The CB650 is the red-headed stepchild because it was changed every year of its 4-year production run. Because of the relatively small production run and constant design changes, aftermarket parts and OEM replacement parts are kind of hard to come by. It also features a couple of half-steps between the "old school" tech of the CB750 and the new tech found on the DOHCs that follow the SOHC lines. The CB650 is considered by some to be the best engine of the SOHC4s because it incorporated newer technology and despite having a smaller displacement than the CB750, it has as much HP and torque as its big brother.
A couple things that make the CB650 unique among the SOHC line include the transistorized ignition - no points, but not quite fully electronic. It also features a hy-vo timing chain usually found on car engines versus the roller-link style in the other SOHCs. The carbs on the 81 and 82 models is a CV style, which was the first and only time they appeared on SOHC. 79 and 80 models used a mechanical slide like the 77-78 CB750 models. I'm sure there's other stuff, but that's what comes to mind right away for me as to what makes the CB650 the red-headed stepchild of the SOHC family.
