The Carter four-barrel carburetor (Figs. 77 and 78) consists of two main assemblies; the air horn and the main body.
The air horn assembly, which serves as the main body cover, contains the choke plate, the fuel inlet, the float assemblies, the accelerating pump assembly, and the vacumeters.
The main body houses the primary and secondary throttle plates, the secondary operating diaphragm assembly, the booster and main Venturis, the dashpot plunger assembly and the fuel passages. The automatic choke housing is mounted on the main body.
To effectively provide the correct fuel-air mixture during all phases of engine operation, the carburetor has a low speed (idle system), an accelerating pump system, a primary high speed system, and a secondary high speed system. In addition an automatic choke system provides the correct mixture necessary for quick cold engine starting and warm-up. Vacuum operated metering rods in the main metering jets control the amount of fuel admitted to the nozzle. An internal dashpot prevents stalling on quick deceleration and slows the closing of the throttle plates. A fuel inlet system provides the various fuel metering systems with a constant supply of fuel.
FUEL INLET SYSTEM
There are two separate fuel inlet systems. Each system supplies fuel to a low-speed system, and a primary and secondary high-speed system.
Each system (Fig. 79) has a fuel bowl, an inlet needle and seat assembly, and a fuel baffle plate. The fuel enters on the left side of the carburetor (as viewed from the rear) through the fuel inlet fitting and screen assembly. A drilled passage through the air horn assembly connects both fuel bowls.
The amount of fuel entering either fuel bowl is determined by the distance the needle valve is lowered off of its seat and by fuel pump pressure. Movement of the valve in relation to its seat is controlled by the float and lever assembly which rises and falls with the fuel level. As the fuel level drops, the float lowers, lowering the valve to allow fuel to enter the float bowl. When the fuel reaches a pre-set level, the float raises the valve to a position where it restricts the flow of fuel, admitting only enough fuel to replace that being used. Thus, any change in the fuel level causes a corresponding movement of the float, lowering or raising the needle valve to maintain the pre-set level of fuel. The fuel inlet system must maintain this pre-set level because the carburetor is calibrated to deliver the proper mixture only when the fuel it at the proper level.
The intake needle seats are installed at an angle to give positive seating of the needles.
The fuel bowls are vented to the inside of the air horn with a connecting passage between the bowls. The bowl vents are calibrated to provide proper air pressure above the fuel at all times.
AUTOMATIC CHOKE SYSTEM
The automatic choke system located (Fig. 80) in the primary side provides the correct mixture necessary for quick cold engine starting and warm-up.
A choke counter-shaft over the secondary barrels connects the choke linkage to the choke plate.
When the engine is cold, tension of the thermostatic spring holds the choke plate closed.
When the engine is started, air velocity against the offset choke plate causes it to open slightly against the thermostatic spring tension. Intake manifold vacuum applied to the choke piston also tends to pull the plate open. The choke plate assumes a position where tension of the thermostatic spring is balanced by the pull of vacuum on the piston and force of air velocity on the offset plate.
When the engine starts, slots located in the sides of the choke piston cylinder allow intake manifold vacuum to draw air from the engine compartment through an air heater tube in the intake manifold heat chamber. The warmed air then enters the thermostat housing and heats the thermostatic spring causing it to lose its tension. The thermostatic spring loses its tension gradually until the choke plate reaches full-open position.
If the engine is accelerated during the warm-up period, the corresponding drop in manifold vacuum allows the thermostatic spring to momentarily, partially close the choke plate, providing a richer mixture.
To combat engine stalling caused by “carburetor icing” during the warm-up on cool, humid days, heated air from the choke housing is circulated through a passage in the base of the carburetor flange. The heat transferred helps eliminate ice formation at the edges of the throttle plates and at the idle ports.
During the warm-up period it is necessary to provide a fast idle speed to prevent engine stalling. This is accomplished by a fast idle cam connected to the choke countershaft. The fast idle adjusting screw on the throttle lever contacts the fast idle cam and prevents the throttle plates from returning to a normal warm engine idle position while the automatic choke is in operation.
