kz 750 twin excessive engine vibration

[ndali]

I recently purchased an 82 KZ 750 csi twin and discovered bad vibration just below 4k rpm or 50 mph increasing exponentially; the vibration from the engine gets too bad to ride for longer then 20 minutes. I would like to get confirmation from another owner of the z750 or kz 750 twin that this just isn't a poor unbalanced engine design (which I can't believe).

If it is something with the engine then some suggestions would be appreciated.

I have cleaned the carbs, spark is fine, engine runs fine and starts up easily. Timing is a little off but can't adjust it on this model, engine mounts are solid, valve clearance is appropriate. The bike only has 7k miles, so not much wear.

I work with a bunch of bikers and guys with more than enough mechanical experience and we are all stumped. Any suggestions or own riding experience with this bike would be great.

[clifftt]

Sounds like you may have a probleem. I have an old KZ750 twin which idles pretty smooth. The engine was designed with a counter-turning balance shaft (I believe) which counters the otherwise occuring vibration....not so great for a race motor, but for street, the engine runs pretty smooth. Might check in the "Vintage" section of this site, ask "kz_750_cafe". He seems pretty knowledgeable about his bike and rides it frequently, probably way more than me! He can give you his first hand account, too!