Another thing to check is that the shift lever and the parts behind it are installed at the correct angle/orientation. My down-tube shifters are mechanically prevented from moving beyond being parallel to the down tube when pointing forwards. When I was fiddling with it once, I reinstalled it and suddenly the shift lever could keep rotating forwards much furthur, so I took it off the mount again and tried a different orientation for the parts, and this made the lever again stop when parallel with the downtube. If the cable is taught when in the lever is in this fully forward position then, as others have said above, the derailleur should limit the amount that it can move when pulled back to about 90 degrees of rotation.