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DELCO-ROCHESTER
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Basic
Principles of Carburetion
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idle
conditions, are relatively inefficient. Therefore,
rich mixtures are necessary to cover up these
deficiencies and insure sufficient fuel to all
cylinders for a smooth idle.
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At
this point, the power system comes into operation.
These
speeds are approximate but may be used as a guide
in trouble-shooting, to decide what system is
at fault on a given complaint.
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As
the throttle valve opens, air flow increases and
engine components work more efficiently allowing
use of leaner mixtures. At 2 to 3 lbs. of air
the curve flattens into the economy range of 16
to 17:1 and remains at that point until the air
flow reaches 12 lbs. per minute. At this point,
the curve richens to 13 to 14:1 and at 15 lbs.
per minute power mixtures are obtained. This represents
wide open throttle at maximum engine speed. Beyond
this point no further increase in air flow is
possible and if the engine load is increased,
the speed will gradually decrease. Air flow will
start to diminish as engine speed decreases and
will eventually reach a point where the main metering
ceases and results in a lean engine stall-out.
This flow curve shows the delivery characteristics
for a typical carburetor on a 6 cylinder engine.
The size, type of engine, operating conditions,
and fuel consumption determines the specific air/fuel
ratio. |
PRINCIPLES OF OPERATION
Vacuum (Fig. 7)
Carburetors
of all types operate on the basic principle of
pressure difference.
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Flow
Curve---Carburetor Systems vs. Miles per Hour
The typical
flow curve (Fig. 6) shows the approximate speeds
at which the various carburetor systems may operate.
The idle system functions alone up to approximately
25 MPH. Between 25-40 MPH the mixture is supplied
by both the idle and the main metering system.
This part of the curve is called the transfer
range. From 40-60 MPH the mixture is supplied
by the main metering system and will continue
until the manifold vacuum drops below a pre-determined
point. (approximately 9" of vacuum). |
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Any
pressure less than atmospheric is considered a
vacuum or a low pressure area. When a straw is
sucked on to obtain liquid from a container. (Fig.
7), a vacuum is produced in the mouth. Atmospheric |
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