Rochester Model BB Carburetor: Accelerating Pump System

The Accelerating Pump System of the Rochester Model "BB" Carburetor

When an engine's throttle is opened suddenly, the air flow and manifold vacuum change almost instantaneously. Because fuel is heavier than air, it tends to lag behind during this sudden shift, which causes a momentary leanness in the air-fuel mixture. To counteract this lag and provide the additional fuel necessary for smooth operation during rapid acceleration, the Rochester Model "BB" carburetor employs a highly responsive accelerating pump system.

Mechanical Operation

The accelerator pump is connected directly to the throttle mechanism, allowing it to move instantly with the slightest change in throttle opening. To ensure the fuel delivery is steady, the unit utilizes a double spring pump plunger. The rates of compression between the top spring and the bottom spring are carefully calibrated to provide a smooth, sustained charge of fuel upon acceleration.

The physical pumping process happens in two stages:

The Intake Stroke: As the pump lifts, fuel passes through the pump inlet screen, unseats the inlet ball, and fills the pump well.
The Discharge Stroke: The downward force of the pump plunger during acceleration seats the inlet ball, preventing fuel from flowing backward. The fuel is forced through the outlet channel, which unseats a spring-loaded outlet ball, and finally discharges through the pump jets directly into the main air stream.

Hot Weather Performance Features

A notable design feature of the Model "BB" pump system is its vented type of pump plunger, which minimizes hard starting and improves initial acceleration in hot weather.

When an engine is shut off, the rising heat from the engine manifold often causes the fuel trapped in a carburetor's pump system to boil. In conventional carburetors, the pressure from this boiling often forces raw fuel into the engine, flooding it. The Model "BB" prevents this by utilizing a ball type check valve built directly into the plunger head. If fuel boils below the pump plunger, this check valve automatically vents the expanding vapor back into the float bowl. This ensures that the pump chamber always remains filled with solid fuel, ready for smooth acceleration even under relatively high operating temperatures.

Service and Maintenance Guidelines

The Model "BB" carburetor was designed for maximum ease of service, and this extends to the accelerating pump system. When maintaining or rebuilding the carburetor, note the following:

The pump jets are pressed into the bowl castings at the factory and never need to be replaced or serviced.
No targeting of the pump jets is required during adjustment or reassembly.
During routine cleaning, the pump plunger must not be immersed in standard carburetor cleaning solvent.
The pump plunger should be cleaned using clean gasoline only.
The pump plunger assembly utilizes leather; during inspection, if the plunger leather is found to be damaged, the entire plunger must be replaced.