USA Pre-War Carburettor Type
Plain tube, downdraft, dual type with 'Cold Idle' (fast idle) control, throttle operated accelerating pump, and vacuum economizer or step-up' device. Carburetters have two independent mixing chambers, main nozzle systems, idle systems, and throttle valves (valves mounted on same shaft and will not require synchronization). A single accelerating pump and vacuum economizer (metering pin and metering jet) serve both carburetter barrels.
Fuel System
The main nozzles are fed directly from the float bowl through the bowl feed jet' and also through the metering pin-and-jet (economy device). Main nozzle is air-bled through a separate nozzle air vent within the mixing chamber. Fuel for idling is taken from the main nozzle channel below the metering pin and jet through the idle fuel channel and is metered by the idle jet. This fuel is mixed with air admitted through the primary air vent in the cross passage and the fuel mixture is then taken down through a passage to the idle ports opposite the throttle valve. The lower idle port below the throttle (when throttle is closed) is controlled by the idle adjusting needle and supplies all fuel for closed-throttle idling, the secondary idle air vent and and upper idle port (in which the idle control jet is located) acting as additional air-bleeds. As soon as the throttle is opened slightly, the upper idle port discharges additional fuel. All fuel for car speeds up to 18 m.p.h. is supplied by the idle system. At this point the main nozzle comes into action and supplies more fuel progressively up to 40 m.p.h. when fuel delivery from the idle ports ceases, the main nozzle then supplying all fuel. At speeds above 75 m.p.h. or whenever the throttle is wide open, fuel supply for main nozzles is automatically increased by the economizer action (see economizer below).
Idle Adjustment
Needle valve type controlling fuel mixture. Engine must be warm when adjustments are made (cold idle not operative). With engine thoroughly warm and running, close throttle, adjust throttle stop screw so that engine speed is equivalent to 7-8 m.p.h. Adjust idle adjusting needle for each carburettor barrel in turn. Turn idle adjusting needle in or clockwise until engine hesitates or misses (mixture too lean), then turn needle out or counter-clockwise until engine rolls (mixture too rich), finally turn needle in just enough to eliminate rolling. This will give the richest setting possible without rolling and will give smoother road performance than a leaner setting. Correct setting should be 1/2 � 3/4 turn open. After adjusting both idle adjustment needles, check idling speed and readjust throttle stop screw to correct idling speed (7-8 m.p.h.).
Performance
Performance should be satisfactory throughout entire driving range if idling adjustment has been correctly made and accelerating pump setting and vacuum economizer operation are correct. All metering jets are 'fixed' type and non-adjustable. Jets should be changed only for permanent operation at elevations greater than 3,000 feet.
Accelerating Pump
Accelerating pump is operated by the throttle valve lever and discharges fuel through a pump discharge jet into the mixing chamber when the throttle is opened. The pump follow-up spring above the pump plunger prolongs the pump discharge. Adjustment - Pump outer lever on countershaft above accelerating pump has four holes (numbered 1, 2, 3, 4) for pump link connection. 1 (outer) provides maximum stroke; 4 (inner) minimum stroke. See tune-up data on car model page for recommended setting and seasonal changes.
Economizer
Economizer consists of a metering pin and jet controlled by a vacuum piston. The lower end of the vacuum piston chamber is connected to the carburettor barrel below the throttle valve. For all part-throttle positions, manifold vacuum will hold the piston down at the lower end of its stroke (against the tension of the piston spring) so that the metering pin is held in position in the metering jet, limiting the fuel flow for maximum economy. Metering Pin Timing Height. To check metering pin timing height (which controls economizer action), remove float bowl cover, hold vacuum piston down on seat (lower end of stroke with metering pin in metering jet), measure distance from top of metering pin to top, of metering pin guide and jet assembly. This distance should be 13/64 in. Adjust by bending metering pin fork or lifter.
Float Level
With engine idling, fuel level in float howl should be 3/4 in. below top face of bowl, or 1/16 in. below the centre of the sight hole in the side of the bowl (sight hole closed normally by a plug). To check float level, remove float bowl cover, hold float up by hand with needle valve seated, measure distance from top face of float bowl to top of float cork. Distance should be 13/32 in.
Throttle Valve Setting
Manufacturer recommends replacement of Throttle Valve Body Assembly, Part No. 227-543 (BD-1), 227-564 (BD-1S), whenever throttle valves or shaft require servicing due to wear or damage. CARS FITTED WITH C2, C3 DOWNDRAFT TYPE. C-2�10-1802 Nash Ambassador Six, Model 3820 (1938). 10-1808�Graham, Std. and Spec. Model 96 (1938-39). C-3�10-1809�Graham, Supercharger Model 97 (1938-39). TYPE. Plain tube, Downdraft, single barrel type with throttle operated accelerating pump and vacuum economizer or 'step-up' device. Fuel System (Idling and Low Speed). - Fuel for idling taken from lower end of main nozzle through Idle Fuel Passage and up through Idle Tube to Idle Emulsion Passage in which it is mixed with air admitted through Primary Idle Air Inlet (above tube) and Idle Air Vent Jet (at end of passage and opening into air horn above venturi). Fuel mixture then taken down through Idle Emulsion Channel and discharged through Primary Idle Delivery Port below throttle valve. This port is controlled by Idle Adjusting Needle. As soon as the throttle valve is opened slightly additional fuel is discharged through Secondary Idle Delivery Port above the throttle edge. Idle system supplies all fuel for closed throttle idling and car speeds below 18 m.p.h. (normal throttle positions). |
