I can't be 100% sure, but I seem to recall ALL CB650 models had twin discs on the front as standard, which means a previous owner has probably removed the second disc/caliper as a cheap bodge as opposed to repairing properly, as brake parts can be pricey. If that is the case, that would explain the poor braking, as would leaving a twin disc master cylinder on the bike with only one disc fitted, lots of pull required at the bars, for relatively little braking force. Also, bikes designed for twin discs often have flimsy mudguards, which have little or no bracing action when the brake is applied. Fine if both calipers are set up and working, but with only one disc fitted the forks will want to twist under braking, and feel horrible. My good lady has a CB650SC (nighthawk) also an 82 model, and that has twin discs.