How The Carter BBD Works Part 2

idle by-pass assist

Idle By-Pass Assist

Some BBD units incorporate an idle by-pass assist. This passage goes through the main body, and the body gasket. Through a passage in the throttle body and enters below the throttle valve. This extra air through the by-pass allows the throttle valve to close a little more for a given idle RPM. This reduces the CFM airflow over the nozzle tips and prevents the possibility of taking fuel from the nozzles during fast idle operation. It also causes turbulence below the throttle valves to aid air-fuel mixture and distribution.

idle solenoid

Idle Solenoid

Many carburetor models use an idle solenoid to prevent “dieseling” or “after run.”

Many things that have been done to lower emissions have enhanced the possibility of dieseling. Higher idle speeds, leaner air-fuel mixtures, retarded ignition timing, higher operating temperature, all contribute to dieseling. When the ignition is turned on, the solenoid is energized moving the plunger outward. The idle RPM is adjusted at the solenoid. When the ignition is turned off the solenoid is de-energized, the plunger moves inward allowing the throttle valves to close enough to virtually shut off the air supply, causing the engine to stop running immediately. Some units have a second adjustment to prevent the throttle valves from closing too tightly.

air conditioner solenoid

Air Conditioner Solenoid

The air conditioner solenoid is used in many applications to maintain idle RPM.

The extra load on the engine when the air conditioner is turned on causes a drop in idle RPM.

The solenoid is energized moving the solenoid plunger outward. This outward movement opens the throttle valves (as specified) to maintain idle RPM.

sis solenoid

SIS Solenoid

Some later models use an S.I.S. (solenoid idle stop) solenoid. When the air conditioning, rear window de­ fogger, or any accessory with a heavy load is turned on, the S.I.S. solenoid is energized and the plunger moves outward to open the throttle valves slightly.

The adjustment or the S.I.S. solenoid is on its inward travel rather than the conventional outward travel. Two adjustments are required and must be made in proper sequence, as specified on the solenoid decal.

When the accessory is turned off, a timer gives a two-second delay in de-energizing the solenoid to prevent the engine die out.



The sol-vac is also used in many applications. The electrical solenoid is energized when the air conditioning is on, when the hedgehog is in operation, rear window defroster, or any heavy electrical load.

The vacuum portion is activated anytime the air temperature in the air cleaner is below 55 degrees, or anytime the idle drops to 450 R.P.M. At 450 R.P.M. the vacuum section is activated and opens the throttle valves to specifications which is above normal idle. A time delay is used to return the throttle valve to normal idle. If idle drops to 450 R.P.M. the second time, the vacuum unit is again activated, however, the time delay is not in operation. A return to idle then requires increasing engine speed to 1150 R.P.M.

The hedgehog replaces the heat riser. It is a finned type heater element located in the manifold just below the carburetor. It is controlled by a wax pellet-type temperature switch located in the engine block. The hedgehog is on any time the water temperature is below 160 degrees.

Three adjustments are required and must be made in the proper sequence.

Air Bleed Circuit

air bleed circuit

High Speed Circuit

Fuel for part throttle and full throttle operation is supplied through the high-speed circuit.

The air-bled circuit used prior to 1974 has an emulsion, tube, or vent tube that extends downward into the high-speed well. This tube mixes air with the fuel before it leaves the high-speed well. The air-bled design always uses “downhill” nozzles. The air bleed in the high-speed circuit also serves as an anti-percolator passage.

high speed circuit

The solid-fuel design, 1974 and Later, takes solid fuel from the high speed well and bleeds air into the circuit at the top through the extended vent tubes located in the cluster, closer to the tip of the nozzle. The solid-fuel design always uses “uphill” nozzles and gives a closer calibration to meet the emission standards and also serves as an anti-percolator passage.

diminishing well bleeds

Diminishing Well Bleeds

Some solid fuel models use diminishing well bleeds. This bleed is subjected to venturi pressure changes that follow engine load conditions. They serve as self-adjusting air bleeds and at or near the wide-open throttle, could deliver fuel.

