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ROCHESTER CARBURETORS Bulletin 9D-8 August, 1951 |
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Bulletin 9D-S
August. 1951
Model "BC"
Page 7
ROCHESTER PRODUCTS, DIVISION OF GENERAL MOTORS.
ROCHESTER, NEW YORK
The pump plunger head has been
designed to eliminate fuel percolation in the pump system. This has been
accomplished by the unique design of a ball check and seat in the plunger
head. When the engine is not operating, any build up of fuel vapors in the
pump well rise and by-pass the ball. This allows the hot fuel and vapors
to circulate up the passage in the plunger head and return to the float
bowl. Without this feature, any vapor pressure built up would evacuate the
fuel in the pump system into the engine manifold, causing poor initial
acceleration due to lack of fuel in the pump system as well as difficult
hot weather starting. |
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CHOKE SYSTEM
To insure proper starting and
driving during cold weather operation, the Model "BC" Carburetor
employs a fully automatic choke. (As shown in Figure 7.) This choke system
is composed of a thermostatic coil, choke piston, choke valve and fast
idle cam and linkage. It is controlled by a combination of intake manifold
vacuum, the offset choke valve, atmospheric temperature, and exhaust
manifold heat.
The thermostatic coil, which is
linked to the choke valve shaft, holds the choke valve closed when the
engine is cold. As the engine is started, air velocity against the
offset choke valve causes the valve to open slightly against the torque of
the thermostatic coil. In addition, as the engine starts, intake
manifold vacuum is applied to the choke piston. which also tends to
pull the choke valve open. |
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Figure 7. Choke System |
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As a consequence, the choke valve
assumes a position where the torque of the thermostatic coil is balanced
against the vacuum pull upon the choke piston and air velocity against the
offset choke valve, thereby causing a regulated air flow into the
carburetor which provides a proper mixture during the warm-up
period.
During warm-up, the choke piston
serves to modify the choking action to compensate for varying engine loads
or acceleration. Any acceleration or increased road load decreases the
vacuum exerted on the choke piston. This allows the thermostatic coil
torque to momentarily increase choke valve closure to provide the engine
with a sufficiently richer mixture for acceleration.
As the engine warms up, hot air
from the exhaust manifold "stove" is drawn into the ther-mostatic coil
cover by the vacuum behind the choke piston. This hot air causes a rise in
temperature which causes the coil to slowly relax its tension. Thus the
choke valve is allowed to move gradually to the full open
position. |
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