Honda CB350 Repair Manual page 105

100 FUEL SYSTEM
The circuit consists of a pilot air pas—
sage, pilot jet, and the throttle slide. Fuel
is provided from the float bowl through
the pilot jet and passage. The pilot air
passage serves a very important function,
even though it doesn't provide most of
the air feed for mixing. It is actually an
adjustable needle valve, and the amount
of air it lets through can be changed to
compensate for atmospheric conditions.
In operation, descending piston suc—
tion creates a low—pressure area behind
the throttle slide. To equalize this low—
pressure, air rushes through the pilot air
passage in amounts determined by the
pilot air screw, and mixes with fuel from
the pilot fuel jet. This mixture is bled into
the carburetor's intake tract through the
pilot outlet. The incoming air under the
throttle slide atomizes and delivers the
mixture to the combustion chamber.
Low—Speed Circuit
(V%¥&—%a Throttle Opening)
This circuit uses the same components
as the idle circuit. There is, however, an
increase in the airflow as the throttle
slide rises, and in the fuel flow as the
needle begins to come out of the needle
jet. This effects a transition to the mid—
range circuit, since the increases in fuel
and air delivered by the carburetor ven—
turi and the needle jet overshadow the
smaller amounts coming from the pilot
outlet, eventually eliminating the idle
circuit from the metering system.
Mid—Range Circuit
(Va—% Throttle Opening)
In this circuit, air is supplied by two
sources: the venturi and the primary air
passage. Fuel is supplied by the float
bowl and metered by the needle jet and
needle. The needle jet is located above
the main jet and works in conjunction
with the needle which is suspended from
the throttle slide. The primary air pas—
sage air serves to atomize the fuel being
drawn through the needle jet.
In operation, the needle jet and needle
work together to supply a corresponding
amount of fuel as the throttle slide is
raised and venturi action takes effect.
The needle is tapered so that, as it is
lifted, morefuel is allowed to pass through
the needle jet.
(1) (2) (3)
Mid—range circuit
. Primary air jet
. Throttle slide
. Secondary air jet
. Throttle plate
Main nozzle
. Primary main jet
Needle jet holder
. Secondary main jet
. Needle
. Needle jet
High—Speed Circuit
(%—Full Throttle)
This circuit is close to that described
for the basic carburetor. The needle has
lifted out ofthe needle jet, and therefore
no longer controls the amount offuel, and
the throttle slide has been raised high
enough so that it has minimal control
over the amount ofair intake.
Venturi action takes over completely.
The amount of air sucked into the com—
bustion chamber is determined by the
size ofthe venturi and the amount of fuel
is determined by the size of the main jet.
The only other component which still has
a significant effect is the primary air pas—
sage which continues to aid fuel atomiza—
tion.
It should be understood that the
operating ranges ofthe various metering
circuits overlap somewhat, and there is a
gradual, rather than an abrupt, transition
from one to another as the throttle is
operated.
This description should also explain