In order to improve the combustion efficiency of the agricultural engine, an ignition timing control system was developed and tested. The control system was composed of the CDI ignition circuit, the microcomputer and the interfacing devices. In this study, the simplicity of the control system and the flexibility of the control strategy were emphasized for the precision, the applicability and the economical efficiency. The hardware was consisted in almost the same compositions as those of the automobile engine. The softwares of the control algorithms were developed to three types depending on the combination of the quasi-adaptive control and the open loop control which had the different spark advance equations according to the input variables such as engine speed, exhaust gas temperature and brake torque. The test results were summarized as follows: 1. By using the computer control system, the fuel consumption efficiency could be improved and the fuel consumption could be reduced by 0 to 57% compared to that of the fixed spark advance system. 2. The fuel consumption of the control mode with the quasi-adaptive algorithm was reduced by average 0.8% compared to that of the control mode without quasi-adaptive algorithm. 3. It was found that the control mode with the quasi-adaptive algorithm adopting single input of engine speed had most applicability and economical efficiency among three types of the control algorithms.