I'm confused because that doesn't sound like an HMH vise. There is no "back screw" on the HMH. There is a cam rod adjustment knob that is adjusted each time you change a hook size, (a quick, one handed operation before locking the cam), and a rotation tension knob that can be locked in position or adjusted as needed while tying if rotation is desired.

Here's a link to a video from their website that might be helpful: http://www.hmhvises.com/videos.html

If you're having problems with these adjustments, contact HMH, they stand by their craftsmanship.

(Been using one, with pleasure, for many years).

Bill