[ Editor's note: I have capitalized "Tempo" to emphasize that this
 [ is the name of the speed control lever, not a metronome setting!
 [ The usual definition is that "Tempo 80" means the initial paper
 [ speed is 8.0 feet per minute.  -- Robbie

John,  It should definitely be possible to adjust the Duo-Art air motor
and governor so that Tempo is reasonably calibrated over a wider Tempo
range than 60-90, although it does take a little time and care to do so.
It may be easier to get slightly better calibration with an electric
pump than with foot pumped system, just because a motor-driven pump is
likely to provide a more consistent vacuum.

First, it is important that the motor and governor be properly rebuilt
with airtight pneumatics and well-fitting and properly adjusted valves.
In particular the shaped opening in the slide plate in the governor
should have the correct "arrow-shaped" opening.  This is usually not
a problem for Duo-Arts if you are using the original governor and motor
combination, since the plate is usually made of durable brass, unlike
Ampico governors that were typically made of celluloid that can crack
and crumble.

Also, the wooden slider should be covered with thin, supple leather for
a good seal to the plate, with the leaf spring on the slider should be
adjusted so that the slider seals the plate properly.  Also check that
the plate is properly attached and sealed to the body of the governor,
without leaks at the edges, and that the bleed hole in the plate has an
unobstructed opening to the outside of the governor when the slider is
pushed all the way to the zero Tempo setting.

Tests for the air motor itself have been discussed here, including that
it should lock up and be impossible to turn in reverse with the air
inlet blocked, and that it runs smoothly without limping when powered
by mouth or a small bleed.

To adjust it, I make my own test roll out of any available 88-note
roll I don't mind listening to for a while, marking out a starting
point in pencil near where the music begins, along with marks at
reasonable distances further down the roll, such as at 2', 4', 6', 8',
and 10'.  I've measured out many Tempo sections of commercial test
rolls, and most are perforated quite far off from the correct spacing
for their timing marks.

With the linkage for the Tempo adjusting lever at some reasonable
adjustment so that settings near 70 sound about right, I then run
the roll a couple of times at each nominal Tempo setting: i.e., 50,
60, 70, 80, 90, 100, 110, etc, and time as precisely as possible when
each pencil mark passes.  From this you can calculate the actual Tempo
at each setting.  (For example, if 10' passes in exactly 118 seconds,
the actual Tempo is 50.8 (calculated as: 10/(118/60)*10).

I check to make sure that at least a few of the different marks
agree on the Tempo reasonably well.  (For example, if the 10' mark
passes at 120 seconds, the 8' mark should pass at 94.4 seconds and
the 6' mark to should pass at 70.8 seconds, since (8/(94.4/60)*10) and
(6/(70.8/60)*10) both equal 50.8.  I then plot in Excel or on graph
paper the actual Tempos measured at each setting versus the nominal
setting.  Ideally, this would be a straight line with slope =1, with
points (50,50), (60,60), ... (110,110), etc., but of course it won't be.

The first step is adjusting the governor tension spring (usually
adjusted with leather nuts) so that the plot you get is as linear as
possible.  Don't worry yet if the calibration at 70 or 80 is exactly
right, but focus on getting it so that there is about 10 Tempo units
difference between each setting (especially at the higher Tempo
settings where the slider is more open).  Increasing governor spring
tension will bend your plotted curve one way while generally increasing
the measured Tempos, while decreasing the spring tension will bend the
curve the other way while decreasing the measured Tempos.  The ideal
adjustment will give you a plot near a straight line, but often with
more deviation at the extremes, like for example: (40,54), (50,63),
(60,73), (70,82), (80,92), (90,101), (100,110), (110,118).

As the final step, adjust the Tempo linkage, either at the leather nuts
on the slider at the governor, or with the rod connector to the arrow
indicator, or some combination of both places, so that you can still
adjust the lever and indicator over the whole range, and so that a
setting of 80 gives you exactly 8.0 feet per minute.  With the example
data here, that means shifting the linkage by 12 Tempo units.  Then do
a final check, but with this data you should get a real Tempo=52 at a
setting of 50, 61 at 60, 71 at 70, 80 at 80, etc., on up to 116 at 120.

If slow Tempos are still always too slow on a relative basis regardless
of the governor spring adjustment, you may then need to consider
reshaping the slider plate to open up the slot a bit at the narrow end,
but I've almost never needed to do that.

Best regards,
Ralph Nielsen
http://www.historicpianos.com/