Saving Old Seagulls

Hi, I,ve brought a 1976 (Bing carb) fourty plus back to life and it runs very well now, only issue i have is that if i'm not carefull when starting the plastic flywheel cover gets pulled of, I nearly lost it today. should there be a washer under the dome nut holding it in place or do i just need to be more carefull? Also the Bing Carb has a aquare cover over the carb inlet, there is a square piece of foam under this, should it have the foam filter, I couldnt see a filter on the diagrams? If anyone can help that would be appreciated, Thanks, Anthony