Mariner Mercury 105/140 JET, Service Manual

Page: 191 / 797

90-824052R3 JUNE 2002 FUEL SYSTEMS - 3B-31
Main Circuit
Air
Fuel
Key
a
b
c
d
e
f
g
h
i
a - From Open Thermal Air Valve
b - Air Flow
c - Venturi
d - Main Discharge Air Inlet Tube
e - Cross Holes
f - Main Fuel Well
g - Main Fuel Jet
h - Main Discharge Nozzle
i - Throttle Plate
As the throttle plate rotates past the off-idle ports, the
low pressure area extends to the main discharge
nozzle. In addition, the increased air flow through the
carburetor bore creates a low pressure area inside
the venturi. These combined forces create a strong
suction over the main discharge tube. Fuel flows:
1. Through the main fuel jet into the main fuel well.
2. Up the main discharge nozzle,
3. Into the venturi.
Air is mixed with the fuel to make it lighter, air enters
the main fuel well through the main discharge air inlet
tube. Cross holes are drilled in the main discharge
tube, allowing the air to mix with the fuel inside the
main well. As the throttle plate continues to open,
additional fuel is drawn out of the main discharge
tube, exposing additional cross holes. At full throttle,
the fuel mixture is controlled by the size of the main
fuel jet.
Back Draft Circuit
Air
Fuel
Key
g
a
b
c
f
e
d
NOTE: Fuel Flow Not Shown For Clarity
a - Back Draft Port
b - Back Draft Jet
c - Air Flow
d - Main Discharge Tube
e - Fuel Bowl
f - Fuel Bowl Vent Passage
g - Throttle Plate
At partial throttle settings, the back draft circuit leans
out the mixture for increased fuel economy. The back
draft circuit uses the float bowl vent circuit and bowl
vent jet to lean out the air/fuel mixture. The bowl vent
jet limits the amount of air entering the float bowl vent
circuit. With the throttle plate positioned correctly, the
low pressure area is exposed to the back draft port
inside the carburetor bore. The float bowl vent circuit
is connected by passages to the back draft port. The
low pressure area pulls air out of the bowl vent circuit.
Due to the size of the vent jet and the air loss through
the back draft port, the air pressure on the fuel inside
the fuel bowl is lowered to below atmospheric pres-
sure. Lower pressure on the fuel inside the float bowl,
lowers the amount of fuel being forced out of the main
discharge tube.