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ROCHESTER CARBURETORS Bulletin 9D-8 August, 1951 |
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Bulletin 9D-8 August, 1951 Model
"BC" Page 4 |
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ROCHESTER PRODUCTS, DIVISION OF GENERAL MOTORS, ROCHESTER. NEW
YORK |
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IDLE SYSTEM
The Idle System controls and
delivers the proper fuel/air mixture for idling and up to approximately 30
MPH.
As shown on Figure 2, the idle
fuel first passes from the howl through the calibrated Main Metering Jet
attached to the bottom of the Main Well Assembly. This fuel is then
drawn up the Main Well by manifold vacuum (suction) to the crossbar in the
Air Horn. Air joins the fuel through the calibrated air bleeds in the
center of the crossbar. This fuel/air
mixture is then calibrated as it passes through the Idle
Restriction and is drawn down the passage in the Float Bowl to the
Throttle Body.
The idle fuel is then metered to
the engine by the idle adjusting needle hole which is below the throttle
valve. As the throttle valve is opened to a greater degree, the idle hole
which was above the closed throttle valve, is exposed to manifold vacuum
and delivers additional fuel to meet the increased engine
demand. |
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Figure 2. Idle System |
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PART THROTTLE SYSTEM
As the throttle valve is opened
further, (as shown in Figure 8), air at a higher velocity is drawn
down the carburetor throat. This creates a pressure drop or
suction in the Venturi at the main discharge nozzle in the cross-bar. As a
consequence, fuel and air begin to pass from the main nozzle to meet the
increased engine demand. Further throttle opening will result in greater
air velocity passing through the carburetor with resultant higher fuel
flow from the nozzle and decreased flow from the idle system
until it eventually cuts out altogether.
The calibration of the Main
Metering Jet and the Air Bleeds in the crossbar maintain economical
fuel 'air ratios throughout the part-throttle or cruising
range. |
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Figure 3. Part Throttle System. |
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