A few items to look at that tend to age poorly with the SOHC CB650s, as long as the battery is in great condition:
Ground connections. This is the first thing to look at, and often the most overlooked. Clean them up and make them SHINY.
Starter solenoid. It might not look worn out because all of the parts that wear out are internal. They're cheap to replace, and doing so with a modern unit will get you a modern fuse instead of the original kind.
After this, a valve adjustment done too tightly can definitely affect hot starting condition to some degree.
Lastly, the starter itself might be in rough shape. Actually having to replace starter brushes is less common than a lot of folks think, but there's more to the starter than brushes. I actually had to replace the internal copper bus bar that takes the solenoid connection to the windings. It is soldered to the inside of the bolt that you tighten the cable nut onto, and that had broken off internally. In opening the starter, I cleaned the ever living #$%* out of it and replaced the brushes while I was in there.
So, to summarize: clean every ground connection and battery connection, consider replacing an original starter solenoid, check valve adjustment to make sure they aren't too tight, and then look to see that your starter is in serviceable shape.
