i have the same problem with my 89 600 its starts and idles perfect with no load on the motor rev good up to 7,000 cuts out for a bit and then revs to 11000 when a load on the motor i can only just barely touch the throttle or it just plain slows down, and the when riding it sounds like its misfiring up to 4000 rpm then cuts in and out forcing me to shift at 4000 and i can only top out at 50, i know nothing about bikes i know about cars but me and my buddy are taking apart my carbs today let ya know if theres anything that could help you

 

rubbers are cracked, and you may not be clean. hehe, so check again.