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CHAPTER 5 The Two-Stroke Cycle Engine
Copyright Goodheart-Willcox Co., Inc.
in place by springs that pull the shoes toward
the center of the clutch. See Figure 5-29. When
the engine is idling, the centrifugal clutch is not
engaged. As the engine speed is increased, the
centrifugal force acting on the shoes overcomes
the tension of the return springs. This causes
the shoes to move outward, into contact with the
clutch drum. As a result, the clutch drum begins
to rotate with the shoes and springs. When the
crankshaft slows down, the centrifugal clutch
disengages.
The amount of spring tension determines
the speed at which the clutch engages. Strong
springs will require high engine speeds to cre-
ate enough centrifugal force to expand the shoes
and engage the clutch drum. Springs with less
tension will allow the shoes to expand at lower
engine speeds. Typically, two-stroke equip-
ment clutches are not engaged at idle. However,
not all two-stroke engines operate in the same
engine speed range, so the strength of the clutch
springs is selected to match the operating speed
of the engine.
Carburetor
The carburetor mixes fuel with air in a precise
ratio so the engine can burn it in the combus-
tion chamber. The combustion chamber is the
sealed area in the cylinder above the piston and
contains the burning air-fuel charge. The car-
buretor supplies fuel at the correct rate for all
engine speeds, from idle to maximum load. Car-
buretors will be discussed in detail in Chapter 6,
The Fuel System.
Air Filter
The air filter system includes the air-filtering
component mounted to the engine and other
components that work with it to filter all the air
entering the engine. The term air filter generally
refers to the container mounted to the carbure-
tor, which consists of a housing, a cover, and an
air filter element. Sometimes, the term “air fil-
ter” is used to describe the device that actually
filters the air, the air filter element. In either con-
text, the air filter removes contaminants from r
the air before the air enters the engine.
Air Filter Housing
The air filter housing is used to securely mount
the air filter element to the carburetor. The edge
of the air filter element must seal against the
housing so that no air can bypass the filter and
enter the engine with contaminants. The sur-
face between the housing and the carburetor
must be sealed to prevent unfiltered air from
A B
Clutch shoe Spring Clutch drum
Goodheart-Willcox Publisher
Figure 5-29. The centrifugal clutch has clutch shoes that move outward as engine speed increases.
A—The three clutch shoes move outward as engine speed increases. When the engine slows, tension
from the three springs pulls the shoes back to their original position. B—A drum is installed over the
clutch shoes. At low engine speeds, the shoes rotate within the drum, but the drum remains stationary.
As engine speed increases, the shoes move outward and contact the drum. The drum begins spinning
with the shoes.