1. Fuel Pressure Is Low
Make sure there is enough fuel in the fuel tank. Look for leaks or severe bends in the fuel line between fuel tank and fuel transfer pump. Look for air in the fuel system. Check fuel pressure. The outlet pressure of the fuel transfer pump at full load speed is 205 ± 35 kPa (30 ± 5 psi). If fuel pressure is lower than above pressure, install a new fuel filter element. Inspect the fuel bypass valve for free movement. Install a new fuel bypass valve if necessary.
2. Air In The Fuel System
Find the air leak in the fuel system and correct it. Remove air from the fuel system. Make reference to Removing Air From Fuel System in Testing & Adjusting.
3. Leak Or Break In Fuel Line Between Fuel Injection Pump And Fuel Injection Nozzle
Install a new fuel line.
4. Wrong Valve Lash
Make adjustment according to Testing & Adjusting.
5. Defect In Fuel Injection Nozzle
Run engine at rpm that gives maximum misfiring or rough running. Then loosen a fuel line nut on the injection line for each cylinder, one at a time. Find the cylinder where loosening the fuel line nut does not change the way the engine runs. Test the fuel injection nozzle for that cylinder. Install new parts where needed.
6. Wrong Fuel Injection Timing
Make adjustment to timing.