Question:What is the choke and what does it do?
Answer:When the engine is first started, a very rich
air/fuel mixture is required because cold fuel vaporizes slowly.
The "choke" at the top of the carburetor provides the
richer mixture by closing and "choking off" the carburetor's
air supply. The choking effect also creates an area of low pressure
inside the throat of the carburetor that helps to pull additional
fuel through the main metering circuit. At idle there isn't enough
air flowing through the venturi to pull fuel through the venturi
discharge nozzle. By temporarily choking off the air supply,
however, manifold vacuum rather than venturi vacuum helps to draw
the extra fuel through the main metering circuit.
When the choked engine is cranked, fuel is pulled into the
engine through the idle port, the transfer port and the main metering
circuit. Combined with the reduced air supply, it creates the
extremely rich mixture that's needed to start a cold engine.
As soon as the engine starts, it needs air to keep running
and to offset the super rich fuel mixture. The choke plate shaft
is offset slightly to one side so incoming air will tend to push
it open. On many older carburetors, a vacuum operated piston
in the choke housing or carburetor casting was used to pull the
choke plate partially open so the engine could receive sufficient
air to keep running. These pistons were prone to gum up and stick,
causing hard starting and stalling problems.
In most later model carburetors (but not all), the vacuum
piston has been replaced with an external "choke pull-off"
vacuum diaphragm. The choke pull-off is also attached to the
choke linkage where it pulls the choke plate open slightly as
soon as the engine starts. On some carburetors, a pair of choke
pull-offs are used to give a more progressive opening. If the
choke pull-off fails to work (because of a vacuum leak or ruptured
diaphragm), the engine may be hard to start or stall. The amount
by which the choke is pulled open can be adjusted by bending the
"U-shaped" pull-off linkage. If the linkage is not
properly adjusted, however, it may open the choke too far, admitting
too much air causing the engine to stall.
Once the engine starts and begins to warm up, the fuel mixture
is gradually leaned out until the choke is fully open. This job
is performed by a temperature-sensitive bimetal spring attached
to the choke plate. The bimetal spring may be located in a well
in the intake manifold where it is heated by exhaust gases flowing
through the crossover passage, or it may be located inside a
plastic housing on the carburetor itself.
The choke housing may be heated one of three ways: by hot
air siphoned up through a pipe in the exhaust manifold that then
flows through the housing and enters the carburetor; by an electrically
heated element inside the housing; or by engine coolant circulating
through a hose attached to the housing.
Whatever the source of heat, the bimetal spring reacts by
unwinding to open the choke (which may occur in as little as 45
seconds or less on an emissions controlled engine). When the
engine is shut off and allowed to cool, the bimetal spring contracts,
winding up tighter to pull the choke shut. But since the choke
linkage is also attached to the "fast idle cam," the
choke won't close until the linkage is pumped at least once to
reset the cam. On automatic chokes that use an electric heating
element, a temperature sensitive contact switch may be used to
break the flow of current to the heating coil when the housing
is hot.
CHOKE PROBLEMS
When the choke is defective or not adjusted properly, it can
make an engine hard to start. The engine may also stall or die
after it is started.
Problems with the operation of the choke will result whenever
heating is lost at the choke housing. On carburetors that draw
heated air through a pipe in the exhaust manifold, rust can corrode
the inside of the pipe blocking air flow. Or the pipe can simply
rust off. When this happens, the bimetal spring doesn't heat
up quickly enough causing the choke to open too slowly. The result
is a rich fuel condition resulting in a rough idle, excessive
fuel consumption and high emissions. On carburetors that use
an electric heating element, a loose wire or poor ground connection
can prevent the choke from opening. The choke housing should
feel hot to the touch within a minute or so after the engine is
started if its heat source is working correctly.
The choke can be adjusted to alter the temperature at which
it closes and opens, which also enriches or leans the starting
fuel mixture. The choke is adjusted by loosening the screws that
hold the choke housing and then rotating the housing. This changes
the relative position of the bimetal spring inside which puts
either more or less tension on the choke.
Rotating the housing to increase tension on the choke (which
you can detect by holding the choke plate or watching it move)
will make it close at a higher temperature and enrichen the mixture.
Rotating the choke housing to decrease tension will make it close
fully at a lower temperature and produce a somewhat leaner mixture.
Notches are provided on some housings for reference. The vehicle
manufacturer will often specify how many notches rich or lean
the choke is to be adjusted.
On many late model carburetors, rivets are used instead of
screws to discourage tampering with the choke housing adjustment.
But this doesn't prevent adjustments because the rivets can be
drilled out. Once the adjustment is made, new rivets or screws
can be installed.
The rate at which the choke opens is critical. If the choke
doesn't open quickly enough, especially during warm weather, the
mixture becomes too rich and increases carbon monoxide emissions.
On the other hand, if the choke comes off too quickly, especially
during cold weather, the mixture can lean out causing the engine
to stall or to stumble when the throttle is opened quickly.
The choke pull-off plays an important role here by modifying
the rate at which the choke opens during warm weather. Some carburetors
are equipped with two choke pull-offs. The combination of two
choke pull-offs provides a progressive rate of opening that changes
according to temperature. The primary pull-off opens the choke
a bit when the engine starts to keep it running. The second choke
pull-off operates through a thermal vacuum switch that senses
the temperature of the heated air entering the carburetor.
During cold weather, the bimetal spring in the choke housing
will open the choke fully before the second choke pull-off has
any effect because the choke housing warms up faster than the
air entering the carburetor. So during cold weather, the second
choke pull-off has no effect. But during warm weather, the air
entering the carburetor is already warm. This causes the thermal
vacuum valve to open, passing vacuum to the second choke pull-off
which pulls the choke open sooner than it would open otherwise.
A vacuum delay valve is often used in the secondary choke pull-off
vacuum line to delay full choke opening for 8 to 20 seconds (to
give the engine a little more time to warm up).
