Black ones better than the brass ones, brass part is now replaced with synthetic float. It's a good thing you mentioned you have a 750 cause I know probably what the problem is.
Your buddy probably looked at the floats you have now and figured they were a match, they certainly do at first glance. However, what I suspect is that he actually replaced your 750 floats with 500/550 floats, which are slightly WIDER. Hence why you are probably rubbing where you shouldn't be.