Good idea because that gaining 500 rpm just by installing the air breather suggests something drastic is wrong. As mentioned by others also check all the linkages and choke and such for something interfering with them. Also check the throttle shafts for wear. If the shafts are loose it can suck air just like a vacuum leak.
BTW: my Holley "bible" lists your 6708 as being a universal 650 double pumper. Sounds like an old version of the 4777.