Rough start and idle

Rough start and idle plus better performance at higher RPM sounds like it is running rich. Those love to leak fuel from the fuel pressure regulator and the fuel lines inside the intake manifold on the spider injectors. They also get the poppet injectors that stick but that usually shows up as a single cylinder miss while the others cause random misfires and rich running. The smell and black smoke both indicate the rich condition. I would try to get it repaired ASAP as running it that way can easily damage the converters.

Replacing both parts is not all that difficult as you only need to remove the upper intake cover. However, if it has not had the lower intake gaskets replaced yet it is not a bad idea to do them as well as you will already have the intake apart.

Also the 1999 usually has the older poppet style injectors. You can get a conversion unit that replaces that with the newer updated MFI injector unit. That gives each cylinder an actual injector instead of the SCPI system. I do that upgrade on most of the 4.3's that have issues and everyone has come back and said they get better mileage and a much smoother engine than they were before the conversion. The updated unit is about $50.00 more but it eliminates a few problems on the older injector units.

Remove the air cleaner outlet duct from the throttle body.
Remove the air cleaner assembly. (Image 1)
Disconnect the following electrical connectors:
-The A/C pressure switch, if equipped.
-The throttle position (TP) sensor.
-The idle air control (IAC) motor.
-The fuel meter body assembly connector.
-The manifold absolute pressure (MAP) sensor.
-The EVAP canister purge solenoid valve.
-Remove the EVAP canister purge solenoid valve.
Disconnect the vacuum hose from the intake manifold, if equipped with A/C. (Image 2)
Remove the vacuum brake booster hose.
Remove the PCV valve hose assembly form the intake manifold and the valve rocker arm cover. (Image 3)
Disconnect the accelerator control cable from the throttle shaft and the accelerator cable bracket.
Disconnect the cruise control cable, if equipped from the throttle shaft and the accelerator cable bracket.
Remove the accelerator cable bracket from the throttle body and the intake manifold.
Remove the accelerator and cruise control cable bracket from the throttle body. (Image 4)
Leave the accelerator and cruise control cables and the engine wiring harness on the bracket. (Image 5)
Remove the bracket for the engine wiring harness from the intake stud. (Image 6)
Remove the fuel lines from the fuel meter body assembly.
Remove the upper intake manifold. (Image 7)
Clean all sealing surfaces.

Disconnect the fuel lines from the SCPI injector and remove the bolts from the clamp that holds it down. Squeeze the injector retainer clips and pull each injector out of the lower intake manifold.
If you do the upgrade be sure to get the bracket and clamp for the new unit. Some include it some do not. The last picture shows the new unit and the hold down bracket.

There are also multiple youtube videos on the process if you want to see it being done.