8
OPERATION
In the “CHOKE” or “START” position, the choke shutter is closed and the only air entering the engine enters through
openings around the shutter. As the engine starts to rotate, downward piston travel creates a low air pressure area (or
vacuum) above the piston. Higher pressure (atmospheric) air rushes into the engine and fills this low pressure area. Since
the majority of the air passage is blocked by the choke shutter, a relatively small quantity of air enters the carburetor at
an increased speed. The main nozzle and both idle fuel discharge ports are supplying fuel due to the low air pressure in
the engine intake. Maximum fuel flow through the carburetor orifices combined with the reduced quantity of air that
passes through the carburetor, make a very rich fuel mixture which is needed to start a cold engine.
At engine IDLE speed, a relatively small amount of fuel is required to operate the engine. The throttle is almost completely
closed. Fuel is supplied through the primary idle-fuel discharge orifice.
NOTE: Dual system carburetors do not have an idle circuit.
During INTERMEDIATE engine operation, a second orifice is uncovered as the throttle shutter opens, and more fuel is
allowed to mix with the air flowing into the engine.
During HIGH SPEED engine operation, the throttle shutter is fully opened. Air flows through the carburetor at high speed.
The venturi, which decreases the size of the air passage through the carburetor, further accelerates the air flow. This high
speed movement of the air decreases the air pressure at the main nozzle opening. Fuel is forced out the main nozzle
opening due to the difference in the air pressure on the fuel in the carburetor bowl and the reduced air pressure at the main
nozzle opening.
TYGON TUBE
LOCATION
2
PRIMER BULB
4
PRIMER BULB
3
MAIN NOZZLE
EMULSION
TUBE
MAIN JET
For the fuel to flow, the carburetor bowl must be either
vented externally or internally. Some internally vented float
style carburetors use a tygon tube and a vent within the air
intake. This tube must be present for the carburetor to
operate properly (diag. 2).
Air is bled into the main nozzle and through the air bleed
located in the air horn. This mixes the fuel and air prior to
the fuel leaving the main nozzle. Atomization occurs as
the fuel mixture contacts the fast moving air stream. This
mist then flows into the intake of the engine.
FUEL PRIMERS
Primers may be mounted remotely or as an integral part of
the carburetor. The basic function of the primer is to supply
a charge of air to the carburetor main well, or carburetor
bowl. On diaphragm carburetors it displaces fuel directly
into the carburetor venturi. This displaced fuel provides a
rich mixture necessary for engines to start easily on the
first or second attempt (diag. 3 & 4).
Primers must be vented either internally (a passage in the
carburetor air horn prior to the venturi) or externally (through
a hole in the primer bulb). The vent allows air to fill the
primer bulb after the primer bulb is released. On diaphragm
carburetors a one way valve in the body prevents the fuel
from being forced back into the fuel tank.
Two different methods are used to prime float style
carburetors, leg prime and bowl prime. The leg prime
system is used only on the dual system carburetor. Air is
forced into the center leg of the carburetor, which then forces
an enriched mixture of fuel up the main nozzle. The bowl
prime method is used on Series 6, 8, 9 and 10 carburetors
and is distinguished by a stepped or hour glass shaped
primer bulb. A good seal of the primer bulbs center lip is
critical to assure that a full charge of air reaches the bowl.
Also critical is a tight seal around the float bowl.
NOTE: Never re-use a bowl gasket.
5
BOWL PRIME
