Carburetor Theory Of Operation - Suzuki Intruder 1978 Service Manual

Carburetor Theory of
Operation
Introduction
The mixing of fuel and air in the amounts required
for efficient combustion is the function of the
carburetor.
A common method for referring to carburetors is
the bore or venturi size . This method is used in
snowmobiles . The measurement is the diameter of
the smallest part of the venturi. (See Figure 2-12.)
The carburetor is the rider's primary control over
the movement of his machine . The carburetor
chooses the engine speed that will propel the
machine at the desired rate . With a squeeze of the
cOQtrol the rider can choose a speed anywhere
from a virtual crawl to flat out. The rider expects
that the engine will respond instantly, anywhere
within its operating range .
Figure 2-12
The carburetor receives the message from the rider
in the form of a pull on, a cable . This lifts a slide
which uncovers the air passage to the engine . This
slide, aided by numerous ports, passages, needles,
and jets, regulates the flow of fuel and air into the
engine. When the slide is lifted, a greater quantity
of fuel / air mixture flows under the slide into the
engine, causing it to produce more power . When
the slide is lowered, less fuel/air mixture is ad -
mitted to the engine, causing it to reduce speed
and power.
The carburetor controls the amount of fuel / air
mixture which reaches the engine . (See Figure 2-
13 .)
The fuel / air ratio must be adjusted to meet the
changing needs of the engine for particular con-
ditions of load and speed . The ideal burning ratio
of fuel to air is about 1 :15 or one gram of fuel to
each 15 grams of air. This is an " ideal " or
"theoretical" mixing ratio, and is only achieved for
a fraction of the time that the engine is running .
Due to incomplete vaporization of fuel at low
speeds or additional fuel required at high speeds,
the actual operational fuel / air ratio is usually
richer .
Within the acceptable fuel / air ratios that can be
burned in the engine, a balance between power
and economy must be reached. The amount of air
entering the engine for combustion is the limiting
factor for maximum performance. To take advan -
tage of the limited amount of air available for com-
bustion, it is necessary to surround each air
molecule with enough fuel molecules to insure
that all of the air is utilized. Maximum power is ob-
tained by gaining maximum burning efficiency of
the available air. Maximum economy is gained by
surrounding each molecule of fuel with several
molecules of air to insure maximum use from a
given quantity of fuel. Maximum economy is maxi-
mum burning efficiency of the available fuel.
Figure 2 - 13
THEORY OF OPERATION 2-13