In modern magnetic solenoid actuated diesel injection equipment, the injection cycle process is regulated by the action of the ball spill valve. This connects the high-pressure region of the needle valve and the low-pressure region of the fuel tank. The precision in timing and hydrodynamic forces determines the amount injected and the amount of the fuel returned back for the following cycle. The sequence of inlet and outlet throttle, through which the high pressure diesel flows whilst the solenoid is actuated, are expected to enhance cavitation due to their microscopic size. In the long-term this may cause the accumulation of damaging internal deposits. Two acrylic models of the spill valve reproducing the geometrical features with different outlet throttle diameters were constructed to allow an optical access of three different fuels – a paraffinic rich model diesel, a 95% - 5% hexadecane-octane mixture and a 80% - 20% hexadecane-octane mixture. Variation of the upstream pressure up to 40 bar and downstream pressures up to 10 bar and manual control of the ball valve lift height were the key elements controlling and producing the flow conditions. Experimental results show the pronounced effect of the upstream pressure on the characterisation of cavitation inception occurring at the throttle entrance. It was concluded that there is a linear relationship between upstream and downstream pressures that control cavitation inception. Moreover the ball valve height plays a significant role in cavitation inception at the beginning of the ball valve lift, where higher upstream-to-downstream pressure ratios were necessary to initiate cavitation. The highest-pressure ratio was obtained with the highest fuel viscosity and largest outlet nozzle diameter, and it consistently decreased as the viscosity and diameter decreased. No cavitation was observed in the inlet throttle passage, but the change in pressure ratio between the two acrylic blocks suggests a change in pressure in the intermediate region; thus it is possible to control the cavitation occurring in the region above the injector needle in spill valves. A high-speed, high-resolution imaging system was utilised to acquire images for inception, partially and fully cavitated flows. The results obtained are presented.