Direct-Control Carburetor - Honda CB350 Repair Manual

FUEL SYSTEM 99
It is important that the fuel level in the
float bowl be maintained at the proper
level, or the engine will receive a lean or
rich mixture as the motorcycle acceler—
ates, brakes, or banks.
It is the difference in pressure between
the float bowl (at atmospheric pressure)
and the pressure in the venturi above it
which causes the fuel to rise through the
fuel nozzle into the air flowing through
the venturi. The fuel nozzlein this case is
either the main jet or the pilot jet. The
venturi is basically an air funnel which is
narrower in the center than it is at the
ends. When air rushes through it, pulled
by the suction of the downward moving
piston through the open intake valve, a
low—pressure area is created in the nar—
rowsection of the venturi. The higher the
rpm of the engine, the faster the air
moves through the venturi, and the
greater the pressure drop in the narrow
section. This narrow section is actually
the area of the throttle slide; thus the
slide allows the venturi size to vary.
Since the fuel in the float bowl is at atmo—
spheric pressure and the venturi above
the main jet is a low—pressure area, the
fuel is forced up through the jet into the
air flowing through the venturi in an ef—
fort to equalize the pressure. The amount
offuel is metered by the main jet, and the
air by the size of the venturi.
Of course, in actual practice, carbure—
tors are not quite that simple. The basic
unit described above makes no provi—
sions for throttle control, cold weather
starting, or the varying needs of the
engine.
The operation of a practical carburetor
can best be described by dividing it into
five circuits, and the components which
control each one.
The carburetor casting is drilled with a
number of air and fuel passages. Among
these are: the primary air passage which
is drilled from the front of the carburetor
just beneath the intake to the needle jet.
The air drawn through this passage helps
to atomize the fuel passing through the
needle jet before it enters the carburetor
venturi.
The pilot air passage is located along—
side the primary air passage. The air
passing through this passage is metered
by the pilot air screw and provides air for
the idle and low—speed operation.
The pilot outlet is a very small drilling
which can be seen on the engine side of
the throttle slide bore. The air and fuel
mixture for idling pass through this pas—
sage and then to the engine.
Other metering devices and jets are de—
scribed in the following sections.
DIRECT—CONTROL CARBURETOR
Starting Circuit
(0 Throttle Opening)
When the choke lever is operated, a
plate closes off the mouth of the venturi
and a low—pressure area drawn in through
the pilot air passage is mixed with fuel
from the pilot jet. This produces a rich
mixture needed for cold weatherstarting.
Idle Circuit
(0O—% Throttle Opening)
At idle and under normal operating
conditions, the engine requires very little
fuel and air. It does require, however,
more accurate metering than pure ven—
turi action can provide while the engine
is still turning over slowly and intake air
velocity is low.
(4) $) \_
Idle and low—speed circuits
1. Pilot outlet
By—pass
Slow air jet
Pilot screw
Primary jet
. Primary main jet
. Main nozzle