I think I've worked out why Mikuni used that thick brass washer. They decided - for fairly good reasons having to do with not picking up too much garbage from the bottom of the float bowl into the main jet - to use a horizontal main jet rather than the traditional vertical one on that carburetor. However they wanted to use as small a diameter as possible for the vertical fuel-pickup tube (on which both the main jet and emulsifier are mounted), in order to save cost and weight. They also wanted to use a standard main jet, rather than invent a new non-standard one for that carburetor (which would have caused inconvenience to their spare parts department). Since the standard main jet was too long to suit their small-diameter fuel-pickup tube, the simple solution was to put a spacer under the head of the jet. All sensible decisions, but the result was an odd-looking design.
