Lesson 1/learning Event 2
Many carburetors, particularly on engines with more than six cylinders, have more than one barrel. Each
barrel of a two-barrel carburetor has separate low- and high-speed circuits.
If the carburetor has four barrels, at least two barrels will have low-speed circuits and accelerating pump
circuits. There are separate throttle valves for each barrel, but two of the valves are mounted on the same
throttle shaft, causing two throttle valves to open or close at the same time.
The purpose of more than one barrel is to split the air-fuel mixture delivery to different cylinders between
semi-independent carburetors. For instance, consider an eight-cylinder, V-type engine with a two-barrel
carburetor. One barrel will take care of the two middle cylinders on the left bank and the two end cylinders
on the right bank. The other barrel will take care of the remaining four cylinders. With such an
arrangement, the intake manifold is divided into two sections, with each section delivering the fuel-air
mixture to four cylinders.
Carburetors are often classified as up-draft, down-draft, or side-draft depending on the carburetor's position
in respect to the intake manifold. If the carburetor is mounted below the manifold, it is classified as an up-
draft carburetor. If it is above the manifold, it is a down-draft. When the carburetor and intake manifold
are at the same level, the carburetor is a side-draft.
When gasoline is vaporized rapidly, a natural refrigerating (cooling) process takes place. If you feel the
outside of the carburetor throttle housing while the engine is running, the housing feels cool even though all
the surrounding parts are very warm. This explains the frost that often collects around the carburetor and
intake manifold during cold weather. The cooling action may cause the fuel vapors to condense into liquid
Careful construction and design of the manifold reduces condensation. The intake manifold should be as
short and as straight as possible to reduce the possibility of the fuel condensing before it reaches the
cylinders. To assist in vaporizing the fuel, some intake manifolds have a section called the hot spot that is