I cannot say for certain if this is your problem, but I helped a friend out with the Amal concentric on his BSA, suffering from a sticking slide. The cause of the problem was that the body was slightly distorted from a "professional" who prevously worked on it overtightened the flange mounting bolts. I successfully squeezed the distortion back out gently with a vise, but this can be a risky thing to do. My concern about filing "high" spots is it accelerating the wear. Good luck!