Mario Solimbergo is having problems with the air motor increasing
speed when the spill valve closes on his Weber Duo-Art.  This valve
is also called the "Atmosphere Intake" valve.

This spill valve on the Duo-Art regulator allows outside air to enter
the regulator.  It relieves the vacuum pump from unnecessary strain
when softer playing levels are required.  Without this valve, it is
possible to stall the electric motor and burn it out.  When louder
playing is needed, this spill is closed allowing a higher vacuum level
to be achieved by the pump.

With the spill value wide open (for soft playing), the vacuum level
at the pump is around 30-35" of water.  When the spill is completely
closed (loud playing), this level can be 45" to 50" or even 65" of
vacuum.  Your air motor regulator sees this change in vacuum and this
is where your problem is.  Your air motor regulator has a leak in it
and it needs to be rebuilt.

The knife valve inside the air motor regulator has two wood surfaces
that regulate how much air passes through the regulator.  If the wood
surface is warped, then unregulated air can pass through the regulator
even though the knife value is completely closed.  Resurface the two
valve faces by lapping them on a glass plate with 400 grit sand paper
and lubricate with graphite and denatured alcohol.  The bottom of the
knife valve assembly is sealed with leather and screwed down with two
screws.  Also make sure this is not where your leak is.

The re-roll port in the motor regulator could also be your problem.
A wood slider covered with thin pouch leather moves across a vacuum
hole.  When in the PLAY position, this hole is sealed closed.  When
put into REWIND, this hole is opened up, which speeds up the air motor
regardless of the tempo setting when rewinding.  If the leather seal
is ripped or torn, unregulated vacuum is sent to the air motor and can
cause the speeding up of the motor when in the play position.

It is 12:00 noon and the thermometer reads 109 degrees F (43 C) in the
shade.

Pete Knobloch (Tempe, AZ)