If during the starting period the engine becomes flooded, the choke plate may be opened manually to clean out excessive fuel in the intake manifold. This is accomplished by fully depressing the accelerator pedal and engaging the starter. The unloader projection on the throttle lever contacts the unloader lug on the fast idle cam and in turn partially opens the choke plate.
LOW SPEED (IDLE) SYSTEM
At idle and part throttle operation, the primary throttle plates are almost closed. This produces strong manifold vacuum below the throttle plates which creates a pressure differential great enough to operate the low speed or idle fuel system (Fig. 81). The low speed system is located in the primary barrels only.
Fuel under air cleaner pressure is forced from the float chamber through the main jet to the bottom of the main well. From here it flows through a short diagonal passage to the low speed jet which meters the fuel for idle and part throttle operation. The fuel flows up the low speed jet where it is mixed with metered air from the by pass air bleed. The by pass air bleed acts as an anti-siphoning device during off-idle operation and when the engine is stopped.
From the low speed jet the air and fuel flow through an economizer passage (restriction) where it is mixed and partially vaporized. After leaving the restriction, air is bled into the mixture from an additional air bleed. The mixture then flows down a vertical passage to the idle passage in the main body to the upper and lower discharge ports. The upper discharge is a vertical slot-type port which is located so the upper portion is above the throttle plate at idle. The throttle plates are milled at the location of the upper discharge ports so a small portion of the port is exposed to manifold vacuum at curb idle. At curb idle the upper portion of the port acts as an additional air bleed. The lower discharge port, which is fitted with an idle adjusting needle, is exposed to manifold vacuum at all times.
As the throttle is opened a larger portion of the upper discharge port is exposed to manifold vacuum and a correspondingly larger quantity of fuel is discharged into the air stream. Further opening of the throttle results in a decrease in manifold vacuum and a decrease in the amount of idle fuel discharged. As the idle fuel tapers off the high speed circuit begins discharging fuel.
When the throttle is closed the pump plunger moves upward in its cylinder and fuel is drawn into the pump cylinder through the intake check valves. The discharge check valve is seated at this time to prevent air being drawn into the cylinder. When the throttle is opened the pump plunger moves downward forcing fuel out through the discharge passage. The fuel is forced through a diagonal passage to the discharge check valve needle. The force of the fuel unseats the needle and the fuel is free to flow through a restriction into the discharge nozzles. When the plunger moves downward the intake check valves are closed preventing fuel from being forced back into the bowl.
ACCELERATING PUMP SYSTEM
The accelerating pump system (Fig. 82) located in the primary side provides a measured amount of fuel necessary to insure smooth engine operation on acceleration at lower car speeds.
At higher car speeds, pump discharge is no longer necessary to insure smooth acceleration. When the throttle plates are opened a pre-determined amount, the pump plunger bottoms in the cylinder eliminating pump discharge.
During high speed operation, a vacuum exists at the pump discharge nozzles. To prevent fuel from being drawn through the pump system, the pump discharge nozzles are vented by a cavity between the pump restrictions and discharge nozzles. This allows air instead of fuel to be drawn through the pump discharge nozzles.
When the pump plunger is stationary the intake check in the plunger shaft is not seated. This permits fuel vapor pressure caused by heat to be relieved through the intake passages located in the plunger shaft.
HIGH SPEED SYSTEM
Fuel for part throttle operation and full throttle operation is supplied through the high speed system.
The position of the metering rod (Vacumeter) in the main metering jet (Fig. 83) controls the amount of fuel admitted to the nozzles. The position of the metering rod is controlled by manifold vacuum applied to the vacumeter piston.
During part throttle operation, manifold vacuum pulls the piston and rod assembly down, holding the large diameter of the metering rod in the main metering jet. This is true at all times that the vacuum under the piston is strong enough to overcome the tension of the vacumeter piston spring. Fuel is then metered around the metering rod in the jet.
Under any operating condition, when the tension of the spring overcomes the pull of vacuum under the piston, the metering rod will move up so its smaller diameter or power step is in the jet. This allows additional fuel to be metered through the jet.
