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Choke System

When the engine is cold, the fuel tends to condense into large drops in the manifold, rather than vaporizing. By supplying a richer mixture (8:1 to 9:1), there will be enough vapor to assure complete combustion. The carburetor is fitted with a choke system to provide this richer mixture. The choke system provides a very rich mixture to start the engine and to make the mixture less rich gradually, as the engine reaches operating temperature. The two types of choke systems are the manual and automatic:

• The manual choke system (fig. 4-29) was once the most popular way of controlling the choke plate; however, because of emissions regulations the possible danger when used with catalytic converters and technological advances in automatic choke systems, manual chokes are not often used today. In the manual choke system, the choke plate is operated by a flexible cable that extends into the operator’s compartment. As the control is pulled out, the choke plate will be closed, so the engine can be started. As the control is pushed back in, the position of the choke plate is adjusted to provide the proper mixture. The following are two features that are incorporated into the manual choke to reduce the possibility of the engine flooding by automatically admitting air into the engine.

• A spring-loaded poppet valve (fig. 4-30) that is automatically pulled open by the force of the engine intake strokes.

• An off-center choke valve (fig. 4-31) that creates a pressure differential between the two sides of the choke plate when it is subjected to engine intake, causing it to be pulled open against the force of spring loaded linkage.

• Automatic chokes (fig. 4-32) have replaced the conventional manual choke. They control the air-fuel ratio for quick starting at low temperature and also provide for the proper amount of choking to enrich the air-fuel mixture for all conditions of engine operation during the warm-up period. An automatic choke system has a choke plate (valve), a thermostatic spring, and other parts depending upon choke design.

• The choke plate is a butterfly (disc) valve near the top of the carburetor air horn. When the choke plate is closed, it blocks normal air flow through the carburetor.
• The thermostatic spring is a bimetal spring (spring made of two dissimilar metals) which may be used to open and close the choke. The two metals have a different rate of expansion that make the spring coil tighter when cold and uncoils when heated. This coiling-uncoiling action is used to operate the choke.

The basic operation of the automatic choke system is as follows:

1. With the engine cold, the thermostatic spring holds the choke closed. When the engine is started, the closed choke causes high vacuum in the carburetor air horn. This pulls a large amount of fuel out of the main discharge tube.
2. As the engine and thermostatic spring warm, the spring uncoils and opens the choke plate. ‘This action produces a leaner mixture. A warm engine will not run properly if the choke were to remain closed.

Various methods are used to control the warming of the choke thermostatic spring. The four methods of providing controlled heat to the thermostatic spring are as follows: electricity, engine coolant, well-type heated, and exhaust manifold.

ELECTRICITY (fig. 4-33) uses an electric coil to heat the thermostatic spring. The heating coil is switched on with the ignition switch. Some systems use a control unit that prevents power from reaching the electric coil until the engine compartment reaches a desired temperature.

ENGINE COOLANT (fig. 4-34) uses a passage in the thermostat housing to circulate engine coolant for heating the thermostatic spring.

WELL-TYPE HEATED (fig. 4-35) mounts the thermostatic spring in the top of the exhaust manifold. As the engine and manifold warms, the thermostatic spring uncoils to open the choke.

The EXHAUST MANIFOLD (fig. 4-36) uses heat from the exhaust manifold to heat the thermostatic spring. The exhaust heat is brought to the choke through the means of a heat tube. The heat tube passes through the exhaust manifold, so as it takes in fresh air via the choke stove, it picks up heat from the exhaust without sending any actual exhaust fumes to the choke mechanism.

When the choke system is operating during warm-up, the engine must run at a faster idle speed to improve drivability and prevent flooding. To accomplish this, fit the carburetor with a fast idle cam (fig. 4-37) that is operated by linkage from the choke.

When the choke closes, the fast idle cam swings around in front of the fast idle screw. As a result, the fast idle cam and fast idle screw prevent the throttle plate from closing. Engine idle speed is increased to smooth cold engine operation and prevents stalling. As soon as the engine warms, the choke opens and the fast idle cam is deactivated. When the throttle is opened, the choke linkage swings away from the fast idle screw and the engine returns to curb idle (normal, hot idle speed).

If for some reason the engine should flood when it is cold, a device is needed to open the choke, so air may be admitted to correct the condition. This is accomplished by the choke unloader (fig. 4-38). The choke unloader can be either mechanical- or vacuum-operated.

A mechanical choke unloader physically opens the choke plate any time the throttle swings fully open. It uses a metal lug on the throttle lever. When the throttle lever moves to the fully opened position, the lug pushes on the choke linkage (fast idle linkage). This provides the operator a means of opening the choke. Air can then enter the air horn to help clear a flooded engine (engine with too much liquid fuel in the cylinders and intake manifold).

A vacuum choke unloader (fig. 4-39). also called a choke brake, uses engine vacuum to crack open the choke plate as soon as the engine starts. It automatically prevents the engine from flooding.

Before the engine starts, the choke spring holds the choke plate almost completely closed. This action primes the engine with enough fuel for starting. Then as the engine starts, the intake manifold vacuum acts on the choke brake diaphragm. The diaphragm pulls the choke linkage and lever to swing the choke plate open slightly. This action helps avoid an overly rich mixture and improves cold engine drivability.

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