The two center holes are the pump discharge windows and also the air bleed to prevent pump pull over. A pump pullover is when gas is coming out of the discharge when the accelerator pump isn’t being activated.

Metering Rod

The position of the metering rod in the main metering jet controls the amount of fuel admitted to the discharge nozzle.

The metering rod has varying step diameters which controls the effective size of the main metering jet in which it operates.

Function of the Metering Rod

The two metering rods are yoked to a single step-up piston assembly which rides in a cylinder in the bowl casting. The jets which work with the metering rods are located in the fuel bowl. Note the solid fuel jets are different than those used in the air-bled system.

At part throttle and cruising speeds, increased airflow through the venturi creates a low-pressure area in the venturi. Since the air above the fuel level in the bowl is near atmospheric pressure, fuel flows to the lower pressure area created by the venturi. The fuel flow moves through the main jets to the main nozzle as it picks up air from the air bleeds.

During heavy road load or high-speed operation, the air-fuel mixture must be enriched to provide increased engine power. Power enrichment is accomplished by movement of the metering rods which are attached to a single yoke and piston actuated by the manifold vacuum. The metering rod piston rides on a calibrated spring which attempts to keep the piston at the top of the cylinder. At idle, part throttle, or cruise conditions when manifold vacuum is high, the piston is drawn down into the vacuum cylinder, compressing the vacuum piston spring. The larger diameter of the metering rods will be positioned in the main jets allowing a calibrated amount of fuel flow to the nozzle. Under any operating condition where the tension of the vacuum piston spring overcomes the pull of vacuum under the piston, the metering rods will move upward so the smaller diameter step is in the jet. This permits the necessary additional fuel flow to be metered through the jets.

metering rod

The metering rods in the solid fuel unit are both mechanically and vacuum operated and must be adjusted. The lifter tab lifts the metering rods mechanically and also limits the amount of lift from the vacuum piston.

Vacuum Step-Up Piston Hex-Head Screw

Never attempt to change the factory setting of the vacuum step-up piston hex-head screw as it will seriously affect performance. This adjustment is made during flow testing and cannot be duplicated in the field.

An air leak past the gaskets sealing the venturi cluster, venturi cover, and tube assembly or the venturi cluster screws will affect both low speed and high-speed performance. To assure a positive seal always use new gaskets and be sure venturi cluster screws are tightened securely.

manual altitude compensator

Manual Altitude Compensator

To meet emission standards at 4,000 feet above sea level, some BBD carburetors use a manual altitude compensator. It consists of a spring-loaded adjustable cap added to the venturi cluster. During pre-delivery of the vehicle for altitude use. The adjusting screw is turned in the counterclockwise direction. The spring forces the cap upwards uncovering the auxiliary air bleeds to the low-speed circuit. In addition to the auxiliary air bleeds, there is an oversized air bleed drilled into the lower section of the venturi cluster assembly and with the cap in its upward position, the air is bled into both the low speed and high-speed circuits to lean out to the altitude calibration required. There is no adjustment. The cap merely opens or closes these additional air bleeds.

pump circuit

Pump Circuit

The accelerating pump circuit provides a measured amount of fuel which is necessary to ensure smooth engine operation for acceleration.

When the throttle is closed, the pump plunger moves upward in its cylinder, and fuel is drawn into the cylinder through the intake check. The discharge check is seated at this time to prevent air from being drawn into the cylinder. When the throttle is opened, the pump plunger moves downward forcing fuel out through the discharge check and out of the pump jets. As the plunger moves downward, the intake check is closed preventing fuel from being forced back into the bowl.

The discharge check ball is 5/32”. The intake check is 3/16.”

The calibration of the pump spring and the size of the jets provide a pump discharge of the desired duration.

The accelerating pump stroke adjustment provides a means to assure the proper pump discharge volume.

