If they are the ones I’m thinking of they will be JIS - Japanese Industry Standard. If you haven’t got a JIS screwdriver already, I recommend you get one and use it in conjunction with the penetrating oil, and patience. I found the JIS made a difference. When they engage in the screwhead to undo they sometimes make a “click”. On my ‘96 carbs I bought one of those JIS screwdriver bits that fit into a holder which then fits a 5/16” t-handle....plenty of solid but careful leverage.