The enricher operating rod has detent holes drilled into it and there is a small steel ball backed with a spring in a recess in each carb that locate in the operating rod detents to hold enricher in the off/half/full positions. The small balls and springs are probably missing as if you don't know they are there when stripping the carb down they are easily lost. If you need some, the balls and springs are still available from Yamaha:
ball - P/N 93501-08001-00
spring - P/N 583-14589-00-00
One of each is needed for each carb.