so when you pull off, is it actually bogging out or sputtering, or is there just no "ooomph"? If the motor is firing fine, then I would tend to agree with the poster above. Even 125s of that era were that way....squat for power except when pushing them. Lots of clutch work to get the RPMs up when leaving a corner and stuff like that.
Having said that, I would think ignition timing or carb. The ignition timing may be such that it is off at low RPMs, but with the natural advance of a faster spinning crank, it comes into phase so to speak. I would almost sneak it forward a bit (the stator plate) and see what you get. How did you set the ignition timing? I do my older points units with a meter, but Ive never come up with a way to do it on a CDI machine except trial and error.