As engine speed increases, the air passing through the booster venturi creates a vacuum. The amount of vacuum is determined by the air flow through the venturi, which in turn is regulated by the speed of the engine. The pressure difference in the venturi and fuel bowl causes fuel to flow through the primary high speed circuit (Fig. 84).
At a predetermined venturi vacuum, fuel flows from the fuel bowl, through the primary high speed jet into the bottom of the main well. The fuel moves up the main well tube past air bleed holes. Filtered air from the air bleed enters the fuel flow in the main well tube through holes in the side of the tube.
The fuel and air mixture continues up the main well tube and is discharged into the booster venturi where it is vaporized and mixed with the air flowing through the carburetor. The air bleed also acts as an anti-per-colating vent when a hot engine is stopped or at idling speed. This will help vent fuel vapors in the main well before the pressure is sufficient to push fuel out of the nozzles and into the intake manifold.
The throttle plates control the amount of fuel-air mixture admitted to the intake manifold, regulating the speed and power output of the engine.
To provide sufficient fuel-air mixture to operate the engine at maximum power, the mixture supplied by the primary stage of the carburetor is supplemented by an additional quantity of fuel-air mixture from the secondary high-speed system (Fig. 85).
This additional supply of fuel-air mixture is delivered through the two secondary barrels of the carburetor. The secondary throttle plates are operated by a spring-loaded vacuum diaphragm assembly attached to the side of the main body and linked to the secondary throttle shaft (Fig. 86).
The secondary throttle plates are controlled by vacuum from one primary and one secondary venturi. When operating at moderate speeds, the primary venturi vacuum, which would normally start to open the secondary throttle plates, is bled into the secondary venturi. This bleed back of primary venturi vacuum prevents the secondary throttle plates from opening too soon.
When the accelerator is fully depressed, the secondary plates are manually opened a few degrees. When primary venturi vacuum increases sufficiently to overcome the bleed back into the secondary venturi, primary venturi vacuum then starts to act on the secondary system operating diaphragm, which in turn starts to open the secondary throttle plates. The bleed back hole in the secondary venturi now becomes a vacuum passage which delivers secondary venturi vacuum to the diaphragm, assisting primary venturi vacuum in further opening the secondary throttle plates.
As the secondary throttle plates begin to open, secondary venturi vacuum causes a pressure differential great enough to cause fuel to start flowing through the secondary high-speed system. Fuel is forced from the fuel bowl through the secondary high-speed jet into the secondary main well tube. The fuel flows up the tube where air is introduced from the high-speed air bleed. The fuel and air are then discharged into the booster Venturis through the secondary discharge nozzles. The high speed air bleed also acts as an anti-percolator vent when a hot engine is stopped or at idling speed. This will help vent fuel vapor pressure in the high speed well before it is sufficient to push fuel out of the nozzles and into the intake manifold.
When decelerating, vacuum at the openings in the primary and secondary Venturis decreases, and the secondary throttle plates begin to close. A mechanical over-riding linkage will overcome any lag in the vacuum system, closing the secondary throttle plates with the primary plates.
To slow the closing of the throttle plates to idle position an internal dashpot is incorporated (Fig. 87) in the carburetor.
When the throttle is opened the dashpot plunger spring pushes the plunger upward. The intake ball check valve opens allowing fuel above the plunger to fill the cylinder below the plunger. When the throttle is closed, the plunger is pushed downward. The intake ball check valve is closed and fuel below the plunger is forced through a small restriction delaying the closing of the throttle plates.
- Remove the air cleaner. Remove the throttle rod from the throttle lever. Disconnect the choke control heat tube, the fuel line, and the distributor vacuum line at the carburetor.
- Remove the four nuts and lock washers that secure the carburetor to the intake manifold, then remove the carburetor. Remove the spacer and two gaskets from the manifold.
- Install bolts about 2 ¼ inches long of the correct diameter through the carburetor retaining bolt holes with a nut above and below the flange (or install carburetor legs) to facilitate working on the carburetor and preventing damage to the throttle plates.
Use a separate container for the component parts of the various assemblies to facilitate cleaning, inspection, and assembly.
