Not quite sure I understand your description about the size of the top loop but it sounds like it's wrong for your bow. Many years ago, I bought a really nice heavy weight Wing Tbird and it came with a string in decent condition, nocking point in the right place, etc..... obviously the guy had used it on the bow. I saw that the upper loop was way too big for the nock, but I couldn't resist shooting a couple of shots before twisting up a new string. Well, the FIRST SHOT that top loop came out of the groove(on recovery) and the bow unstrung! No damage, thank goodness, but that string went right in the trash.
Every old bow I take in gets a new string, for several reasons, but one is that the loops will be the right size for that bow.