03-I 3
Fuel Svstem 03-l 3
ZENITH MODEL #33 CARBURETOR (Continued)
When the temperature drops below a certain point a
thermostatic switch closes and the heater circuit is
complete. Electric current, supplied by circuit, now flows
through the coil heater. Its electrical resistance to current
flow generates the necessary heat.
Choke Pull-Off
After the engine is started, air flow into the carburetor will
open the choke plate a small amount against the tension of
the thermostatic coil. The choke plate wili open this slight
amount because it is mounted off center on the choke
shaft.
This slight opening of the choke plate is necessary to
prevent an overly rich mixture after the engine starts.
Because the velocity of the air entering the carburetor will
vary with engine speed, the choke plate is linked to a
vacuum operated piston that provides a positive pull
against the closing tension of the thermostatic coil.
PISTON
The action of the piston is called choke pull off, and its
purpose is two-fold.
l
It helps open the choke after the engine starts
l
It controls the position of the choke plate depending on
engine load.
When the throttle is opened to accelerate, the air/fuel
mixture has to be enriched. The action of the accelerator
pump provides momentary enrichment, but additional
richness is required, since the engine is cold. This added
richness is provided by the action of the pull off system.
The opening of the throttle valve causes a drop in intake
manifold vacuum which is transmitted to the diaphragm or
piston. With the lower vacuum signal, the choke coil is able
to move the choke valve towards the closed position. The
amount it is closed depends on how much vacuum drop
occurs during acceleration.
Automatic Choke Setup Procedure
The automatic choke assembly contains a bimetallic
(thermostatic) coil spring which regulates the choke plate
opening at different temperatures, and a vacuum piston
which regulates choke pull-off. The piston exerts a pull
against the closing action of the thermostatic spring. This
helps to open the choke after the engine starts, and
controls the position of the choke plate depending on
engine load.
Adjusting the Choke Pull-Off
1.
Remove the choke housing cover.
2. Remove the cork insulation strip.
3. Remove the inner plate.
4.
Position the sliding pin in the slip link slot away from
the piston.
5. Place a 9.52 mm (0.375 inch) gage pin between the
top of the choke plate and the inside of the air intake.
SET OPENING TO 9.52
MM
(0.375 INCH) GAUGE PIN
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