The "washer" you are referring to is the spacer that prevents the slide carrier from making metal-to-metal contact with the carb body.
From my notes I put in my manual - .5mm thick, 13mm I/S dia., 17mm O/S dia. I believe this spacer also allows for a correct clearance between the end of the vacuum-post and the bottom of it's bore. This I believe maintains an adequate gap to allow responsive changes in vacuum, which is crucial for correct slide/mixture operation.
The spacer is required. They are difficult to obtain. Assembly without them will seriously affect the high-speed needle performance because the slides and needles will be .5mm lower (leaner) in the jets. A set of carbs I obtained for rebuilding with the intent to re-sell, had one (1) missing. I couldn't find anything I could use as a substitute but got very lucky when another member offered one. I would suggest that a more thorough Google search may get you something that could be used.
If you've got a '79 ( sorry - can't recall) then you'll need the specific part. These needles have only one slot for the c-clip so no easy possibility of adjusting the needle height. If you have an '80 and up with 5-slot needles, then you at least have a better chance of maintaining the correct mixture, if you substitute.
So keep in mind: If you end up making some (plastic/fiber) spacers, the difference in the OEM thickness vs the material you use, must be given consideration to avoid "chasing" your mixture..
