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Two-Stroke Engines
Copyright Goodheart-Willcox Co., Inc.
The Reed Valve Engine
If the carburetor is mounted on the crankcase
of a two-stroke engine, the engine is identified
as a reed valve engine. The crankcase of the
engine houses and supports the crankshaft. The
crankshaft receives the up and down movement
(called “linear motion”) of the piston and trans-
lates it into rotary motion used to perform work.
The “reed” component of the reed valve, also
called a “diaphragm,” is a thin sheet of spring
steel or flexible composite material that seals
the crankcase from the carburetor, but opens
into the crankcase when the engine needs the
air-fuel mixture from the carburetor.
Figure 5-4 shows one location of a reed valve
mounted on a crankcase. Different engine mod-
els will have the reed valve at different loca-
tions on the crankcase, depending on the engine
application.
Figure 5-5 shows the reed valve mounted to
the insulator block. The insulator block is used
to minimize heat transfer from the crankcase to
the carburetor. The insulator block has a large
hole through it that allows air and fuel from the
carburetor to be delivered to the engine. One
gasket on the insulator block prevents outside
air from leaking in between the insulator block
and the crankcase. Another gasket on the insu-
lator block prevents air from leaking in between
the insulator block and carburetor. The back-
ing plate limits the amount the reed can open
during engine operation. This prevents the reed
from opening too wide, which would cause it to
weaken in time and eventually break.
On some engines, the reed valve is mounted
to a metal plate called a reed plate. Just like
the insulator block with reed valve attached,
the reed plate is mounted to the crankcase and
sealed with a gasket.
Typically, a reed valve engine runs at a lower
engine speed than a piston ported engine.
Because of its mechanical properties, a metal
reed can only open and close a limited num-
ber of times per second. Thus, a reed valve
engine’s maximum speed is limited. Some engine
Cylinder block Carburetor Insulator block
Goodheart-Willcox Publisher
Figure 5-3. The carburetor for a piston ported
engine is mounted on the side of the cylinder.
A B
Carburetor Crankcase Cylinder block Carburetor Reed valve Crankcase
Air intake Insulator block
Reed valve and
insulator block
Goodheart-Willcox Publisher
Figure 5-4. The reed valve engine has the carburetor mounted to the crankcase. A—The assembled
engine. B—The same engine has been partially disassembled to show the various parts. Note that the
reed opens in toward the crankcase.