TM 5-4240-501-14&P
Since no accelerator pump is used on this carburetor
and since many of these engines are used on lawn
mowers where rapid acceleration is needed, the mixture
should be rich. Turn the needle valve in until the engine
begins to lose speed, indicating a lean mixture. Then,
open the needle valve past the point of smooth operation
until the engine just begins to run unevenly. Since this
setting is made without load, the mixture should operate
the engine satisfactorily under load.
These carburetors do not have an idle valve, but the
mixture at idle speed is controlled in a different way. As
the throttle closes to idle, the leading edge takes a
position between the two discharge holes. The larger of
the discharge holes is now in the high pressure area, and
the flow of fuel through it will cease. The small hole will
continue to discharge fuel but the amount will be
metered by the hole size and will be in proportion to the
reduced air flow. For this reason it is important that the
small discharge hole be of the proper size. The needle
valve will allow much more fuel to pass than should go
through the small discharge hole. A number 68 drill can
be used as a plug gauge to check the small hole. A
number 56 drill can be used to check the larger hole.
This can be done with the needle valve and seat
removed. See Fig. 20.
Figure 20.
THEORIES OF OPERATION
Carburetion
You will notice a small section is milled out of the throttle
where it meets the discharge hole. This concentrates the
flow of air past the hole and assures good vaporization.
The idle speed adjusting screw should be set to obtain
an idle speed of 1750 RPM. This may seem fast to
people accustomed to auto engines, but it is necessary
in order to have fast acceleration. It also helps cooling
and lubrication.
A slight unevenness may be noticed at idle speed, but
this is normal and no readjustments of the needle valve
should be made.
The choke is the sliding plate mounted at the outer end
of the carburetor. Fig. 20 and 21. The choke is pushed
in to close the air intake for starting but should be pulled
out as soon as the engine starts. The use of this choke
should be understood clearly. Many complaints of
engine trouble, upon investigation prove to be nothing
more than failure to properly use the choke, especially
where the choke is operated by a remote control. The
choke must close fully.
Figure 21.
The latest engines with Vacu-Jet carburetors incorporate
a ball check in the fuel pipe which assures a steady flow
of fuel to the needle valve and discharge holes.
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