Hi, the OS 5B carb is basically the same as the 7B. It will work in the 60FSR. I think you might have a few problems with your carb. I will give you a few extra things to check out.
#1: remove the whole needle valve assembly and look in the end where you removed the assembly. There should be a small brass hole. Make sure it is completely round and hasn't been damaged. If it is okay replace assembly.
#2: Put a piece of tubing on the fuel inlet and blow air from your mouth into the carb. If you cover both ends of the carb, as well as the threaded mounting holes to see if there is an air leak somewhere. Another thing to try is to turn the mixture screw all the way in and blow again into the tube to see if the needle valve is still working. Back off the needle valve until air starts to move.
#3: Make sure that the 2 mounting screws are secured and not leaking. One guy at my club had major problems and that's what it was.
Try these few things and let me know the results. Also, email me and I can send you the operating instructions for the 7B carb. I might have a few spare parts around as well. Good luck and all the best..... Bob