TM 5-4240-501-14&P
THEORIES OF OPERATION
Carburetion
Airfoil
Now, what is an airfoil ? Here is a picture of a tube in an
air stream. When still, the pressure is equal on all sides.
Under movement, an air pattern is formed, Fig. 14, so
that we have a high pressure area and a very low
pressure area.
Figure 14.
Now how does all this apply to Briggs & Stratton engines
that employe three types of carburetors, the Flo-jet
(gravity feed or float type), the Vacu-jet (suction feed)
and the newer Pulsa-jet (fuel pump) type?
FLOW-JET CARBURETORS OR GRAVITY FEED
First, let us consider the gravity feed system. The tank is
above the carburetor and fuel flows by gravity. Notice an
air vent hole in the tank cap so that air can flow in as fuel
flows out and a vent hole in the carburetor bowl so that
air can flow out as fuel flows in. If one or both of these
holes were plugged, the flow of fuel would cease and
stop the engine. See Fig. 15 and 16.
Figure 15.
Figure 16.
As the fuel enters the bowl, it raises the float. The float in
turn raises the needle in the float valve. When the
needle touches the seat, it shuts off the fuel flow, and the
position of the float at this time is called the float level.
Float Level
The float level in general should be high enough to afford
an ample supply of fuel at full throttle and low enough to
prevent flooding or leaking.
To set the level on the carburetor, invert the upper body
as shown. See Fig. 16. The float and the body cover
should be parallel. If not, bend the tang on the float to
obtain this position. The actual distance on the small
carburetors is 5/16 of an inch between the float and the
gasket. On the larger models it is 3/16 of an inch. It is
seldom necessary to measure this distance. The float
level is not as critical as on some carburetors.
Remember, however, that there should be one gasket
between the float valve seat and the carburetor. No
gasket or two gaskets will change the float level.
Now, the fuel is down into the bowl but how does it get
into the cylinder?
Here is shown the position of the nozzle and the fuel
level. See Fig. 16. The fuel in the bowl seeks its own
level, which is well below the discharge holes. Notice
that the discharge holes are in the venturi, the place of
greatest air velocity. As the piston in the cylinder
moves down with the intake valve open, it creates a low
pressure area that extends down into the carburetor
throat and venturi. Two things start to happen.
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