Carburetors can have a single barrel, or two, or four barrels.
Extra barrels improve performance, particularly at high speeds, letting more air enter the cylinders than with a single barrel.
Two barrel-carburetors have two outlets to the inlet manifold, and have two basic designs.
One has a common float chamber, but each barrel has a complete set of all other circuits, and the throttles can open simultaneously. The float chamber may be straddled by two connected floats that almost surround the air passage.
This leaves it unaffected by cornering, climbing, accelerating or braking.
The other basic design has throttles that open in two stages. It combines the two barrels to act as a single carburetor. The two stages combine good low speed operation of a single-barrel design, with the extra air flow of two barrels.
One-half of the carburetor has all the circuits needed to supply mixtures for the whole range of operation. This is called the Primary side. The other barrel, the Secondary side, supplies extra mixture, but only at high speed or full throttle. It normally has a main metering system and a non-adjustable idling system. The primary side has a choke for cold starting.
When the engine is being started, the throttle on the secondary side is already closed, so a choke isn’t needed.
From idle to medium speeds, only the primary throttle is open. When engine speed rises to where additional breathing capacity is needed, the secondary throttle opens to admit more air-fuel mixture. By the time the primary throttle is wide-open, so is the secondary throttle. This can be controlled mechanically.
Or by a vacuum unit, connected by pullrod, to a lever on the secondary shaft. When air flows past ports in the venturis, it produces low-pressure areas. A hose transmits this low pressure to a diaphragm chamber. This low pressure acts on the diaphragm and opens the secondary throttle.
Large capacity V-8 engines may use a four-barrel carburetor of two-stage design - effectively two, two-stage carburetors combined, with each side supplying four of the eight cylinders.
A central molded plastic fuel bowl and suspended metering system can be incorporated into the design. This gives lower fuel temperatures with more precise fuel metering and closer control over air-fuel ratios.
