Hi All, Most men (and some women) understand why a spark plug is fired
prior to top-dead-center (TDC). However, in an air motor the vacuum
must be applied to each bellow shortly after the camshaft lobe for each
bellow passes TDC.
The further past TDC that the vacuum is applied, the lower the torque
will be. If the vacuum is applied before TDC, the air motor will have
a noticeably jerky nature.
The easiest way to insure that the vacuum is applied shortly after TDC
is to apply a medium level of vacuum (15") to the motor and then hold
the camshaft with your hand, preventing it from turning. Then, allow
the camshaft to turn very slowly and watch each bellow as its
associated slider valve moves over its associated vacuum port.
As the cam lobe for each bellow passes TDC you can see the bellows
cloth react to the applied vacuum. Then adjust the slider (up or down)
so that the bellow 'pulls' the camshaft 'forward'. Then, allow the
camshaft to move 180 degrees and check to make sure that the atmosphere
is allowed to enter the bellow before the next bellow starts 'pulling'
the camshaft 'forward'
Another thing to bear in mind is that if you get the 'start' point of
each bellow too close to TDC, the felt bearings in the connecting rod
will wear out much faster.
I realize that the term 'shortly' is not very precise. Frankly, I've
never attempted to measure the actual number of degrees past TDC that
produces optimum performance. I imagine there is such a number, but
I've never seen it in print in any reference material. Most technical
literature relates the optimum position of the slider to the physical
location of the vacuum supply port, which varies from manufacturer to
manufacturer. With a little bit of trial and error, you'll find the
I do know that a well timed (and well rebuilt) air motor will operate
smoothly with the main vacuum hose moved away from the supply flange
as much as 3/4", if the supply vacuum is about 20".
This information appears in the web page at:
John A. Tuttle