Service Instructions; Description; High Speed System; Operation - Wisconsin vg4d Repair Manual

ZENITH MODEL 87A8
CARBURETOR
L-57, L-77
Series
SERVICE INSTRUCTIONS
DESCRIPTION
The Zenith 87-Series is a horizontal carburetor with a con-
centric fuel bowl. It is a ~balanced
~ carburetor,
because
all air for fuel chamber and metering well ventilation and
idling must come through the air cleaner. Air cleaner
restrictions have a minimum influence on the fuel-air ratio
when a carburetor is thus ~balanced
".
The main jet and discharge jet are centrally located. The
metering well which completely surrounds tlhe discharge
jet is in the center of the fuel bowl assembly. This con-
struction permits extremely high angle operation in any
direction.
The venturi, which is part of the throttle body casting,
measures the volume of air that passes through the car-
buretor. In selecting the venturi size, the smallest size
that will permit full power development should be used.
PITOT TUBE
BOWL VENT
PASSAGE
FUEL
ALVE
SEAT
FUEL
VALV E
NEEDLE
FUEL
LOAT
INLET
Fig. 1 FUEL SUPPLY SYSTEM
OPERATION
FUEL SUPPLY SYSTEM (Fig. 1)
Fuel under normal pressure entering the float chamber
through the fuel valve seat is controlled by the twin float
which, moving on its axle, closes the needle valve when
the fuel reaches the proper level in the bowl.
IDLE SYSTEM (Fig. 2)
At idling speeds the throttle plate is almost closed, thus a
very high suction exists at the edge of the throttle plate
where the idle discharge holes are located. All fuel for
idling and part throttle operation is supplied through the
IDLE AIR
IDLE AIR r'~
BLELD PAS,SAGE ~ THROTTLE
~ // PLATE
, n
METERING
// "
~
~ MAIN JET
Fig. 2 IDLE SYSTEM
main jet.
Fuel from the float-chamber flows through the
main jet into the metering well. Fuel for idling is drawn
{tom this well through the calibration, or metering orifice,
in the center of the idling jet. As the fuel reaches the
idling channel it is mixed with air which is admitted
through a calibrated orifice in the channel from the inside
of the air intake to form an emulsion. This emulsion is
discharged into the air stream, to form the idling mixture,
through two holes one of which is controlled by the idle
adjusting needle. Turning the adjusting needle counter-
clockwise (out) permits more of the emulsion to reach the
air stream and make the idling mixture richer while turning
the needle in (clockwise) cuts off the amount of the emul-
sion reaching the air stream and makes the mixture leaner.
HIGH SPEED SYSTEM (Fig. 3)
As the throttle is opened, the suction on the idling system
diminishes, but the increased volume of air entering the
engine through the venturi creates sufficient vacu~un
(suction) on the discharge jet to draw an emulsion of fuel
and air from the metering well which receives its fuel from
the main jet and its air from the well vent. The flow
characteristics of the discharge jet are influenced by the
size, location, and number of holes in the sides of that
part of the jet which is in the metering well, as well as by
VENTURI
W
Fig. 3 HIGH SPEED SYSTEM
the sizes of the discharge jet orifice, the size of the main
jet, and the size of the well vent. The well vent is located
in the air intake and permits air to enter the top of the
metering well around the outside of the discharge jet. The
flow of fuel through the main jet is controlled by the size
of main jet opening.
CHOKE SYSTEM (Fig. 4)
Starting a cold engine requires a much richer mixture of
fuel and air. Moving the choke lever to close the choke
plate restricts the air entering the carburetor (except at the
pitot tube, Fig. 1, to the bowl vent) and increases the
suction on the idling system which makes the mixture
richer.
STARTING THE ENGINE
Before cranking the engine, the carburetor throttle should
be opened a little to expose both idle discharge holes to
suction. The choke should be fully closed until the engine
starts, then opened a little to prevent stalling from being
over-choked, then when the engine is fully warmed up the
choke can be returned to wide open position and the
throttle closed to the idling position.
ADJUSTMENTS
Adjust the throttle stop screw to obtain the desired idling
speed by turn[ng screw in (clockwise) to increase speed
and out (counter-clockwise) to decrease engine speed.
37