The process of deposit-induced, fuel-flow reduction in multiport fuel injectors of the director-plate type was investigated both experimentally and analytically in order to understand both the flow-reduction mechanism and the non-linear variation in flow reduction with engine operating time. Injectors that had accumulated deposits in previous extended engine testing utilizing a rapid-plugging fuel were flow-tested, and the orifice plates from two of the injectors containing deposits were examined using a scanning electron microscope. A thin, carbonaceous deposit protruding into the flow paths of the orifices was found to be present on the downstream face of the orifice plate, but not present within the fuel orifices or on the upstream surface of the plate. Based upon these observations, calculations were made using a fluid flow model of an orifice plate with deposits. These calculations verified that deposit protrusions of a few microns result in flow reductions of the order of 10 percent. The analysis also confirmed that the deposit-induced fuel flow reduction is greater than would be expected based upon the decrease in flow area alone, and is caused by the combined effects of a decrease in orifice outlet area and a reduction in orifice discharge coefficient. Equations relating the fuel flow reduction and deposit geometry were obtained from the director-plate flow model. For a restricted injector for which detailed deposit geometries were measured on the scanning electron microscope, the flow reduction predicted by the analysis program agreed closely with the bench flow measurement.