I own a '04 1600 classic Kawasaki, bought it about 2 months ago. I is an awesome bike but there is a lot of vibration in the backrest making it very uncomfortable for the passenger. It have counter ballancers in the motor to make it a smooth running bike but still there is the vibration in the backrest. My question is: does anyone out there have the same problem and have you found a solution for it?

Make sure that all the bolts are tight, and that your wheels are correctly balanced.

The thing you have to remember is that the engine is hard bolted to the frame, and each one of those cylinders is 800cc's. It's big, it's going to vibrate, and it's not a car where the engine is hydrolically damped.

What do you mean you installed counterbalancers?

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Of course I'm generalizing from a single example here, but everyone does that. At least I do.

I didn't install the counter ballancers, Kawasaki did in '03 when they came up with the new 1600 motor. Because of it's size they installed bual counter ballancers in the motor to counter act the vibration from the engine and rubber mounted the motor on top of that. Harley Davidson does this to their soft tail line of bike and they have no vibration at all. In fact I have an '07 1200low sportster and it has no vibration on the tail end.
I did put some rubber between the sissybar and the backrest the other day and reduced the vibration about 50%. But I'm wondering why it does it in the first place. At this point I'm seriously considering selling it and getting another Harley soft tail.

As per what Sev first said.... check all nuts and bolts. Not just the seat/sissy bar mounts, all the engine mounts, swingarm pivots, wheels, calipers, covers, bars, pegs, clamps.... everything. But most obviously the engine mounts, a loose or busted engine mount can make all sorts of odd things vibrate. While you're checking them out, make sure that any mounting plates or braces are not cracked, and that the bolts haven't stretched and are being torqued against their own threads, instead of the mounting hardware.

Although, if you did have to sell it and buy a Harley to reduce your vibe problems, I for one, would laugh my "O Ring" off.....

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