I bought a CSR 305 for 500 bucks.  Guy said the carbs needed cleaning.  I could get it to run (barely) for a few seconds.  Cleaned carbs, changed plugs, etc. etc. then I looked in the little window (for adjusting valves) and on the right cylinder, the exhaust valve opens and closes just fine, but the left one barely opens and closes -- it moves, just not much.

I'm thinking about whether I want to bother tearing it apart or parting it out, but I'm wondering what the possible causes of the valve problem might be.  Parts are available and most are cheap, but the camshaft is not (almost worth the price i paid for the whole bike!)  Nobody would confuse me for a mechanic, but I'm fairly brave.

It either has a worn cam lobe, or the rocker is way out of adjustment. Might as well dig in and see what you find.

What have you got to lose? If your going to part it out then you might as well
look at the valve adjustment. You might just luck out and fine something easy
to fix. If not you have not lost much.

Here's something to think about...
The valve may be opening properly but may not be closing. That could be why its hard to start. If that's the case if its been run long like that it could have a burned valve.
Then again, might that model have the same problem with the valves like the early 86 ZGs?
Just sayin'.....

You might as well pull the top and have a look. That is the old 250 bored out to 305. Good luck trying to find a belt for the belt drive.
Here is a manual for you.
http://inkandwood.files.wordpress.com/2009/08/kawasaki_kz_250_-_305_79_a_82_-_service_manual.pdf