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ENGINE DIVISION SERVICE MANUAL
TM 5-4210-230-14&P-1
GENERAL
Power Enrichment System
The enriched air/fuel mixture required under heavy load
conditions is provided by a supplementary fuel metering system
and the power valve which is controlled by manifold vacuum.
Since manifold vacuum is proportional to engine load, the
enrichment can be triggered at any required point in the load
range. Some versions of this carburetor contain a two-stage
power valve which can be calibrated to enrich the air/fuel ratio
in two increments in the load range in order to provide more
precise calibration where required for emission control (Fig.
10).
The power valve in this carburetor is a diaphragm
actuated, spring loaded conical valve normally closed under
high manifold vacuum conditions (cruise or idle) by the force of
the vacuum acting on the diaphragm. When the manifold
vacuum is reduced to a given level (acceleration or load), the
spring force will open the valve, providing a path for fuel to flow
from the fuel bowl to the main metering well, bypassing the
main metering jet. As engine load decreases, manifold vacuum
increases and the valve returns to the closed position,
eliminating the supplementary fuel.
Figure 10 Power Enrichment Fuel Air Flow
1. Main Well
6. Power Valve Diaphragm
2. Main Jet
7. Main Well
3. Fuel Flow
8. Channel Restriction
4. Metering Body
9. Spring
5. Power Valve Assembly
Two-Stage Power Valve
The added supply of fuel necessary for high speed or full
power operation is delivered by a fully automatic power
enrichment system. Manifold vacuum on the power valve
diaphragm actuates the power enrichment system.
To provide a leaner light load and light part-throttle
operation, a twostage diaphragm-actuated power valve is used
in some model series carburetors (Fig. 11).
During light load and light part throttle operation where
manifold vacuum drops to a pre-determined level, the first stage
of the power valve opens and enriches the mixture slightly. As
manifold vacuum decreases on acceleration or steeper grades,
the second stage opens, providing full power enrichment. The
two stage power valve is serviced as an assembly and is not
adjustable in the field.
Figure 11 Two-Stage Power Valve
1. First Stage Orifice
2. Main Passage
3. Main Stage
Power Valve Modulator
The power valve modulator is incorporated in the power
valve vacuum system to ensure fuel enrichment during high
speed, wide open throttle operation. As wide open throttle
engine speed approaches the maximum engine RPM, intake
manifold vacuum may rise to a level sufficient to close the
second stage of the power valve, thereby reducing fuel flow and
engine power. The power valve modulator system is designed
to bleed off power valve
CGES-125-T Page 8
PRINTED IN UNITED STATES OF AMERICA
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