- Remove the air cleaner anchor screw. Remove the dashpot arm, dashpot operating lever, and the choke countershaft lever from the choke side of the carburetor. On the fuel inlet side, disconnect the fast idle connector rod at the inside choke countershaft lever, and disconnect the acceleration pump operating rod at the accelerating pump arm. Remove the ten retaining screws, then remove the air horn from the main body.
- Remove the accelerating pump plunger, the vacumeter assemblies, the floats, the fuel inlet needle and seat assemblies and gaskets, then remove the air horn gasket. Remove the fuel inlet fitting, gasket, and screen. Remove the choke plate connector link, then slide the choke plate countershaft out of the air horn assembly. Remove the choke plate, then slide the choke shaft out of the air horn assembly.
- Remove the accelerating pump spring, and the dashpot piston and spring. Remove the venturi assemblies and gaskets, and remove the accelerating pump discharge nozzle assembly, gasket, and discharge check valve. Remove the primary and secondary main jets, and the idle mixture adjusting needles and springs.
- Remove the thermostatic spring housing and gasket, then remove the choke housing from the main body. Remove the choke connector rod from the choke housing shaft, then remove the shaft, and piston and lever assembly.
- Remove the throttle connecting rod. Disconnect the secondary operating diaphragm lever, then remove the secondary throttle operating lever. Remove the secondary diaphragm housing cover, return spring, and diaphragm, then remove the diaphragm housing and gasket from the main body. If it is necessary to remove the secondary throttle plates and shaft remove them at this time.
- Remove the accelerating pump inlet check ball retainer, ball seat, and ball from the bottom of the carburetor. If it is necessary to remove the primary throttle plates and shaft, remove the primary throttle shaft dog, operating lever, and flex spring, then remove the throttle plates and slide the shaft out of the main body.
Carburetor Cleaning and Inspection
Wash all the carburetor parts (except the accelerating pump piston assembly, dashpot piston, the secondary operating diaphragm, and the choke thermostat housing) in clean solvent. Rinse the parts in hot water to remove all traces of the cleaning solvent, then dry them with compressed air. Be sure all dirt, gum, and other foreign matter and carbon are removed from all parts. Force compressed air through all passages of the main body, air horn assembly, choke housing, the secondary housing, the booster venturi assemblies, and the accelerating pump discharge nozzle assembly. Wipe all parts that can not be immersed in solvent with a clean, soft, dry cloth. Do not use a wire brush to clean any parts. Discard all gaskets.
Check the choke shaft and countershaft for wear, and excessive looseness or binding in the air horn. Inspect the choke plate for nicked edges.
If the primary or the secondary throttle shafts are excessively loose or bind, or if the plates are burred preventing proper closure, replace the plates and shaft(s).
Inspect the main body, air horn assembly, booster venturi assemblies, accelerating pump discharge nozzle assembly, choke housing and thermostat housing, and the secondary diaphragm housing for cracks.
Inspect all gasket mating surfaces for nicks and burrs.
Check the floats for leaks by holding them under water that has been heated to just below the boiling point. If bubbles appear the float leaks, replace it. If the float is satisfactory, polish the needle contact surface of the float arm. Replace the float if the arm contact surface is grooved.
Inspect both fuel inlet needle valves and seats. Replace both the needle and seat if either part is defective as they are matched assemblies.
Inspect all screws and nuts for stripped threads, replace any that are defective.
Replace all distorted or broken springs.
Inspect the accelerating pump discharge needle, the inlet ball in the base of the carburetor and the pump piston for corrosion or sticking, replace as necessary. Be sure the pump plunger leather is in good condition and the intake and discharge checks and pump jet are free of lint, gum or other foreign matter.
Be sure the dashpot plunger leather is in good condition and the intake check and discharge restriction are free of lint, gum, or other foreign matter. The plunger shaft must operate freely in its guide in the air horn.
All by-passes, economizers, idle and main ports, idle adjustment needle ports, as well as the bore of the carburetor flange must be clean and free of carbon. Obstructions will cause poor operation. Replace worn or damaged idle adjustment needles. The idle speed jets, air bleed, economizer and by-passes, and main and secondary wells, are pressed in place. If replacement is necessary, use a new venturi assembly.
