How weather affects a carburetor and the problems it causes.

Gasoline became the fuel of choice for the internal combustion engines early in its infancy. It has many attributes such as a high energy density, is easily refined from crude oil, a long storage life, and, most importantly, could be brought to market at a low price, even today. A downside of gasoline that is still relevant is its poor vaporization rate when cold. Gasoline engines do not like to start in cold weather unless the mixture is greatly richened from what would be required during normal operating temperature.

In an engine, liquid fuel will not burn. Three things need to occur to the fuel before it becomes a candidate for ignition. These are atomization, emulsification, and vaporization: broken down into small particles, mixed with air, experience a phase change from a liquid to a gaseous state — a rarefied form.

The booster in the carburetor venturi is responsible for atomizing the fuel, making it into small droplets. Depending on the carburetor design, the emulsification process occurs within the circuits, usually via a tube connected to a passage with a series of minute holes to introduce air. At this point, the mixture enters the intake manifold. It can be described as a recipe of tiny fuel droplets among air molecules. It is now technically called a charge, defined as the air/fuel mixture.

Due to the atomization process, there is more surface area for the air to interact with since the fuel is no longer in a sheet or film but in droplets. The problem being the particles of fuel are tiny, but they are still in liquid form. Therefore, heat is required to change the liquid fuel into a rarefied state so the engine can burn it when the spark plug arcs. This phase change occurs while the charge is traveling through the intake manifold.

The rate of vaporization (phase change) of the gasoline is directly attached to the amount of heat available. Therefore, as the ambient temperature drops, so does the vaporization rate. For example, at minus 45 degrees F, only 20 percent of the fuel will vaporize. Under these conditions, a gasoline-powered engine will stop running unless an auxiliary heat source is supplied. At 60 degrees F, the vaporization rate is only around 50 percent.

Conversely, another problem is if the underhood temperature becomes too excessive such as during a hot summer day, the fuel vaporizes before entering the venturi area of the carburetor. It does not allow the engine to run. This is called vapor lock. The oil companies address the hot fuel situation by altering the gasoline blend with additives and modifying the refining process. And though the cold vaporization rate can be altered at the refinery, it only changes the facts slightly. For example, race gasoline has extremely poor vaporization in the cold. It most likely will not allow an engine to run.

A choke is required to start a cold engine, a means to richen the mixture significantly. Without it, the charge that would reach the cylinders would be too lean to ignite. Even if the engine did start until heat transfer occurred, fuel distribution would suffer since the gasoline would puddle in the runners of the intake manifold.

Another concern is the slow cranking speed of the engine during the start and the excessive friction that must be overcome by the electric cranking (starter) motor. To conquer these obstacles, a choke richens the air/fuel mixture. Once the intake manifold is hot enough for good fuel vaporization, the choke function must be negated.

An excellent rule to identify the mixture strength during engine crank with the choke evoked would be an air/fuel ratio of 2:1. This compares to around 6:1, just as the engine fires, and 14.7:1 when fully warm. The balance is parts of air to one aspect of fuel. When the number goes lower, there is less air to the standard amount of fuel.

The choke is simply a butterfly or plate located in the air horn of the carburetor. When evoked during crank, the choke is closed and exposes almost full manifold vacuum to the booster venturi, drawing a large amount of fuel through the main metering system.  

The first choke systems were cable operated by the driver from inside the vehicle. The choke would be pulled out, which would close the butterfly. The operator would adjust the opening amount to keep the engine running smoothly and allow the vehicle to be driven while the engine was still cold. If the mixture was too lean, the engine would buck, spit through the carburetor, and possibly flame out.

As the heat was being built, the driver’s task was to slowly cancel the choke and allow the carburetor to provide the proper mixture. Often this did not occur, and the choke would still be evoked with the engine fully warm. This would cause an excessively rich mixture, foul the spark plugs, pollute the lubricating oil with gasoline and build excessive carbon in the cylinders, all while wasting a good amount of gasoline. A better way that worked independently of the driver’s memory would be required.

The automatic choke that operated via a thermostatic bi-metallic spring was the answer. It would naturally be exposed to the underhood temperature, which in the cold would be ambient or near that. As the engine warmed, it would self-cancel. All the driver would need to do is push the throttle to the floor once before cranking the engine– this would close the choke and allow the throttle to go against a cam to evoke a fast idle. As the engine built heat, the spring tension would loosen, and the choke plate and the fast idle cam would come off. But there was a problem: How would enough air be supplied after start-up since the temperature at the bi-metallic spring was still the same? With a manual choke, the driver would adjust the plate angle for a smooth engine, but the spring could not do that.

Vacuum is the answer.

When the automatic choke closes, it also evokes the fast idle cam to significantly increase the engine’s curb idle speed. This is to help the engine overcome internal friction when cold and improve the signal strength in the venturi booster while increasing the air speed through the intake manifold. The faster the charge travels, the less chance the extremely dense mixture will fall out of suspension. But the main reason for the fast idle is to blow open the choke plate against spring tension.

The high idle speed will cause the incoming air to rush against the choke plate and force it open slightly to lean the mixture. This is identified as being blown open. The problem was the choke plate could not be blown sufficiently open to allow the engine to run clean and smoothly during warm-up — it needed more air.

A vacuum-operated diaphragm with a linkage attached to the choke plate is then used to sufficiently alter the mixture until heat relaxes the choke spring. Depending on the carburetor manufacturer, this device can be identified as a choke pull-off, vacuum break, choke break or choke dashpot. Regardless of the name, they all function in the same basic manner.

A hose connects the diaphragm to manifold (full-time) vacuum. As soon as the engine fires, the rushing air from the fast idle blows the choke plate open while the break pulls in, imparting a slight opening of the choke plate. The specified opening of the choke break, along with the proper fast idle specification, allows a carburetor-equipped engine to start quickly and run smoothly without any stalling or drive-away hesitation.

The amount of authority the choke break had on the butterfly was a precise specification. During the 1960s, when the automatic choke became standard on almost every engine, a drill bit index was used as the specification for the amount of opening created by the vacuum signal. Later, General Motors made it more exact and required a dedicated choke angle gauge to set the break properly.

Most early choke pull-off designs were adjusted by expanding or contracting (bending) a “U” in the linkage. Others, namely later model Rochester carburetors, had a screw adjustment that allowed for a finite setting of the pull-off opening.

A common problem with the choke break was a deterioration of the diaphragm or swelling as the years went by. A swollen diaphragm will hold a vacuum but hardly evoke any motion of the choke plate. At the same time, a ruptured canister will not move the plate at all.

A symptom of a misadjusted or non-functioning choke pull-off would be loading up the engine shortly after a cold start that is usually accompanied by rough running, bucking, and, if very severe, black smoke from the tailpipe. When the author was a child, in the winter, if I saw a black spot on the snow behind a car, I knew that the choke pull-off was not functioning correctly.

Over the years and still today, many mechanics never fully understood the importance of the choke pull-off, along with its companion adjustment and fast idle speed. Thus, the carburetor was given an undeserved reputation as a fussy part that did not allow the engine to run well in the cold. Unfortunately, that could not be further from the truth. To this day, I remember a neighbor that had a 1966 Buick Skylark with a V-8 and dual exhausts. I believe it was a 400. That car would start so cleanly, fast idle beautifully, and warm up smooth as silk with an even amount of vapor smoke from each tailpipe. The pull-off and automatic choke came from Flint adjusted like a fine watch. A simple but marvelous invention.