Get yourself a piece of 256 rod threaded on one end and a 256 nylon clevise. Screw the nylon clevise onto the rod and use that end to attach to the carb arm. Use the other end to attach to the servo arm with the fastner that you have. If the set screw does not stay tight when you tighten it up with a screwdriver then apply a small amount of blue loctite to the screw threads and then tighten it up.