Install all the new gaskets and parts furnished in the overhaul kit. Make sure all holes in the new gaskets have been properly punched and that no foreign material has adhered to the gaskets.
Refer to Fig. 88 for the correct location of parts.
AIR HORN ASSEMBLY
- Slide the choke shaft into the air horn, then install the choke plate. Slide the countershaft inside lever on the shaft so that the flange will face the air horn when the countershaft is installed. Slide the countershaft in the air horn from the fuel inlet side. Connect the choke plate connector link to the choke plate and to the countershaft center lever, then install the lever on the countershaft.
- Install the fuel inlet filter screen, gasket, and fuel inlet fitting.
- Place a new air horn gasket on the air horn, then install the fuel inlet needle valve gaskets and seats. Install the needle valves in their matched seats. Do not install the left needle in the right seat or vice versa. Inlet needles and seats are matched assemblies.
- Install the floats in their original positions, then refer to “Carburetor Bench Adjustments” and adjust the float alignment, float setting, and float drop.
- Place the springs in the vacumeter pistons, then place the pistons and metering rods in the vacumeter chambers. Install the vacumeter covers.
- If the throttle plates have been removed, install both the primary and the secondary plates with the part number toward the bottom of the carburetor (toward the intake manifold) and facing the center of the carburetor.
Hold the throttle plates tightly closed. Loosen the throttle plate attaching screws slightly and tap both plates with a screw driver handle. When the plates are correctly seated, tighten the attaching screws.
- Install the primary throttle shaft lever and flex spring. Compress the ends of the flex spring with needle nose pliers, then install the throttle shaft dog with the lugs toward the main body. Release the flex spring so that the ends of the spring fit in the notches in the shaft dog lugs. Install the washer and retaining screw.
- Install the secondary diaphragm housing and gasket on the main body. Install the secondary throttle operating lever on the secondary shaft with the pin facing out. Install two flat washers on the throttle operating rod, then secure it to the secondary lever and to the primary lever with a flat washer and pin retainer.
- Place the gasket on the secondary diaphragm housing with the vacuum port hole at the top. Position the diaphragm in the housing with the vacuum port opening at the top, then hook the end of the diaphragm lever over the pin on the secondary throttle operating lever. Close the secondary throttle plates. Smooth the diaphragm fabric and be sure that the holes m the diaphragm, gasket, and housing are aligned. Place the diaphragm return spring and cover in position and install the screws and lock washers finger tight. Be sure the cover is positioned so the vacuum passage is at the top. Unhook the diaphragm lever from the secondary throttle operating lever and allow the diaphragm lever to go the full length of its travel, then tighten the cover screws. Hook the end of the diaphragm lever over the pin on the secondary throttle lever and secure it with a pin retainer.
- Position the choke piston and lever assembly in the choke housing, then install the choke housing shaft and secure the piston assemby to it with the flat washer and screw. Actuate the assembly to be sure it moves freely. Install the choke connector rod on the choke housing shaft lever. Position a new gasket in the vacuum port recess in the choke housing, then install the housing on the main body. Position the thermostat housing gasket on the choke housing. Engage the thermostat spring and piston lever, then secure the thermostat housing to the choke housing. Align the index mark on the thermostat housing with the second mark (rich side) from the center on the choke housing.
- Install the idle mixture adjusting needles and springs and turn them in gently until they just touch the seat, then back them off 1 turn for a preliminary idle adjustment. Install the primary and secondary main jets. If the fuel bowl baffle plates have been removed, install them at this time. Install them with the trade mark toward the front of the carburetor.
- Drop the accelerating pump discharge needle into its passage, then install the gasket and pump discharge nozzle assembly. Install the gaskets and venturi assemblies. The venturi assemblies are marked for correct installation. Be sure they are installed in their correct location. Install the accelerating pump spring and pump in its chamber on the left side of the carburetor. Install the dashpot plunger spring and plunger in its chamber on the right side of the carburetor.
