When I had timed the air motor in my last restoration, I tested it
with a vacuum cleaner, holding the vacuum hose an inch or so away from
the air motor suction tube.  I'd adjust the distance from the vacuum
cleaner hose to the suction hose until I got the vacuum so low that the
air motor would just "creep", barely moving.  I was dismayed that the
air motor appeared jerky.  No way I could make it smooth, no matter
what I did.

I finally decided that my test technique was flawed, because if I just
sucked on the air motor suction line, very lightly, the air motor was
absolutely smooth, even at the slowest speeds.   The difference in
techniques is that the former method is not a positive displacement
technique.  When you suck on the suction with no leakage path, the
motor response is smooth.  The message here is that the way you test
your air motor affects how it performs.

Ray Fairfield