Constant-Velocity Carburetor; Carburetor Overhaul; Direct-Control Type - Honda CB350 Repair Manual

FUEL SYSTEM 101
why it is fruitless to make random
changes in carburetor settings without
first determinging the nature ofthe prob—
lem, and the range in which it occurs.
CONSTANT—VELOCITY
CARBURETOR
The constant—velocity carburetoris ba—
sically the same as the direct—control type
carburetor, except that the throttle twist—
grip is not connected directly to the throt—
tle slide. Instead, in the CV carburetor,
the throttle grip and cable are connected
to a butterfly valve located between the
intake manifold andthrottle slide. As the
throttle butterfly is opened, the manifold
vacuum evacuates air from the top of the
slide chamber through a passage in the
slide. Consequently, on demand from the
engine, the slide is raised and more air is
admitted, and the tapered needle is pro—
portionally lifted out of the jet tube to
admit more fuel.
The term "constant—velocity" (or con—
stant vacuum) refers to the speed of the
air passing over the main jet tube and the
vacuumin the carburetor throat (between
the butterfly and slide), which remains
constant due to the movement of the pis—
ton in relation to the vacuum. As the
engine demands more air and the mani—
fold vacuum increases, the slide re—
sponds by lifting in proportion to the vac—
uum. Thus the carburetor air speed and
vacuum remain constant, because an in—
crease in vacuum means an increase in
slide lift, which in turn increases the
amount ofair passing through the carbu—
retor by altering the size of the air pas—
sage (venturi), and compensating for the
increased engine demands with a larger
flow ofair. A constant vacuumindicates a
constant—velocity, and vice versa.
The only other basic differences be—
tween the direct—control type of carbu—
retor and the CV type is that the pilot
screw controls the amount of fuel enter—
ing the system from the float bowl in—
stead of the air as in the direct—control
type. The CV carburetor also has two
main jets (primary and secondary) with
the needle jet and needle above the sec—
ondary main jet. The primary main jet
functions mainly in low—range operation.
Carburetor Overhaul
DIRECT—CONTROL TYPE
Removal and Installation
1. Unscrew the carburetor cap and
pull the throttle slide assembly out ofthe
carburetor. If the slide assemblyis to be
disassembled, see the procedure below.
If not, place a small plastic bag around
the assembly to keep dirt out, and place it
out of the way. Note that mishandling the
assembly may necessitate replacement
ofthe slide or needle.
2. Turn the fuel supply off at the pet—
cock, and disconnect the fuel line from
the carburetor.
3. Remove the float bowl drain plug,
and drain off any gas in the float bowl.
Disconnect any overflow or breather
tubes.
4. Loosen the intake manifold and air
cleaner connecting bands. Remove the
carburetor.
5. Installation is in the reverse order of
the above, but the following points
should be noted:
a. Lubricate the throttle slide if de—
sired with a molybdenum or graphite
lubricant. Insert it into the carburetor
with the slide cutaway facing the air
cleaner. Be sure that the slot in the
slide is engaged with the tab in the car—
buretor body;
b. Be sure that the slide goes in eas—
ily, and especially that the needle
enters the needle jet. Do not push or
force the slide into place. If resistance
is noted, it is probable that the needle
is cocked to one side and is not enter—
ing the needle jet;
c. Tighten the carburetor cap and
check throttle operation. The slide
must move freely up and down. The
throttle return spring must have
enough tension to snap the slide to its
fully closed position if the throttle is
opened and released;
d. Connect the fuel line from the
right carburetor to the rear outlet on
the petcock, and the fuel line from the
left carburetor to the front outlet on the
petcock. Secure the fuel lines in place
with a spring clip;