The potential of vegetable oils as an alternative to diesel fuel has already been proved. Nevertheless, the high viscosity of vegetable oils poses some problems concerning their use in unmodified vehicle diesels. In particular, distortion of the speed characteristics of a distributor fuel injection pump with a maximum-minimum speed mechanical governor when using sunflower oil-diesel oil blends has been observed. A diesel engine fuelled by a viscous blend could not develop its rated power. After fuel delivery was increased to retain the rated power, the engine was overfuelled at lower speeds and the smoke limit was exceeded. In the present paper, measures required to retain the characteristics of an injection pump fuelled by viscous blends are described. This can be done by readjusting the pump governor and changing the hydraulic characteristics of the fuel delivery system.