The paper deals with the existing Diesel nozzles optimisation of the conventional injection system pump - high pressure pipe - nozzle. The aim of the optimisation is to increase the injection rate of the nozzle by 9 %, lower the fuel consumption and reduce the emissions of the engine. Existing and optimized nozzles should feature the same inlet pressure, i.e. the same load on the pump allowing that the same pump may be used also for the redesigned nozzle.Redesigned was the tip of the nozzle. The nozzle was optimized by computer simulation. The prototype of the redesigned nozzle was then tested on the test stand and in the four - stroke air - cooled engine according to the 13-mode ECE R49 cycle. Explained is also the influence of fuel injection parameters on engine performance and emission. All test results are presented in the paper.