These Cup Washers are mandatory to obtain a air tight seal on the Idle Mixture Adjusting Screw.  The Washers hold the o-ring tightly preventing the washer from being squeezed away from the carb body.  When air leaks occur in this area the carbs will spit, cough and backfire at cruise or steady speed but will run great when accelerating hard New O-Rings are needed often as they deteriorate due to heat! (While these washers are similar to Finish Washers they are different! -BTDT)