Air enters the idle system through the main (anti-siphon) air
bleeds and through
the idle air bleeds at the top of the main venturis.
The idle fuel/air mixture moves through restrictors at the
bottom of the idle
down channels, and then mixes with more air drawn through the
transfer slots just
before passing through the idle discharge ports into the
The amount of the idle fuel/air mixture that enters the
throttle bores is
controlled by the idle mixture adjusting screws. The tapered
tips of the
idle mixture screws protrude into the discharge ports and
effective metering area of the ports.
Above: (1) Idle down channel and (2) idle air bleed.
At off-idle, when the throttle plates begin to move, the
transfer slots deliver
fuel when exposed to the vacuum below the throttle plates. The
allow a smooth transition from the idle system to the main
metering system as
the throttle plates open.
As the throttle plates continue to open, airflow velocity
increases through the
main venturis and throttle bores, and the pressure differential
in the booster
venturis brings the main metering system into operation. As
fuel begins to
flow through the main metering system, it tapers off and stops
the idle system.
Above: Bimetallic hot-idle compensator.
Some later model Motorcraft 2150 carburetors have a hot-idle
valve mounted on the back of the air horn, behind the choke
tower. The HIC system
consists of a bimetallic strip attached an air valve, and
special air passages
inside the carburetor’s main body. When the temperature rises,
strip opens the valve and allows air to enter the throttle
bores under the
throttle plates, which leans out the idle mixture and raises
the idle speed.
Higher idle speed makes the water pump move more coolant
radiator and makes the fan move more air, thus preventing
extended periods of idling.
Most mid-to-late '70s Ford vehicles used a hot-idle
compensation system that was
independent of the carburetor. It consisted of a ported vacuum
switch (PVS) that
switched the vacuum signal for the distributor’s vacuum advance
system between the
normal carburetor spark port (i.e., ported vacuum) source and
full manifold vacuum.
The PVS uses coolant temperature to determine when to switch.
When ignition timing
is advanced by the higher manifold vacuum, idle speed
The fuel/air ratio produced by the idle system is determined by
The amount of air drawn through the main air bleeds, idle
and transfer slots
The amount of fuel drawn through the calibrated opening at
the bottom of the
idle pickup tube
The amount of fuel/air mixture that flows past the idle
needles and through the idle discharge ports
Since no fuel flows through the idle system at engine speeds
above idle, the idle
mixture adjustment screws have no affect on fuel/air ratio,
except at idle.
Accelerator pump system
When the throttle plates open for acceleration, airflow through
the venturis and
throttle bores responds almost immediately. However, because
fuel is heavier (and
more dense) than air, the fuel flow takes longer to respond to a
Above: External accelerator pump components. (1) Fuel bowl
(2) accelerator pump lever, (3) fuel bowl vent valve, (4)
pump rod, and (5) over-travel lever.
The accelerator pump system provides an instantaneous “shot” of
extra fuel in the
main venturis as the throttle plates open. This extra fuel
prevents a severe
lean-out in the fuel/air mixture that would otherwise occur.
The accelerator pump system is activated by rotation of the
throttle shaft when
the throttle plates open. A lever on the top of the throttle
shaft (called the
over-travel lever) pulls the accelerator pump rod, which pulls
pump lever on the pump housing.
The accelerator pump lever pushes the pump diaphragm in, which
pushes fuel into
the pump discharge passages inside the carburetor. When the
throttle closes, the
accelerator pump rod moves the pump lever forward, releasing
the force on the
Inside the accelerator pump housing, the pump diaphragm has a
spring under it to
push the diaphragm back out when the pump lever is released. A
(elastomer) valve allows fuel to be pulled into the pump’s fuel
chamber when the
diaphragm is pushed out, and keeps fuel from returning to the
bowl when the
diaphragm is pushed in.
Above: (1) Air/fuel bleed, (2) pump discharge passage, and
(3) holes for elastomer valve (lower hole is fuel inlet).
An air bleed orifice near the top of the accelerator pump fuel
vapor pressure buildup under the pump diaphragm. When the
diaphragm is pushed in,
this orifice bleeds any vapor or air trapped under the
diaphragm, as well as a
metered amount of fuel, back into the fuel bowl. The size of
this bleed orifice
is a factor in determining the pump’s discharge volume.
When the diaphragm pushes fuel into the pump discharge
passages, the pressurized
fuel pushes the pump discharge weight and check valve ball off
their seat below the
hollow screw in the middle of the booster venturi assembly.
