Rochester BB Choke

The Rochester Model "BB" Choke System

The Rochester Model "BB" carburetor utilizes a fully automatic choke designed to ensure proper engine starting and smooth driving during cold weather operation.

Core Components and Controls

The automatic choke system is composed of several key parts:

• A thermostatic coil.

• A choke piston.

• An offset choke valve.

• Fast idle cam linkage.

The system's operation is dictated by a combination of intake manifold vacuum, atmospheric temperature, the physical offset of the choke valve, and heat drawn from the exhaust manifold.

Operational Phases

• Cold Starting: When the engine is cold, the thermostatic coil's calibration holds the choke valve closed. As the engine cranks, air velocity pushes against the offset choke valve, forcing it to open slightly against the coil's torque. Simultaneously, intake manifold vacuum pulls on the choke piston to open the valve further. The choke valve settles where the vacuum pull and air velocity balance against the coil's torque, providing a richer fuel mixture for starting.

• Warm-Up Period: As the engine warms up, hot air from the exhaust manifold "stove" is drawn into the thermostatic coil housing. This temperature rise causes the coil to slowly relax its tension, gradually allowing the choke valve to reach the full open position.

• Dynamic Adjustments: The choke piston modifies the choking action during warm-up to account for engine loads. If vacuum decreases due to acceleration or increased road load, the thermostatic coil momentarily closes the choke valve further to provide a richer mixture. To prevent stalling, a fast idle cam linked to the choke shaft holds the throttle valve open slightly, raising the engine's RPM until the choke fully opens.

• Choke Unloader (Dechoking): To relieve a flooded carburetor, the system features a mechanical unloader. If the driver fully depresses the accelerator, a tang on the throttle lever contacts the fast idle cam linkage, mechanically opening the choke valve to allow extra air into the manifold.

The Choke Modifier

Some Model "BB" carburetors incorporate an additional component called a choke modifier.

• Its primary purpose is to prevent the engine from "loading up" (receiving excessively rich mixtures) during the warm-up period.

• The modifier is linked directly to the throttle.

• If a cold engine experiences a wide throttle opening (such as driving up a steep hill), there may not be enough vacuum to draw sufficient heat to relax the coil. The throttle movement actuates the modifier, which mechanically rotates the thermostatic coil, relaxing its tension and opening the choke valve enough to prevent loading up.

Troubleshooting and Maintenance Guidelines

When servicing, disassembling, or troubleshooting the Model "BB" choke system, the bulletin specifies the following procedures to ensure proper operation:

• Cleaning Precautions: You must never immerse the choke coil and housing in carburetor cleaning solvent.

• Component Inspection: Inspect the choke shaft for wear within its bores; if it is worn excessively, the part must be replaced.

• Vacuum Integrity: Ensure the 1/2" brass fitting on the choke suction tube is tight. The choke suction tube nut must be tightened securely to the choke housing to prevent any loss of vacuum. Additionally, inspect the suction tube nut packing; if it is compressed or out of round, it must be replaced.

• Assembly Alignment: When reinstalling the thermostatic coil and cover, you must rotate the cover until the index marks on the cover and the choke housing are perfectly aligned.

• Linkage Positioning: When installing the trip lever, ensure its tang rests on top of the counterweight tang when the choke valve is fully open.

• Free Movement: After assembly, always check the choke valve to ensure it moves freely without binding.

• Fast Idle Cam: Inspect the steps of the fast idle cam for wear. If wear is present, it should be replaced, as it can disrupt the engine's idle speed during the choking period.