ASSEMBLE AIR HORN TO MAIN BODY
- Carefully, position the air horn on the main bodyBe sure the dashpot plunger stem and the accelerating?pump stem enter their respective holes in the air horn.
Do not force the air horn into position, as the metering rods will bind. Install the ten retaining screws and lock washers. Install the two Phillips head screws at the center retaining screw hole locations. Tighten the screws evenly working from the center to the outside.
- Place the dashpot arm and dashpot operating lever, in position on the air horn and secure them with a washer and pin retainer. Connect the dashpot connector rod to the dashpot arm and the primary throttle shaft lever with pin retainers.
- Connect the choke connector rod to the countershaft lever, then slide the lever on the countershaft with the trade mark facing out.
- Install the accelerating pump connector link and secure it to the pump arm with a pin retainer. Insert the accelerating pump operating rod in the correct adjustment hole on the pump arm and secure it with a pin retainer. Install the fast idle connector rod and secure it to the inside countershaft lever with a pin retainer.
- Refer to “Carburetor Bench Adjustments” and make the following adjustments in the order listed:
- Accelerating Pump.
- Choke Linkage Setting.
- Fast Idle Linkage Setting.
- Fast Idle Throttle Valve Clearance.
- Unloader Setting.
- Primary Throttle Shaft Dog Adjustment.
- Dashpot Setting.
- Be sure all old gasket material is removed from the manifold heat riser flange, then place the spacer between two new gaskets and position them on the manifold. Position the carburetor and install the lock washers and retaining nuts. Tighten the nuts evenly in a criss-cross pattern.
- Connect the accelerator linkage, and the choke heat tube. Install the fuel line, distributor vacuum line, and the air cleaner.
- Refer to “Carburetor In-Chassis Adjustments” and adjust the idle speed and the idle mixture.
Carburetor Bench Adjustments
There are three adjustments that should be made on each float and lever assembly.
Sight down the side of each float shell to determine if the side of the float is parallel to the outer edge of the air horn casting (Fig. 89).
To adjust float alignment, bend the float lever by applying pressure to the end of the float shell with the fingers while supporting the float lever with the thumb. Apply only enough pressure to bend the float lever.
After aligning the float, remove as much clearance as possible between the arms of the float lever and the lugs on the air horn by bending the float lever. The arms of the float lever should be as parallel to the inner surfaces of the lugs on the air horn as possible.
With the air horn inverted, the air horn gasket in place, and the needle valve seated, check the clearance between the top center of each float and the air horn gasket (Fig. 90). The clearance should be %2 inch. Bend the float arm (up to increase the clearance or down to decrease the clearance) by applying up or down pressure to the end of the float shell with the fingers. Apply only enough pressure to bend the float
With the air horn held in an upright position, measure the distance between the top of each float and the air horn gasket at the center of each float (Fig. 91). The distance should be 19/32 inch. Bend the stop tab on the float bracket as necessary to adjust the drop.
ACCELERATING PUMP ADJUSTMENT
Back out the hot engine idle adjusting screw until the throttle valves seat in the bores of the carburetor. Be sure the choke is wide open so the fast idle cam does not hold the throttle valves open. Place the pump link in the center hole (medium stroke) of the pump arm. Measure the distance from the top of the air horn to the top of the plunger shaft (Fig. 92). The distance should be 15/32 inch. To adjust, bend the accelerating pump operating rod at the lower angle as necessary.
CHOKE LINKAGE SETTING
Loosen the countershaft lever clamp screw. Hold the choke plate tightly closed (Fig. 93). Take the slack out of the linkage by pressing the countershaft lever towards the closed position until there is 0.086-inch clearance between the choke lever and the stop in the choke housing. While maintaining this clearance, tighten the clamp screw.
FAST IDLE LINKAGE SETTING
Hold the choke plate tightly closed and the fast idle cam against the stop on the main body (Fig. 94). Check the clearance between the lug on the outer countershaft lever and the stop on the inner countershaft lever. The clearance should be 0.010 inch. To adjust the clearance, bend the lug on the outer countershaft lever.
