TOPIC:

I'm in the final stages of my engine rebuild on my 1981 GPZ550.

I examined the clutch plates and replaced bad. Cover back on.

New clutch cable. Pivot inside cover inspected and re-greased.

When cable is put on initially there is no pull tension at all. I take all of the adjustment out of the cable by adjusting it and finally there is some tension, clutch cable does not hang limp as ... well.

So, then I give it one pull on the lever to the bar. Something goes BONK and now there's little to no cable tension.

Thoughts?

 

Sounds like you have the actuation ramp mechanism incorrectly indexed.  Position it like shown below.  The actuator arm should be at the base of the ramp, not already traveling up.  Then you adjust the mechanism and cable per the manual.  If you don't have a factory manual, you can download one here for free.



 

In case you didn't install it, the cotter pin helps to keep the cable end seated. 
Some manuals are ambiguous about what direction the clutch adjusting screw should be turned; some models are clockwise first until hard to turn, but others are counterclockwise first.  From the loudhvx motorcycle stuff page: s3.amazonaws.com/gpzweb/ClutchReleaseMech.GIF
The owner's manual and factory service manual for my motorcycle specify turning it counterclockwise first, then clockwise 1/4 turn.