3-06
Fuel System
3-06
POWER ENRICHMENT SYSTEM MODEL 1940 (FIG. 7)
The power enrichment system consists of a power valve Installed near
the center of the carburetor body and a vacuum piston installed in the
bowl cover. A vacuum passage leads from the top of the piston down
to the manifold flange.
When the manifold vacuum is high, the vacuum piston is raised
to the top of its cylinder and the spring on the piston is compressed.
When the manifold vacuum drops to a predetermined level. the
spring overcomes the vacuum and pushes the piston stem down. The
piston stem In turn pushes the power valve down. opening the power
*valve and permitting fuel to flow through the power valve. through the
power valve channel restriction and into the main well located near the
power valve.
The power valve originally used in the model 1940 is a three-
piece valve sold as an assembly Later model 1940 carburetors used a
one-piece, two-stage power valve (Figure 7).
FIG. 5. Idle System. Model 1940
MAIN METERING SYSTEM (FIG. 6)
As the engine approaches cruising speed. the increased air flow
through the ,venturi creates a greater vacuum (low pleasure area, in
the venturi of the carburetor Near atmospheric pressure present In the
bowl above the fuel causes the fuel to flow to the lower pressure area
created by the venturi and is magnified by the dual booster venturi.
Fuel flows through the main jet into the main w ell. air enters
through the main well air bleed and into the main well through holes in
the main well tube The mixture of air and fuel being lighter than raw
fuel responds faster to changes in venturi vacuum and is also more
readily vaporized when discharged into the venturi.
The main discharge nozzle passage is a part of the dual booster
venturi. which is an integral part of the main body casting. Distribution
tabs in the main venturi provide the proper distribution of the air-fuel
mixture between cylinders for specific engine applications
FIG. 7. Power Enrichment System
The main metering system is calibrated to deliver a lean mixture for
best overall economy. When additional power is required, a vacuum-
operated power system enriches the air-fuel mixture.
ACCELERATOR PUMP SYSTEM (FIG. 8)
When the throttle plates are opened suddenly, the air flow
through the carburetor increases almost immediately; however. there
is a brief time interval or lag before the fuel can overcome its inertia
and attain required flow to maintain the desired air-fuel ratio.
The piston type accelerating pump system mechanically
supplies the fuel necessary to overcome this deficiency for a short
period of time.
Fuel enters the pump cylinder from the fuel bowl through the
pump cup stem clearance hole when the pump is lifted to a refill
position. The fuel level is above the normal position of the pump
piston. This is known as a wet pump system.
As the throttle lever is moved. the pump link operating through
a system of levers and a drive spring, pushes the pump piston down
seating the pump cup against the face of the stem. Fuel is forced
through a passage around the pump discharge jet which is drilled in
the main body.
When the pump is not in operation, vapors or bubbles forming in
the pump cylinder can escape through the stem clearance hole of the
floating piston cup and past the pump stem.
FIG. 6. Main Metering System. Model 1940
