Bulletin 9D-6 August, 1951

Bulletin 9D-6
August, 1951
Model "B"
Page 3



The new Rochester Products' Model "B" Carburetor as used on the Chevrolet engines, presents a number of distinct new features, features which show themselves in performance for the car owner and ease of service for the mechanic.
Foremost of the features contributing to the improved performance is the unique design of the carburetor float bowl. The new carburetor has a concentric float bowl as contrasted to an eccentric or one-sided bowl on other type units. This float bowl concentricity in conjunction with the centrally located main discharge nozzle prevents any fuel from spilling from the nozzle on road inclines. Regardless of any angle the car may assume, the fuel level is at all times below the nozzle spill point.
A second feature of the new Model "B" Carburetor is the design of the Main Well support Assembly. This design eliminates any need for an anti-percolator. This assembly contains the Main Metering Jet and Power Valve. It is attached to the Cover and is suspended in the fuel of the Float Bowl.
When the engine is not operating, the rise in heat from the engine manifold is applied to the Float Bowl, causing fuel percolation or boiling. When the main metering jet is attached directly to the Float Bowl, it allows the heat to percolate the fuel in the main passageways. As a con­sequence, the boiling fuel seeks escape through the main nozzle and into the engine. This often times causes difficult hot starting.
However, in the Model "B" Carburetor, the heat applied to the Float Bowl may cause minor vapors atop the fuel level but the entry to the main passageways through the main metering jet is actually insulated and surrounded by the cooler solid fuel near the bottom of the Float Bowl. This prevents any fuel percolation in the passageways.
For smoother performance at low speeds the new carburetor has been designed to permit a continuous fuel flow to the engine. In conventional type carburetors, there is a distinct transfer point when the direction of fuel flow changes from the idle to the main metering system. This causes a momentary lag in fuel delivery to the engine. The Model "B" carburetor by the design of a common passage for both the idle and main metering systems, enjoys the smoother performance due to this continuous fuel flow from the idle to the main metering system.
During the period of engine warm-up, it is often advantageous to operate the engine with the power mixtures, without advancing the accelerator to the full wide-open position. By the use of a vacuum operated power system in the new carburetor, these power mixtures are readily available. This is due to the ability of the power valve to function at any drop in mani­fold vacuum below 5" Hg., regardless of the degree of throttle opening. In carburetors which employ a mechanical operated power system, the throttle must be advanced to the full wide open position to obtain the power mixtures.
To prevent poor initial acceleration and to minimize hard starting in hot weather the model "B" unit employs an anti-percolator type of pump plunger. When the engine is not operating, the rise in heat from the engine manifold causes the fuel in the pump system to percolate.







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