The ABCs of Carburetion

UM-900 (1959)


Page 11 of 19

Models B and BC
The six general systems in a Rochester Carburetor are Float, Idle, Part Throttle, Power, Acceleration and Choke.
Without getting too technical, here briefly is what each one of the six systems does:
The Float System keeps the fuel at the right level in the bowl. Gasoline coming in through the fuel inlet rises in the bowl and lifts the float until it closes the needle valve. As gasoline is used, the surface is lowered and the float drops to allow the needle valve to re-open, and so on.
At Idle speeds, the throttle valve is nearly closed and there is so little air flow that even the venturi won't lift the fuel from the bowl. So to get fuel to flow, manifold vacuum is ap­plied directly to the fuel in the bowl, from a point below the throttle valve. Now there is atmospheric pressure on the fuel surface and manifold vacuum (very low pressure) applied to the idle passages, so that fuel will be forced through the idle passages where air is picked up through small holes, or air bleeds, and the mixture is eventually fed to the carburetor bore below the throttle valve. The Idle Mixture Adjusting Screw regulates the amount of mix­ture through this idle discharge hole, to com­bine properly with the small amount of air still passing the nearly closed throttle valve.



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