Maybe one with a blade that's, say, 3 - 4 times the width of eyeglass screwdriver blades. If it fits into the hole in the carb casting, it'll probably be the right size. Get a short one, since there isn't much room under the carbs to move around. The screw isn't hard to turn. BTW - I'd suggest wearing some sort of heat proof gloves, since you'll be making your adjustments with a warm (hot) engine. I use an old pair of summer weight riding gloves.