1200's came original with 'blind plugs' over mixture screws. To access the screws, drill a small hole in the middle of the plug and use sheet metal screw/slidehammer to remove. The washers and orings tend to stick in the carb when the screw and spring come out. I use a cotter with the tip bent to 'fish em' out. All carbs have a mixture screw. I always remove them when I do carbs with rare exceptions. I prefer the OE gasket sets...they're pretty cheap for 1200's and usually come with everything I need. I've found the standard mixture screw setting to be way lean, more to meet emissions than drivability. I usually start around 3 turns out and fine tune them by ear after a test ride...seems I more often end up around 3.5 turns for the best idle.