Crazypj, that's not always true.
Most CV slides "bounce" at idle, feeding some extra fuel in the process. Look for yourself, its easily visible. There are some carbs with heavy slides that don't of course, british strombergs for example.
A common Ninja 250 problem is where it will suddenly jump from idle to a high rpm and hang there if the idle circuit is lean.
This happens because the CV slide bounces, it suddenly runs richer, all cylinders start firing, and it hangs there (with the slide partly open).
The op's Kawasaki may be doing this. If so, try running a thin guitar e string through the carb's idle circuits to clear things out.