A CTUALES SISTEMAS DE GESTIÓN AMBIENTAL

Electric and hot air chokes operate almost identically – the only difference is the heat source they use. Just like the manual choke, the choke plate is connected by a link rod that passes down through the air cleaner mounting boss to the choke assembly. The rod is connected a choke housing lever. The lever is put under tension by a coil spring which holds the choke in the closed position when the coil spring is cold. If the coil is warmed up, it expands and slowly moves the choke housing

lever/choke link rod/choke plate to the open position. The amount of tension in the spring determines how much heat is required to get the spring to start moving. The tension can be adjusted (by manually winding the spring up or down) by rotating the choke housing cap a few degrees either way. A series of marks on the cap show just how far it has been turned. One mark (the “index mark”) is normally bigger than the others and is used as the reference point. Rotating the cap

anticlockwise to make the choke stay on longer is referred to as “richer”, whilst rotating the cap clockwise to make the choke open earlier is referred to as “leaner”. Both RICH and LEAN are cast into the choke

cap as a reminder. The choke coil must be adjusted so that it opens the choke plate at the proper speed as the engine warms up - slowly enough to prevent lean stumbles; yet fast enough to prevent over-rich mixtures.

The heat source for a hot air choke is the engine warmth. A small amount of air is drawn from a clean place, often by tapping into the air cleaner (the image to the right shows a Model 2300 carburettor where the air cleaner bosses have been drilled and a tube fitted to allow clean air to be drawn from the air cleaner). The clean air is then routed past a source of heat – often a choke stove in the exhaust manifold or inlet manifold crossover. If the engine is cold, the air is also cold, As the engine warms up, so too does the air flow.

The “hot air” then passes into the choke housing past a small brass piston (we’ll come back to that small piston later) and past the coil spring. The “hot air” provides the heat to warm up the coil spring. The spent air then flows through a small vacuum port in the side of the carburettor (see red arrow in diagram to the right above). This port is present in all 350 Holley carburettors,

though with manual chokes fitted the port is either covered with a gasket or is lead-filled. The air flows down a channel in the main body, through a channel in the throttle body and into the inlet manifold (manifold vacuum is used to “suck” the air along). A small screw-in brass restrictor (0.055” diameter or #54 drill) in the throttle body is used to control the rate of the air flow (the orifice is present in all 350 Holleys, including those with manual chokes – see red arrow in the diagram to the right). For an electric choke, the same air flow occurs, but the air source is not heated (it is often still drawn from the air cleaner, but does not

pass through a hot air stove). Here the air flow is used to prevent the electric choke coil from burning out. Heat for the electric choke is supplied by electricity warming the coil (just like a toaster element). The electrical power is turned on when the vehicle ignition is switched on, starting to slowly warm and unwind the choke spring. Even when the engine is hot, the choke spring normally has power supplied to it. Without the air flow, the choke coil would soon burn out.

The air flow path can be seen in the image to the right:

1. hot air enters the choke housing,

2. passes through a channel and into the main housing compartment,

3. flows over the choke coil (removed in this photograph) which warms the coil,

4. flows past the choke pull-off piston, 5. passes through a further channel, and

6. exits the choke housing to flow to the throttle body.

Having the choke plate fully shut is fine for initial starting as it puts a lot of vacuum on the fuel system (at the low engine speeds developed by the starter motor there is precious little vacuum, so closing the choke plate helps conserve what little there is). However, when the engine first fires the vaccum available is much larger, and the engine requires to be fed some air. A fully closed choke plate will cause the engine to be overly rich, and can stall or flood. To prevent this, the choke plate is cracked open slightly as soon as the engine fires. This is done by the small brass piston mentioned above, called the choke pull- off. The choke pull-off has the air flowing past it for the choke supply. At low engine speeds (when the starter motor is turning) there is not enough vacuum (and hence not enough air flow to the choke) to move the small brass piston. Once the engine fires, vacuum increases (as does air flow to the choke). This causes the choke pull-off piston to move, overriding the choke coil spring and cracking open the choke plate. After a minute or so of operation, the choke coil spring has

warmed up and opens the choke plate even more, making the choke pull-off redundant. Note that the choke pull-off piston only just cracks the choke plate open – if it opens too much, the engine could hesitate, backfire through the carburettor or stall from having too little choke function. The amount the choke plate is cracked open by the choke pull-off is adjustable, often by bending a linkage rod or by an adjustment screw. The choke pull-off can be seen in the image to the right.

The choke linkage also incorporates a fast idle cam (the red plastic item in the image to the right). The fast idle cam bumps open the throttle a small amount when the choke is opened, increasing engine speed. The fast idle cam has a number of “steps” that are ridden by the fast idle screw. As the engine warms and the choke closes, the fast idle cam rotates, the fast idle screw drops down to lower “steps” and the throttle closes back to the curb idle speed. When the choke pull-off cracks the choke plate open, pushing the accelerator pedal allows the cam to rotate so that the fast idle screw will drop from the highest (fastest) step and align with the second highest (second fastest) step of the fast idle cam. Note that adjusting the choke pull-off may also change the fast idle cam position and vice-versa.

If the vehicle has flooded, the spark plugs will be wet and will prevent the engine firing. If the accelerator pedal is pushed all the way to the floor (and held there), the throttle will rotate open ready to let lots of lean fuel/air mixture in to dry the plugs. However, the closed choke plate will prevent this air getting in. To accommodate this, the fast idle cam lever has a small unloader tang on one side. By pushing the accelerator pedal all the way to the floor, the rotating throttle shaft rotates the fast idle cam lever, the tang moves forward and butts up against the end of the “steps”, pushing the fast idle cam partly around. This drives the choke plate manually open (not all the way, but a little bit more than the choke pull-off would open it). This allows the airflow in to clear the flooded engine. Releasing the throttle allows the choke spring (or choke pull-off) to resume controlling the choke plate position.