Pontiac FIREBIRD 1972 Service Manual page 717

ENGINE FUEL 6B-63
NOTE: Plug the compensator hole with some-
thing that will be seen, as the plug must be
removed before the air cleaner is installed. 0th-
erwise, the compensator will not function if the
plug is left in the hole.
MAlN METERING SYSTEM
The main metering system (Fig. 6B-78) supplies fuel to the
engine from off-idle to wide open throttle operation. The
two primary bores of the carburetor meter fuel through
the venturi principle. This type design allows the use of
multiple venturi for finer and more stable metering con-
trol during light engine loads.
The main metering system is in operation at all times
when air flow through the venturi is high enough to main-
tain efficient fuel flow from the main fuel discharge noz-
zles. The main metering system begins to feed fuel when
the idle system can no longer meet the engine require-
ments.
The main metering system consists of main metering jets,
vacuum operated metering rods, main fuel well, main well
air bleeds, fuel discharge nozzles and triple venturi. The
system operates as follows:
During cruising speeds and light engine loads, engine
manifold vacuum is high. Manifold vacuum holds the
main metering rods down in the main metering jets against
spring tension. Manifold vacuum is supplied through a
channel to the vacuum operated power piston connected
to the primary main metering rods. Fuel flow from the
float bowl is metered between the metering rods and main
metering jet orifice.
As the primary throttle valves are opened beyond the
off-idle range allowing more air to enter the engine mani-
fold, air velocity increases in the carburetor venturi. This
causes a drop in pressure in the large venturi which is
increased many times in the double boost venturi. Since
the low pressure (vacuum) is now in the smallest boost
venturi, fuel flows from the main discharge nozzles as
follows:
Fuel flows from the float bowl through the main metering
jets into the main fuel wells and is mixed with air from the
adjustable air bleed at the top of the main well and also
from side bleeds (one which leads from inside the bore
area above the venturi and another from the cavity around
the main fuel nozzle into the main fuel well). The fuel then
passes through the main discharge nozzle into the boost
venturi and on into the engine as a combustible mixture.
It should be noted that there is an adjustable part throttle
air bleed system which supplements the other main well
SECONDARY MAIN ACCELERATOR
WFI
I A I R
R I F E n W F l I
II
TI IRE
-----
.
.-. .
-999-
w . 9 - 9
-
.
" I 9
PRIMARY
MAIN^
TUBE
I
METERING ROD
n
I
I
POWER
(
,
PISTON
POWER PIST(
SPRING
L d
METERING
I
ROD LEVER
I
SECONDARY
/
METERll
ENGINE
METERING RODS
ORIFICE
MAN1 FOLD
VACUUM
SECONDARY
THROTTLE VALVE
Fig. 68-79 Power Enrichment System