C.
Yamaha
Duo
Intake
System
(YO IS)
For this system, the primary carburetor and the secondary carburetor
are
equipped
as
a unit:
A
cable-operated-slide type functions
as
the primary carb, and a vaccum-controlled-slide type
provides a secondary bore. The engine has a
4-valve head, and each carb
has
its own intake port
and
valve. Although each carb bore and intake port
is
smaller than that
of
a more conventional
engine, their conbined
area
is
about 20% greater than a single, larger carb and intake layout.
Flow
efficiency
of
both the intake and exhaust sides
is
substantially improved.
From
idle to about
half
throttle, the primary carb supplies the air-fuel mixture, and almost the entire intake charge
enters through just one
valve; since the 4-valve layout offsets the intake ports relative
to
the
cylinder-bore axis, a strong YICS-type swirl
is
produced
in
the
low-to-medium rpm range.
Combustion efficiency
is
significantly enhanced, resulting
in
improved fuel economy.
In
addi-
tion,
the single small bore provides excellent low- and medium-speed throttle response due
to
the
high air velocity in the venturi. This eliminates the need for a mechanical accelerator pump,
allowing minimal carb height and ample room for the Monocross rear suspension. As the throttle
is turned from half to wide open, a linkage between the carburetors gradually opens the
secondary-carb
butterfly. The vaccum-controlled slide
in
the secondary carb opens
as
engine
demand
builds, providing superb mid-range smoothness. With both slides fully open, the engine
receives more mixture and produces more power than a
regular single-carb machine.
SECONDARY
CARB
PRIMARY
CARB
1-4
Top view
EXHAUST
INTAKE
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