Finally, fuel is sprayed into both main venturis through the
on the booster venturi assembly.
Above: (1) Check valve ball and (2) check valve weight.
The purpose of the check valve ball and weight inside the
hollow screw is to keep
air from being sucked back into the discharge nozzles (and
when the accelerator pump diaphragm is released. Since the
diaphragm can’t pull
air into the discharge passages, the negative pressure pulls
fuel from the bowl,
past the one-way elastomer valve, to refill the pump chamber so
it’s ready for the
The volume of fuel and the rate at which it is discharged by
pump system is determined by several factors:
The position of the accelerator pump rod in the throttle
The amount of accelerator pump rod movement produced by the
The position of the accelerator pump diaphragm when the
throttle is closed
The lever ratio of the accelerator pump lever
The size of the air bleed orifice in the pump fuel chamber
The size of the discharge nozzles
A poorly calibrated or improperly adjusted accelerator pump
system can cause
off-idle stumbling or hesitation, and weak throttle response.
High-speed bleed system
Whenever the engine is running, the Motorcraft 2150 carburetor
into the main fuel wells through the main (anti-siphon) air
bleed in the
booster venturi assembly. The high-speed bleed system is
designed to allow
less air to be drawn into the fuel stream as the airflow
This enriches the fuel/air mixture for high-rpm operation.
There were two types of high-speed bleed systems used on
Fixed high-speed bleed system
Variable high-speed bleed system
Above: (1) Main air bleeds and (2) variable high-speed
air bleeds on later model carburetor.
In early 2150 models (mid-to-late 1970s), the main air bleed is
integral with the
high-speed air bleeds. In later 2150 models (early-to-mid
the booster venturi assembly has separate main air bleed and
high-speed air bleed
The fixed high-speed bleed system uses fixed-orifice air bleeds
in the booster
venturi assembly. As fuel is drawn through the booster venturi
air is drawn into the fuel stream through the bleed orifices,
up to the airflow
limit of the fixed orifices.
The fuel/air mixture
is enriched when the airflow velocity through the booster
venturis increases beyond
the point at which the fixed bleed orifices reach maximum flow.
With the fixed high-speed bleed system, the point at which
enrichment begins is
determined by the size of the fixed bleed orifices, and the
rate of enrichment is
determined by the rate of airflow velocity change.
Above: (1) Lift rod and yoke and (2) tapered metering
The variable high-speed bleed system consists of tapered
metering rods that move
up and down in high-speed air bleed holes to regulate the
amount of air drawn into the
main fuel well and the fuel stream flowing to the booster
The metering rods are raised and lowered by a cam on the
shaft. When the throttle plates are closed, the cam does not
contact the lift rod
and the lift rod is held down by a spring.
When the throttle plates open, the cam engages the lift rod,
which raises the
metering rods to close off the bleed holes and enrich the
The variable high-speed bleed system provides more precise
control of the fuel/air
mixture over a wider range of engine speeds by linking mixture
with throttle position.
Above: (1) Throttle shaft, (2) lift rod spring and
(3) throttle shaft cam, and (4) lift rod, yoke, and
With the variable high-speed bleed system, the point at which
and the rate of increase in enrichment is determined by the
taper of the metering
rods and the position of the metering rods relative to the
throttle. The degree of
enrichment is determined by the amount of air drawn through the
air bleed holes
at a particular throttle position.
The variable high-speed bleed system gives you more options for
fine tuning the
performance of the carburetor. If your Motorcraft 2150 doesn’t
have the variable
high-speed air bleed system, I recommend you get one that does.
High-speed pullover system
The high-speed pullover system consists of discharge nozzles in
the front of
the choke tower connected to a tube that extends from the
body cover down into the fuel bowl. Some Motorcraft 2150
carburetors also have
an air bleed orifice between the discharge nozzles and the
Above: Pullover discharge nozzles.
At high airflow velocity, the pressure differential created on
nozzles in the choke tower draws fuel up from the bowl, through
the pullover tube,
to be released into the intake airstream. This enriches the
fuel/air mixture for
more high-speed power.
The fuel/air enrichment produced by the high-speed pullover
system is determined
by amount of fuel drawn through the calibrated opening at the
bottom of the
pullover tube, the size of the air bleed orifice (if used), and
the size of the
discharge nozzle openings.
Main metering system
As airflow velocity increases through the venturis and throttle
creates vacuum inside the booster venturis. Fuel is drawn
through the main
metering system by the pressure differential between the vacuum
booster venturi main discharge ports and the air pressure
inside the fuel