Small diesel engines, specially S195 type diesel engines, are widely used in rural areas of China. According to the results of investigation and inspection of 21000 engines, it is found that the working conditions of the engines in use are relatively bad in general. The average specific fuel consumption increased by 25.6 % and the average power decreased by 18.2 % compared with the rated specific fuel consumption and the rated power respectively. A new suitable technology, i.e., optimal adjustment and inspecting technology was put forward to improve the working conditions of small diesel engines in use. The test showed that the average specific fuel consumption decreased 11.21～29.47 g/(kWh) and the average power increased 0.18～0.97 kW respectively after the engines were inspected and optimally adjusted. This technology has been listed as one of the major popularizing projects of state commission on science and technology of China. It was estimated that over 939.8 thousand tractors in use have been inspected and optimally adjusted and 97.7 thousand tons of fuel have been saved and 959 thousand kilowatt of power have been regained from 1991 to 1994.