Hey ziggy,
For a 4t (fore stroke) I would keep the RPM's no more then 8K.
Now you'r not going to blow it up at 8K-- but if you ride it for long at 8k.. you might be walking.
6k is ideal at full WOT, on a flat/slight upgrade, given your weight is around 120-160.
And no fat girlfriends on the back with you, trust me i know.... errrr and so did my friends at work and it was ONLY a sunday ride but come work day monday.... well never mind.
Just kiddin about the sunday ride it really was saterday