FAST IDLE THROTTLE VALVE CLEARANCE
With the choke valve tightly closed, insert a 0.020-inch feeler gauge between the throttle valve plate and the bore of the carburetor (the side opposite the idle port), then tighten the fast idle adjusting screw against the high step of the cam (Fig. 95).
With wide open throttle, check the clearance between the upper edge of the choke plate and the inner wall of the air horn (Fig. 96). The clearance should be 0.067 inch. To adjust the clearance, bend the unloader lip on the throttle shaft lever.
PRIMARY THROTTLE SHAFT DOG ADJUSTMENT
Put the primary throttle plates in the wide open position and check the clearance between the edge of the secondary valves and the bore of the carburetor adjacent to the primary plates (Fig. 97). The clearance should be 0.015 inch. To adjust the clearance, bend the short tang on the primary throttle operating lever as necessary.
Now close the primary and secondary throttle plates and check the clearance between the primary throttle shaft flex spring and the long tang (Fig. 98). The clearance should be 0.067 inch. To adjust the clearance, bend the long tang on the operating lever as necessary.
ANTI-STALL DASHPOT SETTING
With the primary throttle plates in the wide open position check the distance between the top surface of the air horn assembly and the top of the plunger shaft (Fig. 99). The distance should be 7/16 inch. To adjust the measurement, bend the lug on the dashpot operating lever.
Carburetor In-Chassis Adjustments
Refer to Fig. 100 for the location of the adjustments.
IDLE SPEED ADJUSTMENT
The engine idle speed must be adjusted to the proper hot and cold settings.
Hot Engine Idle Speed
Set the hand brake. Place the transmission selector lever in the neutral position. Operate the engine until the temperature has stabilized, and the choke fast idle cam is in the slow position. Turn the hot engine idle adjustment screw in a direction to obtain 475-500 rpm. Clockwise rotation increases the engine idle speed and counterclockwise rotation decreases it.
On cars equipped with Fordomatic, set the handbrake and place the selector lever in Dr (Drive) position. Check the engine idle speed. Adjust it to 425-500 rpm in Dr (drive) position if necessary.
Adjust the cold engine idle speed setting.
Cold Engine Idle Speed
The cold engine idle screw contacts steps on the fast idle cam during the engine warm-up period and controls the cold engine idle speed.
Adjust the hot engine idle speed to the recommended rpm before attempting to set the cold engine idle speed. Make this adjustment with the engine at normal operating temperature.
With the fast idle cam in the slow position, turn the cold engine idle speed adjustment screw in until it just touches the lowest step on the fast idle cam. With the fast idle screw on the first step of the fast idle cam, turn the cold engine idle adjusting screw to obtain 650 rpm. In localities where normal setting of the cold engine idle speed may be considered unnecessarily high, the cold engine idle speed may be reduced by backing off the adjusting screw not in excess of one full turn.
IDLE MIXTURE ADJUSTMENT
The idle mixture is controlled by the idle mixture adjustment needles. Turn the needles “in” to lean the mixture, and “out” to enrich the mixture. Make the initial adjustment by turning the needles in until they lightly touch the seat, then back them off 1 turn. Do not turn a needle against the seat tight enough to groove the point. If the needle is damaged, it must be replaced before a proper mixture adjustment can be obtained.
Run the engine at fast idle speed for 20 minutes to bring it to normal operating temperature.
Turn the mixture needles in until the engine begins to run rough from the lean mixture. Turn the needles out until the engine begins to “roll” from the rich mixture. Then, turn the needles in until the engine runs smoothly. Always favor a slightly rich mixture rather than a lean setting.
It may be necessary to reset the idle speed after the correct idle mixture is obtained.
ACCELERATOR PUMP STROKE
To satisfy acceleration requirements in various climates or altitudes, the accelerator pump operating rod can be placed in one of three positions in the accelerating pump arm.
Placing the rod in the top hole will give the longest stroke which is suitable for cold weather operation. The center hole should be used for average conditions. The bottom hole will give the shortest pump stroke which is recommended for warm weather or high altitude operation.