An easy check can be made to find the cylinder that runs rough (misfires) and causes black smoke to come out of the exhaust pipe.
Run the engine at the speed that is the roughest. Loosen the fuel line nut at a fuel injection pump. This will stop the flow of fuel to that cylinder. Do this for each cylinder until a loosened fuel line is found that makes no difference in engine performance. Be sure to tighten each fuel line nut after the test before the next fuel line nut is loosened. Check each cylinder by this method. When a cylinder is found where the loosened fuel line nut does not make a difference in engine performance, test the fuel injection pump and fuel injection nozzle for that cylinder.
Temperature of an exhaust manifold port, when the engine runs at low idle speed, can also be an indication of the condition of a fuel injection nozzle. Low temperature at an exhaust manifold port is an indication of no fuel to the cylinder. This can possibly be an indication of a fuel injection nozzle with a defect. Extra high temperature at an exhaust manifold port can be an indication of too much fuel to the cylinder, also caused by a fuel injection nozzle with a defect.
The most common defects found with the fuel injection nozzle are:
1. Carbon on tip of the nozzle or in the nozzle orifice.
2. Orifice wear.
3. Dirty nozzle screen.