ain two sets
of marks indicating the position of keyway, as shown in fig. 39, and it
is obvious that we must give the lever less play by screwing up the set
screws shown in fig. 11. The effect of this is to cause the valve to
open earlier and close later than it would if the play were greater; as
it would were the operating portion of cam larger. A minimum amount of
play must always be allowed, however. When two sets of marks are
obtained, the mean must be taken and the keyway cut as shown by the
thick lines in fig. 39. The exhaust cam in larger engines is usually
made with a swelling on the opening portion, as shown in fig. 40, so
that the valve is _very slightly_ opened some time before the crank has
reached the position shown in fig. 41. Fig. 42 shows position of crank
at the close of exhaust valve, and the two last-mentioned diagrams
correspond with the two positions in which the exhaust cam is shown in
fig. 34. The small lump on the back of exhaust cam, fig. 40, is only
required on engines above 3 B.H.P. to relieve the compression on the
compression stroke when starting up. By moving the roller R on valve
lever longitudinally, so that it engages both parts of cam as they pass
in front of it, the exhaust valve is held open during a small portion of
the compression stroke, usually closing when the crank has reached the
bottom centre.

Referring again to fig. 26, this gas or governor cam may be set out, and
the keyway marked on the same principle as already described for the air
and exhaust valves. An end view of the three cams keyed up on the side
shaft is given in fig. 40A. In small engines it is convenient to have
the air and exhaust cams made in one casting, when one key only will be
required. On some engines, instead of employing a movable roller or
valve lever, the exhaust cam is fitted on side shaft with a
"feather"--_i.e._, a headless key--and the cam being capable of
longitudinal movement, such movement being controlled by a small lever
or handle, called the half-compression lever.

[Illustration: FIG. 40.]

[Illustration: FIG. 40A.]

Having once thoroughly grasped the important part the cams play in the
working of the engine, it will be an easy matter to adjust the valve
settings, and to keep them adjusted correctly. The effect of a wrong
setting will then be strikingly apparent. On small engines a separate
cam to operate the gas valve is not a necessity; and the practice of
fitting the gas valve spindl