Why is my 94 Silverado have black smoke coming from tail pipes and runs rough?

First check the fuel filter, then check the spark plug wires, rotor, and distributor, you need to check resistance contact your local auto parts store or dealer for the correct specification. Were doing the simple things first. From their check the Map sensor, your checking resistance, then one of the injectors ( im assuming a TBI - throttle body injector setup) sometimes the TPS throttle Positioning Sensor goes bad! Engine Temperature Control sensor can also have an effect on performance its located by your thermostat housing! It informs computer when engine is COLD and sends signal to run rich! If its the fuel pump or the fuel regulator these two device usually make it run lean when they go bad and will cause truck to stall. Check the MAP sensor & Engine Temperature Control sensorFIRST these are the two primary culprits. IF this helps remember to hit Mark Helpful button I will post another paragraph on the subject if this doesnt remedy the problem!

TESTING Test MAP SENSOR Backprobe with a high impedance voltmeter at MAP sensor terminals A and C. With the key ON and engine off, the voltmeter reading should be approximately 5.0 volts. If the voltage is not as specified, either the wiring to the MAP sensor or the ECM may be faulty. Correct any wiring or ECM faults before continuing test. Backprobe with the high impotence voltmeter at MAP sensor terminals B and A. Verify that the sensor voltage is approximately 0.5 volts with the engine not running (at sea level). Record MAP sensor voltage with the key ON and engine off. Start the vehicle. Verify that the sensor voltage is greater than 1.5 volts (above the recorded reading) at idle. Verify that the sensor voltage increases to approximately 4.5. volts (above the recorded reading) at Wide Open Throttle (WOT). If the sensor voltage is as specified, the sensor is functioning properly. If the sensor voltage is not as specified, check the sensor and the sensor vacuum source for a leak or a restriction. If no leaks or restrictions are found, the sensor may be defective and should be replaced! Again if this helps hit MARK HELPFUL

Testing the ECT ENGINE TEMPERATURE CONTROL Remove the ECT sensor from the vehicle. Immerse the tip of the sensor in container of water. Connect a digital ohmmeter to the two terminals of the sensor. Using a calibrated thermometer, compare the resistance of the sensor to the temperature of the water. Refer to the engine coolant sensor temperature vs. resistance FOR EXAMPLE AT 77 Degrees the resistance will be 2796 OHMS chart can be found online or at parts store or Dealer Repeat the test at two other temperature points, heating or cooling the water as necessary. If the sensor does not met specification shown in the temperature versus resistance chart, it must be replaced. The sensor may also be checked in the vehicle. Unplug the sensor and attach a digital ohmmeter to the two terminals of the sensor. Using a calibrated thermometer, compare the resistance of the sensor to the ambient air temperature. Repeat the test at two other temperature points, heating or cooling the water as necessary. If the sensor does not met specification shown in the temperature versus resistance chart, it must be replaced

One correction your checking voltage on MAP sensor!

Check two sparkplugs one from the left bank the other from the right bank. If both sides fouled, inspect the injectors for that side on the TB LEFT BANK FOULED CHECK LEFT INJECTOR IOW, right side fouled right injector. Mark helpful link if this helps you! Thanks!