Well Ray;
It sounds like any of the following:
Carb adjusted to lean or to rich.
Vacuum line to advancer leaking
Obstructed fuel line
Bad plugs - or wrong heat range
Plug wires - or ends are not tight - If your using original plug wire ends
check to see if the wire inside (the one that holds the plug upper threads)
is used to hold in place might not be holding them all.
Rich carb fouling spark
Point breaker plate has slop - which is allowing the points to reach their
highest rpm level sooner than a tight (no slop) breaker plate will give.
Rocker arms have a flat spot - Causing a timing slump at speed
Cam teeth are becoming low in the grooves between the teeth - causing the
engine to be out of time before 6K rpm's.
Poor Ground
Weak Coil
Excessive carbon in cylinders
Valves sticking
Broken inner valve spring
Fuel Pump not holding consistently.
Vent hole in fuel tank cap not open and venting.
I know this is a lot of things to look at, but some like plugs and grounds
show up during idling speeds too. Normally a well tuned Honda 600 engine
will rev to rpm levels of 7.5K before dropping off. Unless you are using
CB100 points, then the acceleration arc becomes lower.
Bill