Fuel System Description; Carburetor; Types Of Carburetor; Principle Of The Vacuum Piston Operated Cv Type - Honda Motorcycle Service Manual

FUEL SYSTEM
SYSTEM DESCRIPTION
CARBURETOR
As the piston begins its descent at the start of the induction
phase (the period when the air-fuel mixture is drawn in),
pressure in the cylinder drops, causing air to flow from the air
cleaner, through the cartiuretor and into the cylinder. The
function of the carburetor is to atomize the fuel and create an
air-fuel mixture.
As in the figures on the right, air drawn into the carburetor
passes through constriction A, where it gains speed. The con-
striction is known as the venturi section of the carburetor. This
increase in flow speed is accompanied by a fall in pressure in
the venturi, whichis used to draw off fuel from the outlet. The
fuel is atomized as it is drawn into the venturi under the in-
fluence of atmospheric pressure, and is mixed with the incom-
ing air.
Carburetors are also equipped with mechanisms for regulation
of the air and mixture volumes. A throttle valve is used to
regulate the flow of air-fuel mixture, and a choke is included
for adjusting the air flow under starting conditions.
Types of carburetors
Carburetors which alter the diameter of the venturi by
throttle valve movement are known as variable venturi types.
The variable venturi cotinuously changes in diameter from low
to high speed in proportion t o the intake air volume t o give
smooth aspiration at low speeds and improved power output
in the high speed range. Honda motorcycles, scooters and
ATVs use one of t w o variable venturi designs.
1 . The constant velocity type (CV): the venturi diameter is
altered automatically by vacuum piston that rises and falls
to alter the diameter. (The throttle valve is installed as a
separate mechanism.)
2. The piston valve or flat slide type: a throttle-controlled
piston is used t o alter the venturi diameter.
Principle of the vacuum piston operated CV type
As the engine is started and the throttle valve opens, the air
flow in the main bore exerts a strong negative pressure on the
lower section of the vacuum piston (see Carburetor theory). At
this point, air is drawn out of the carburetor's vacuum chamber
and pressure in the chamber drops. The diaphragm is lifted due
to atmospheric pressure, and the vacuum piston is raised.
When the throttle valve is closed, air flow in the main bore is
obstructed. Pressure returns to that of the atmosphere and the
vacuum piston is lowered by spring force.
,
Larqe drop in pressure
small drbp in pressure
/
CV TYPE
PISTON VALVE TYPE
(SLIDE TYPE)
VACUUM PISTON
VACUUM
DIAPHRAGM
CHAMBER
MAIN BORE
ATMOSPHERIC
PRESSURE
AIR FLOW O
ROTTLE VALVE
Date of Issue : March,
1995
O
HONDA MOTOR CO.,LTD