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mine vibrates pretty hard when im on the gas hard. (not like the sportster i once had but...) the other 4 cyl. bikes ive had have been extremely smooth so i dont know if somethings not right or thats just how this bike is. maybe the carbs need to be synched?

Try to find out where the vibration is coming from. Does it buzz when you rev it up in neutral?
Check your engine mounts.
Also, lots of other stuff can rattle on the bike. My right mirror makes a helluva racket at 5000 rpm. Brake and clutch levers are notorious for rattling. Foot pegs, battery box, just about anything thats attached to the frame can buzz at certain engine rpms.

im pretty sure its something that happens only under load but ill check to be sure.

i think i will also add that my chain has a few "kinks" where it is not very adjustable. i just bought a new chain and a set of sprockets, just havent put them on yet. ive heard that a poor chain and some sprocket wear can cause the vibes.

When was the last time the carbs were synchronized? My 650 gets buzzy enough over 5500 RPM to cloud the mirrors. but is otherwise quite smooth.

Are the wheels in balance?

Anything that rotates is a suspect. The question is whether the same vibration is present on the road or stopped at a given RPM. That will tell you where to look.

Yes, a kinky chain might be the problem. If it just happens under load thought it may be a slipping clutch. Have you checked your adjustment lately?

My 650 is very smooth. If your engine is running properly check your motor mounts.
Also carb synch is only going to effect your lower throttle settings such as idle and right off throttle.

My 77 KZ650 is very smooth at all speeds. I would suggest replacing a badly kinked chain to start with, and the sprockets. Make sure the wheels are properly aligned, the chain tension is set correctly and make sure the wheels are balanced and your tires aren't out of round (or a rim is dented). My old girl with Progressive Suspension front and rear rides smoother than some more current bikes I've sampled!