I have rebuilt a number of these wonderful motors and I feel that
they are probably more efficient than any other wind motor when
properly done.  You should be able to inhale gently on the supply
and spin that sucker!

I don't want to go in to the ongoing mystery of how Lauter dished
those green cloth pouches.  I use pneumatic cloth.  You don't say if
the motor was rebuilt or not, and if the pouches are supple and free.

That not withstanding, in my opinion, the slide valve side play is
not an issue.  Is the motor mounted properly, tilted back?  Is the
small leather insert between the halves of the slides valves intact
and tight?  Is the cloth on the valves air-tight?

There is a leather gasket between the valve ports and the body of the
motor, sometimes gets overlooked in a rebuild.  You could have leaks
in other parts of the system..  Valve block gaskets, valves, governor.
Isolate and check each component for leaks.

I have a Lauter player sitting in my living room.  Can offer any help
or pictures if you need them.

Arthur Marino