1. Not Enough Air For Combustion
Check air cleaner for restrictions [Max. 635 mm (25 in) of water].
2. Defective Fuel Injection Nozzle(s)
Install new fuel injection nozzle(s).
3. Wrong Fuel Injection Timing
Make adjustment to timing.
4. Fuel Ratio Control Has A Defect
Check fuel ratio control setting. Adjust if necessary. Check for broken springs.