High air velocity passing over the pump jets causes a low-pressure area. An air bleed located between the discharge windows and the pump jets prevents the pump pull-over. Pump pull-over is when gas is dribbling out of the venturi and you are not accelerating.

pump plungers

Pump Plungers

Solid Cup Plunger

After engine shutdown heat can cause vapors to accumulate within the pump cylinder. The BBD pump plunger is designed to relieve this vapor pressure and to maintain solid fuel in the pump cylinder at all times.

sliding cup plunger

Sliding Cup Plunger

The air-bled unit uses a “solid pump plunger” with a vapor vent passage through the plunger. The solid fuel burnt takes advantage of a sliding cup that gives no bleed during acceleration. When at rest, it serves as a release for any vapor pressure in the pump cylinder.

cup plunger

Some 1978 models do not use the intake pump circuit or intake check ball. These models take advantage of the sliding “pump plunger cup” and fill from the slots at the top of the pump cylinder.

choke circuit

Choke Circuit

The automatic choke circuit provides the correct mixture necessary for quick cold engine starting and warm-up. Same BBD carburetors use an integral choke, while others use the cross-over (Remote mounted type).

When the engine is cold, the tension of the thermostatic coil holds the choke valve closed. When the engine is started, air velocity against the offset choke valve causes the valve to open slightly against the thermostatic coil tension. The intake manifold vacuum applied to the choke piston also tends to pull the choke valve open. The choke valve assumes a position where the tension of the thermostatic coil is balanced by the pull of vacuum on the piston and force of air velocity on the offset valve.

When the engine starts, slots located in the sides of the choke piston cylinder are uncovered, allowing the intake manifold vacuum to draw warm air heated by the exhaust manifold through the choke housing. The flow of warm air, in turn, heats the thermostatic coil and causes it to lose some of its tension. The thermostatic coil loses its tension gradually until the choke valve reaches a full open position.

If the engine is accelerated during the warm-up period, the corresponding drop in the manifold vacuum allows the thermostatic coil to slightly close the choke which provides a richer mixture.


When the cross-over type choke is used, the carburetor mounting gasket is most important. If it is not to specified thickness, it upsets choke calibration due to the length of the choke rod. Most cross-over, or divorced chokes are non-adjustable.

choke pull-off

Choke Pull-Off

On many BBD units, the choke piston is replaced by a device called a choke pull-off. The choke pull-off is a diaphragm-type unit that performs the same function as the choke piston. It opens the choke valve to a pre­determined opening when the engine starts. The amount of pull-off is adjusted by shortening or lengthening the choke pull-off rod.

Modulated Choke Pull-Off

Many units use a modulated-type choke pull-off. In addition to the regular diaphragm spring, the diaphragm shaft incorporates a spring within the shaft to provide better warm-up fuel economy by allowing the amount of choke valve opening to vary with the torque of the choke coil spring. This spring-loaded diaphragm shaft merely allows a temporary tighter closed choke valve during the very early stage of the warm-up period.

electric assist choke

Electric Assist Choke

Electric-assist chokes are. used to help reduce HC and CO emissions during starting and warm-up. It gives a closer choke calibration during the warm-up period. This device consists of a heating element located in the choke cap on integral chokes or is built into the remote choke assembly on manifold mounted chokes. A wire from the heater element is connected to an electric control switch. It is designed to shorten choke duration at temperatures above approximately 60 degrees. The switch serves several purposes. Below 60 degrees it will provide the choke heater with partial power or heat, allowing it to stay on longer. Above 60 degrees it provides full heat to get the choke off quicker. The switching temperature is controlled by engine temperature and a small internal electrical heater.

To check the electrical heating element an ohmmeter is used. Resistance of 4 to 12 ohms is normal; check specs for the particular applications.

Some models use a 100 percent electric choke.

tamper proof choke

Tamper Proof Choke

To meet federal regulations on tamper-proofing, some late models use rivets or breakaway screws to attach the thermostatic choke coil and housing. If you need to replace the thermostat, drill out the rivets and replace them with self-tapping screws.

For a period of time, regulations required tamper-proofing the choke pull-off linkage. On these units the choke pull-off. is spot welded to housing which serves as the mounting bracket and also a part of the tamperproof enclosure?

The outside cover plate is riveted on to enclose the choke pull-off link.

Updated on 04/13/2022

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