when you had it apart , you say the flap was sticking , firstly were there any burs on the brass flap or any sideways movement as posibly sticking on the sides of the alloy casing , after i modified my amf one and had it welded up i noticed my flap would stick slightly at fully closed position , on inspection i found a tiny bur on the outer edge prob from me porting out the insides i must of knocked against it and being brass its very soft , i used some wet and dry and curved the whole flaps edges very slightly and it cured my issue .. may be of some help to you