I have never had the feed dog problem you have, but I assume you have to screw the stitch length adjuster knob all the way in, and that should give maximum movement of the feed dogs. After a quick look under my 99 (1934) I can only detect one screw involved with the feed dog part? The feed dog sort of rolles in a metal fork thing that turns on the movment of the rod there. Sorry amout my terminology, but I am interested in the old type 99.