Z4000 repaired!
Once again thanks for all of your replies. Turns out the right handle worked itself loose and only partially engaged the throttle gears. Once that happened the potentiometer would not rotate to the zero position. The Z4000 would turn on but not leave the "short" status.
This Z has set screws to adjust the handle tightness on the underside of the case. I guess one had worked itself loose and allowed the gears to slip - causing the problem. 
The contact between these two white gears is adjustable through the set screw. Backing it out most of the way I was able to rotate the smaller gear to a zero or off position without moving the handle gear. Once set, the handle was rotated to the off position and the set screw re-adjusted bringing the gears back together to mesh. Since both potentiometers were now at zero, the Z started up normally. Funny how something so simple causes the Z to stop working - I suppose its a safety feature. In my opinion this gear design provides a lot of sloppiness in fit and ensures the pot will eventually work its way into a non-zero position and show a fault. Expensive to ship and repair - and unnecessary since nothing is really wrong with the transformer.
CAUTION - as mentioned in the comments, the wiring harnesses do not leave a lot of room to move things around, be careful in lifting off the cover. While you can disconnect the harnesses from the cover you really do not need to do so in order to re-align the pot gear. You will need a long hex key (allen wrench) to remove the 7 screws holding the cover on. Mine were barely finger tight. Remember to pull the power plug before opening this up. I put the cover back on without the screws to make sure the pots were zeroed, powered it up to make sure it worked, pulled the plug and then screwed the cover back on. Also be careful on handling the cover and base, I find it is brittle and found some pieces broken off internally.
Thanks to the following for their help
