Duaine, I suspect that you will get a number of replies to your
question, so my response may not be that useful.  But I have certainly
had some experience with Ampico A Tempo regulators and their issues.
The symptoms you describe, of it "going from 0 to full speed" in the
first little bit of control lever travel "from 40 to 80" or so is very
typical of the most common failure mode, although a couple of other
rarer adjustment issues might also cause the same symptoms.

In the most common configuration, the control box is mounted at the
rear of the right drawer compartment, spanning almost the full width
of the space.  Removing it isn't very simple, since there isn't enough
clearance to remove it from the installed drawer.

For me the easiest method is to remove the wheeled drawer mounts from
one side under the keyboard that support the glide rails, and lower the
entire drawer a few inches to some stable support like a padded piano
bench, so that the control box can be lifted out vertically without
disconnecting all of the tubing harness connections or the electrical
connections for the motor switch.

It sometimes helps to disconnect the swinging stabilizer brace that
connects to the sides of the drawer, but that isn't always necessary.
It is also usually necessary to disconnect and remove the Tempo lever
assembly mounted to the drawer bottom.  And it helps to loosen and
shift some of the nearby components like the tracker pneumatics, even
if they don't need to be removed entirely, to get clearance to
disconnect the tubing connections.

Usually the left two of three large tubes near the treble end of
the drawer are the supply vacuum to the control box, while the third
supplies the reroll, tracking and other functions in the drawer.
A single large tube from control box runs behind the tracker bar to
the air motor in the left chamber.  If tubing is stiff or brittle,
disconnecting these can be a challenge.

Usually two horizontal mounting screws are deeply recessed in the holes
in the top board of the Tempo control box, and they screw into the rear
wall of the drawer.  The fairly long bent hose nipples project toward
the back, so the whole box needs to pivot forward before it can be
lifted out of the drawer.

It is very common for the original Tempo control plate with its
"arrow shaped" opening to fail in Ampicos, since it was typically made
of unstable celluloid plastic, rather than the more stable brass of
Duo-Art and most other Tempo regulators.  The Tempo control lever moves
a leather-faced slider block across the face of the plate, with a leaf
spring supplying a little pressure to hold it in place.  When the plate
cracks or crumbles with age, the slider opens up too much air channel
in the first bit of travel, giving the symptoms you describe.

It is also common for the rough edges of the broken plate to cut or
damage the leather of the slider or even the wood underneath.  It might
also be possible that the control pneumatic spring tension is very
badly adjusted so that excessive supply vacuum behind the plate results
in too much air flow through an intact plate, but that is much less
likely than plate failure.

As far as I know, there are no current commercial supplier of
replacement Tempo plates, so I make my own out of brass shim stock or
out of high-quality plastic like keytop material.  I have photographs
or scale drawings I could send you, along with detailed measurements
from several originals if this one is too damaged to figure out how it
was shaped.  The plate also needs a bleed hole in the proper location
to vent the chamber to air and avoid motor creep at the Tempo=0
setting.

Ralph Nielsen
http://www.historicpianos.com/