as
valve.

This behaviour is very undesirable, as the small quantity of gas so
admitted to the cylinder is quite useless, and a sheer waste is
incurred. With the governing arrangement shown in fig. 26, this trouble
does not exist. The cam is so designed that the first rise from X to A
determines whether or not the valve is to be opened; the curve from A to
Y is struck from the centre of the side shaft; thus, during that
portion of the revolution the arm L is stationary, and the pecker at the
same instant takes up a definite position either in the notch in B or on
top of it, and is ready to open the valve if the speed of the engine is
such as to require an explosion, or simply to slide over the top of B,
allowing the valve to remain closed. It is most interesting to observe
the action of this governor; when an engine fitted with one is running
very slowly, the three distinct movements of the pecker P may be clearly
discerned as the respective portions of the cam pass over the small
roller R.

[Illustration: FIG. 27.]

CHAPTER VII

CAMS AND VALVE SETTINGS

With the gas, as with any other kind of engine, the valve settings are
of primary importance. On very small engines it is often the case that
only the exhaust valve is operated mechanically.

Again, there are several well-known makes which operate the gas and
exhaust mechanically while the air valve is opened by suction alone.
Though opinions differ as to which is the best course to take, there can
be little doubt that, with all three valves mechanically operated, a
greater nicety of adjustment is obtainable than would be otherwise
possible. And provided the working parts are neatly made and finished,
they will take but little power to drive them; and such loss would be
compensated by the additional power and efficiency obtained from the
engine, due to satisfactory and correct adjustment.

In fig. 28 we give a diagram showing the exact positions of the crank
when the gas, air, and exhaust valves open and close respectively, under
normal conditions of working. The solid circle represents the first
revolution of the crank shaft, starting from the commencement of the
suction stroke, and the dotted circle the second revolution, during
which the explosion and exhaust strokes take place; the dotted
horizontal line shows the position of crank at the back and front dead
centres.

As a clear conception of why certain things happen under certain
conditions is