I've been looking for the location of the fuel filter.
At first I thought it might not have one, and only the Model E ZZR's have one, because there seems to be no explicit mention of it or diagram in the Haynes Manual. It seems to me that it would be somewhere between the fuel tank and the carburettor, but I can't find it. A Model E also has a fuel pump, which the Model D does not, so could it be something to do with that?
On the other hand, it does seem to have one, because you can buy them, such as here: http://www.patternparts.net/p-Kawasaki_ZZR_600_Fuel_Filter_93_05-33376.aspx