Motorcycle Repair: carb problem, suzuki gs 650, air intakes

Question
I have been told by several people that most motorcycle carbs work on the same principals.  I know that you are a honda person, but if most carbs work the same, perhaps you could help me.  I have a 1982 suzuki gs 650.  It has four carbs, and an adjustment knob in between the two middle carbs.  The bike accelerates and idles fine, but when I am going down the interstate, it chugs going up hills, and I can barely get it to go 70.  The only time I have this problem is when I am between 3/4 throttle and full throttle. What could cause this, and how can I fix it?

Answer
This could be carbs, but could also be bad air intakes, false air (perished carb inlet mounting rubber) or indeed even a blocked exhaust (less likely but possible if there is corrosion inside)  Have the carbs been messed with, has somebody changed the jets?  If so I'd go back to stock jets first.  Good luck,  Jan