Knobs, or screws? Usually there are two small screws that control how far the derailer will travel for high gear and low gear. There's usually a little "H" and "L" marked next to each of these. These settings should be tuned so that you can change to any gear combination, but not shift so far that the chain pops off. If the settings are too far in one direction, the chain may never change to one of the gears. If the settings are too far in the other direction, the chain can change and keep going past the gear and pop off.
I hope this helps. If this is what you're talking about, write back and I (or maybe someone else) can provide some more detail on how to tune these. Although your bike's owners manual (if you have one) should tell you how to do this.
X_HOBBES
"mp3p0" <bob3823@cox.net> wrote in message news:1113898969.00ac56521f41b30c5d51b9a6f9bbaa2b@teranews...>
