Cold-Starting System
Since the first person struggled with a hard-starting engine on a cold morning, the humble choke-butterfly system has helped our engines. When engines are totally cold, they need more fuel to support combustion. The choke closes off the air supply and enriches the fuel mixture for improved cold-starting performance.
We tend to think suction (manifold...
We tend to think suction (manifold vacuum) is the reason fuel comes from the bowl via the boosters, but it's actually the atmospheric pressure difference between the fuel bowl and throttle bore that draws the fuel through the boosters. Whenever we have less air pressure in the throttle bores than we do in the fuel bowl, fuel will flow toward the low-pressure area.
Here is a close-up illustration...
Here is a close-up illustration of the main fuel nozzle, also known as a main discharge tube. Note the air bleeds which help atomize the fuel.
This close-up of a Holley...
This close-up of a Holley 4160 shows the air-bleed holes for improved fuel atomization. They are factory-sized on most Holley and Demon carburetors, but some newer models have adjustable air bleeds that can be changed as necessary for finite carburetor tuning. Note the accelerator-pump discharge nozzle between the primary bores. Also note the fuel-discharge boosters that get fuel from the main-metering jets in the fuel bowl.
There are two basic types of choke systems, manual and automatic. Manual chokes are operated by hand via a control cable. High-performance Fords were generally equipped with manual choke systems from the factory. The rest were fitted with automatic chokes that engage when the engine is cold and disengage as the engine warms up. Automatic chokes rely on exhaust-manifold heat and a simple bimetallic coil spring to get the message that the engine is warm. Heat is drawn by manifold vacuum from the exhaust manifold or header to the choke-coil package. As the bimetallic spring expands with the heat, it gradually opens the choke and brings the throttle off the fast-idle cam.
Edelbrock, Carter AFB/AVS,...
Edelbrock, Carter AFB/AVS, and Rochester Quadra-Jet carburetors have a different fuel-metering system design than Holley, Autolite, and Demon. Fuel metering rods are either vacuum or mechanically operated. These rods are tapered and hang in the middle of the main metering jet to control fuel flow through the jets. We can change jet and metering-rod sizes depending on performance demands.
Proper cold-starting calls for a one-time application of throttle before starting. This causes the choke to close and the fast-idle cam to kick in. A separate throttle-stop screw jumps onto the fast-idle cam, allowing the engine to run at a fast warm-up idle. When the bimetallic coil expands and pulls the choke off, it also rotates the fast-idle cam to normal, allowing the normal-idle throttle-stop screw to close to normal at a warm idle.
Not all automatic chokes get heat from a hot exhaust manifold. Today, most aftermarket carburetors have an electric choke that gets power when the ignition is turned on. As the heating element warms the bimetallic coil, the choke begins to come off. General Motors used hot-water automatic chokes long ago, where hot coolant circulated through the choke assembly, heating the bimetallic coil for choke pull-off. Chrysler was big on choke stoves, mechanical linkages, and bimetallic coils. With V-8 engines, this kept the intake manifold decidedly hot, causing more than its share of hot-weather driving problems.