bad it can even idle, sure. (Drops)
the cause, is too rich.
the 171 means the PCM sees it stuck at 0v forEVER.

there are in fact other causes.
but these 2 are top cause.

on all 3 of my 4.0L engines
the exhaust manifold cracked on the under side.
side facing block. (i use a hose to ear to hear the putt putt puts, (with FULL PPE protection from burns from a dead cold engine.) ask how.
even the SST exh. man. did that.

or the header flange gasket is TOAST. (that too putts when bad.)

Black smoke carburetor has too rich of fuel mixture. Blue or white smoke is oil getting into the exhaust because of either over filling with oil, engine was tipped on its side or piston rings are bad and is getting blow by through the cylinder.

here try review this.
Car Exhaust Advice Good Smoke vs Bad Smoke

Yes

$1500 spent on what , if I may ask? Obviously a rich mixture, but need to know what was done for $1500, plus year , make and model of Chassis, and engine size.

Black smoke from the exhaust is an indication of the engine running too rich. Probably bad fuel injection system.

head gasket

Sound like a rich by a long way; are you check if have smoke in exhaust? Black smoke is fuel, blue smoke is oil.

Also, your injectors are to big and is running rich.

Could have a bad map sensor. The Map sensor is located on the throttle body. I have seen countless Jepp that start blowing black smoke when they go bad. That would be a good starting place.