Carter BBD 2 Barrel Choke System

The Carter BBD usually includes the choke thermostat along with a choke pull-off. This 2 barrel carburetor generally uses a divorced choke type of thermostat. This means the choke thermostat is mounted in the intake manifold and is connected to the carburetor choke valve by a choke rod.

Carter BBD Carburetor Choke

When the engine is cold, the choke valve should be closed. You may have to open the throttle a bit so that it can close. The fast idle cam may be holding it.
As the engine gets hot, the thermostat will gradually open the choke valve.
A lot of the Carter BBD carburetors will have a choke pull-off attached to the carburetor. This is used to open the choke valve slightly when the engine is cold and you want to quickly accelerate. At idle the choke pull-off piston is pulled back by vacuum. As you stomp on the gas pedal, the vacuum drops allowing the piston to move out and when far enough will open the choke valve slightly (when cold), letting in some air. This is so that the carburetor doesn’t flood out when cold.
Watch the choke valve. When cold it should be closed. When hot, it should be fully opened. Again, you may have to move the throttle slightly in case the fast idle cam is holding the choke.
Carter BBD Choke Circuit:

BBD Choke Fast Idle Lever
This is the fast idle cam on the early BBD carburetor. When the engine is cold and the choke is closed, the fast idle cam moves so that the high step is on the idle screw. This increases the RPM so that the engine doesn’t choke out. As the choke warms up, the lever moves to smaller and smaller points. To adjust – Adjust the warm idle 1st, then put the fast idle cam on the high step and adjust the idle screw to the RPM recommended for your engine.
Free Carburetor Manual

What is the Choke Unloader?

Although the choke unloader should be set to specifications, this dimension is not as critical as most other carburetor adjustments. Mis­understanding or improper use of the choke unloader by the owner is undoubtedly more common cause of trouble than incorrect un­loader adjustment. If a customer has starting problems because of engine flooding, maybe the driver’s engine starting habits need fixing. Be sure that an owner with this kind of trou­ble understands that he must hold the accel­erator wide open and do quite a bit of crank­ing to clear a flooded engine.
All too frequently, an owner doesn’t crank the engine long enough with accelerator fully de­pressed. Another common mistake is to re­lease the throttle every time the cranking is temporarily interrupted. When the accelerator is floorboarded and cranking started again, the accelerator pump gives the engine another shot of fuel which only aggravates the flooded condition.

1 1/4″ BBD Unloader

The choke unloader on the 1¼” BBD is ad­justed by bending the unloader tang on the throttle lever. With throttle valves wide open, the unloader should open the choke one ­quarter inch.

Carter BBD Unloader

1 1/2″ BBD Choke Unloader

The choke unloader arrangement on this car­buretor is a bit different. On this model, a tang on the fast-idle lever actuates the un­loader. However, the unloader specification is the same. The unloader must open the choke valve one-quarter inch when the throttle is wide open. To change the choke opening, bend the tang on the fast-idle lever.

Updated on 04/13/2022

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