There are two “auxiliary” systems that are external on the
All Motorcraft 2150 carburetors are equipped with one of three
systems. They are all similar in operation and components.
Major components of the
automatic choke system mount on the right side of the
carburetor’s main body.
Throttle positioners used on Motorcraft 2150 carburetors are
more varied. Some
are simple dashpots (pneumatic buffers), while others are
solenoid/dashpot combinations, vacuum kickers, and
combinations. In general, the simple dashpot or
is the easiest to use and maintain. Throttle positioners are
mounted on the
left side of the carburetor’s main body, in front of the
Automatic choke systems
There were three automatic choke systems used on Motorcraft
Hot-air choke with electric assist
Both hot-air choke types use air tubes to get clean air from
the back of
the air horn (inside the air filter element) and pull it
through a coiled
heat exchanger tube mounted in the M-block’s intake manifold
Above: (1) Choke pull-down diaphragm, (2) hot-air tube, (3)
spade terminal, (4) choke plate lever, and (5) choke housing
The air picks up heat from the exhaust gas in the manifold
enters the coke housing where it warms the bimetallic
being pulled into the carburetor main body. When sufficient
heat reaches the
bimetallic thermostat, the choke opens.
Both the electric assist and all-electric chokes have an
electric spade terminal
on the end of the choke housing cap and one or more ceramic
heaters inside the
cap. The spade terminal is supplied with +12V from the
terminal. Whenever the engine is running, the ceramic heater
bimetallic thermostat to open the choke.
To set (or load) the choke, you press the accelerator pedal all
the way to the
floor before cranking the engine. This allows the choke plate
to be closed and
the fast-idle cam to be set by the tension in the cold
Closing the choke plate temporarily enriches the fuel/air
mixture to help start
the engine in cold weather. Setting the fast-idle cam increases
idle speed to keep it running and help it warm up more quickly.
Above: Choke components off carburetor body.
As soon as the engine starts, vacuum is supplied to the choke
and it opens the choke plate slightly to adjust the fuel/air
mixture for warm-up.
The amount that the choke pull-down opens the choke plate is
one of the
adjustment specifications for the carburetor. It is usually
documented in rebuild
As the choke bimetallic thermostat warms up, it opens the choke
plate and releases
the fast idle cam so the engine can return to its normal idle
speed when the
throttle is cycled.
There are four choke adjustments you must make when you rebuild
Choke cap position — this sets the degree of enrichment
provided by the
bimetallic thermostat in the choke housing cap.
Pull-down clearance — the clearance between the choke plate
and the inside
of the choke tower when the pull-down diaphragm receives a
Fast-idle clearance adjusting cam screw — this synchronizes
the fast-idle cam
position with the choke-closed position of the bimetallic
Fast-idle adjustment screw — this sets the idle speed
produced by the
Specifications for all these choke settings are in either the
kit instructions, or on the engine calibration label.
There were four common types of throttle positioners used on
Solenoid/vacuum kicker combination
A dashpot causes the throttle plates to close slowly for the
last few degrees
of throttle shaft rotation, as they approach full-closed
throttle (e.g., under
deceleration). Dashpots were introduced as an emissions control
device to reduce
excess hydrocarbon (HC) emissions caused by over-rich fuel/air
Above: Typical solenoid/dashpot combination device. (1)
Dashpot, (2) solenoid,
(3) throttle cable, and (4) idle speed set screw.
A dashpot is a small air chamber with an internal diaphragm, a
push rod, and a
bleed orifice. As the throttle closes, the throttle lever
contacts the push rod
and applies force from the throttle return spring to the
diaphragm, which pushes
air out of the chamber through the bleed orifice to slowly
allow the throttle to
drop to its idle position.
A vacuum kicker is a linear vacuum motor that uses a manifold
vacuum signal to open
the throttle plates slightly upon high-vacuum deceleration to
reduce excess HC
Solenoids allow throttle position to be changed by an electric
signal. They can be
used with air conditioning (A/C) to increase the idle speed
when the A/C
compressor clutch engages. They can also be used as
allowing the throttle plates to close farther than the curb
idle position when
the ignition is switched off.
Solenoids can be combined with either a dashpot or a vacuum
kicker, depending on
With a solenoid throttle positioner, you set the curb idle with
the solenoid in its
“normal” position while the engine is running. In the case of
an A/C solenoid, you
set curb idle with the solenoid de-energized. In the case of an
solenoid, you set the curb idle with the solenoid energized.
With a dashpot or vacuum kicker, you set the curb idle by
turning the adjustment
screw built into the back of the chamber.
Internal carburetor systems