Service Manual

39
ENGINE
General Information
In this section, we will cover both versions of the E
series en
g
ine. The DuraForce is a reed valve en
g
ine,
and the R tek is piston ported. The desi
g
n difference
affects the path which the incomin
g
fuel/air char
g
e
takes to
g
et to the firin
g
chamber.
Two-cycle en
g
ines have special advanta
g
es which
make their use more practical in certain applications.
Two-cycle en
g
ines are li
g
htwei
g
ht with an excellent
power-to-wei
g
ht ratio and can be operated in any
position, limited only by the carburetor and fuel supply
system. They are also notably easy to maintain and
service because of their uncomplicated desi
g
n.
Theory of Operation
Reed Valve Engine
The illustration below (see Fi
g
ure 64) shows what
happens inside the en
g
ine durin
g
one full crankshaft
revolution. Fuel intake, fuel i
g
nition, and the exhaust of
burned
g
ases all take place durin
g
a sin
g
le 360-de
g
ree
rotation of the crankshaft.
Fi
g
ure 64
3297-022-2
Be
g
innin
g
at a point where the top of the piston is just
below the exhaust port, the piston moves forward
toward the crankcase. As the transfer port is
uncovered, the fuel/air mixture stored in the
pressurized crankcase is forced into the combustion
chamber. (See Fi
g
ure 65.)
Fi
g
ure 65
3297-021-2
The crankshaft continues its rotation, and the piston
be
g
ins its travel toward the spark plu
g
. As it moves,
the piston seals off the transfer port first, then the
exhaust port. When both ports are sealed, the
remainin
g
travel compresses the air/fuel mixture to
prepare it for i
g
nition. The travel of piston towards the
spark plu
g
is called the compression stroke. (See
Fi
g
ure 65.)
The movement of the piston towards the spark plu
g
also has an important effect in the crankcase. Once
the transfer port is sealed, a vacuum is created in the
crankcase. This action pulls the reed valve open and
draws in a fresh char
g
e of fuel/air mixture.
Just before the piston reaches top dead center (TDC),
the spark plu
g
fires. When the en
g
ine is functionin
g
properly, the fuel/air mixture i
g
nites, causin
g
a burn
that occurs evenly throu
g
h the power stroke. The
piston chan
g
es direction and be
g
ins its travel towards
the crankcase.
NOTE: If the compression ratio is too hi
g
h, the fuel/air
mixture actually explodes before it is i
g
nited by the
spark plu
g
. This can be heard and is identified as
detonation (also called “knock”).
When the piston opens the exhaust port, hi
g
h-pressure
exhaust
g
ases exit the exhaust port. Further
movement of the piston towards the crankcase
uncovers the transfer port which allows a fresh char
g
e
of air/fuel mixture to enter the combustion chamber.
As the exhaust
g
ases continue to exit the exhaust port,
the en
g
ine is readied for another cycle.
A. Reed Valve Closed B. Exhaust Gases
A. Reed Valve Open