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Automatic chokes

History

Automatic chokes were introduced at least by the early 1930’s. These early chokes were of the “divorced” or remote style, so named because the choke unit usually resided on the exhaust manifold and was remote from or divorced from the carburetor. These units consisted of a metal box that contained a coil of bimetallic strip which would expand/contract with heat/cool and which was connected to a rod that also connected to the choke arm on the carburetor. These units were not overly reliable, and in fact, most manufacturers build a manual override into the carburetor so that the operator has positive control over the choke. One issue with the divorced chokes was the choke had to move the mass of the connecting rod as well as operate the choke plate.

Integral chokes were introduced at least by the mid-1930’s. The integral choke was an integral part of the carburetor, and had a bimetallic strip with an external heat source. These early chokes used both electric and hot air as the external heat source. The electric proved to be much less functional than the hot air, as the electric choke operated on time, not on the actual temperature of the engine. Stalling was a problem with the electric chokes. The electric was discontinued after a few years, except on aftermarket replacement units. The hot air integral choke used a hot air source that was a tube that would be either pressed into the exhaust manifold or extended into a blind external chamber of the exhaust manifold. The integral hot air choke has proved to be by far the most functional and reliable of all automatic chokes. The Achilles heel of the hot air choke is the tube pressed into the manifold. In time, the exhaust burns through the tube, requiring that the manifold be removed, and the tube replaced. This is not a normal service function, and normally did not require service more than once every 20~30 years! Most current restorers of older vehicles have learned that this must be done at time of restoration.

In 1937, Delco introduced a divorced choke that eliminated the solid connecting rod from the choke unit to the choke plate, and replaced the rod with a flexible cable, similar to a speedometer drive cable, that pressed into the end of the choke shaft. These were horribly dysfunctional, and equally unreliable. This unit was discontinued after the 1938 model year; and factory service carburetors with chokes that worked were offered to customers with these units.

Various manufacturers through the 1940’s and 1950’s used both integral and divorced chokes.

In the late 1950’s, there was some concern about unfiltered air entering the engine through the hot air choke, as the supply to the heat tube was generally unfiltered. At this time, a tube was pressed into the airhorn of many carburetors. This tube was then attached to the supply end of the heat tube pressed into the manifold. As the air supply now came through the carburetor air filter, this improvement eliminated “dirty” air from entering the engine through the automatic choke. This modified hot air integral choke was and is the most functional and reliable of all automatic chokes.

In the early 1960’s, a couple of different manufacturers tried using water as the external heat source to the integral choke. Functionally, these worked extremely well, for a couple of years. After a couple of years, electrolysis had completely destroyed the automatic choke assembly. The water heated chokes were quickly discontinued.

In the mid-1970’s, the electric choke was reintroduced, due to government smog regulations. The electric chokes were timed to go off much sooner, thus somewhat reducing emissions because of a leaner mixture. As in the 1930’s, drivability suffered until the engine completely warmed to operating temperature. Being a service manager in a new car dealership in the late 1970’s was a frustrating occupation when one had to convince the customer it just didn’t get any better.

Operation – divorced chokes

The divorced (a.k.a. remote) choke closes due to a coiled bimetallic strip that rotates with temperature change. When the engine is cold, the coil acting through the choke connector rod, closes the choke. Contrary to popular belief unit does NOT open the choke (except for the 1937-1938 Delco unit, which never worked anyway, so the statement is still true). External heat to the divorced choke unit caused the coiled bimetallic strip to “relax” or “unwind” removing the closing tension to the choke plate, and allowing the choke plate to be opened by some other means. Early chokes were opened by a counterweight assembly on the choke arm, or by an offset on the choke plate to choke shaft connection, causing the choke plate to fall open when the tension was relaxed. Beginning in the late 1950’s, external choke pulloffs were used. The choke pulloff had a vacuum source attached, which pulled directly against the tension of the bimetallic strip. As the tension relaxed, the choke pulloff would open the choke plate.

Operation – integral hot air chokes

The integral choke, like the divorced choke, closes due to a coiled bimetallic strip that rotates with temperature change. The coil is located inside the integral choke housing. Also inside the choke housing is a vacuum piston assembly. An internal vacuum source enters the choke housing behind the vacuum piston, and escapes by the choke piston to cause a negative pressure (vacuum) on the entire integral choke housing. As the housing is connected to the hot air choke tube, this negative pressure pulls air heated by the exhaust through the heat tube. This hot air causes the tension of the coil to relax, allowing the vacuum piston (this may be thought of as an internal choke pulloff) to open the choke plate.

Operation – integral water chokes

The integral water choke works similar to the integral hot air choke, with the exception of the heat source, which is a water compartment, heated by the engines cooling system coolant, that is a part of the integral choke housing.

Operation – integral electric chokes

The integral electric choke works similar to the integral hot air choke, with the exception of the heat source, which is a coil of resistance heating wire inside the integral choke housing. This element is connected to an external electric source.

Pros and Cons (opinion of the author)

Divorced chokes have a reliability issue due to the mass of the connector rod weighing on the coil. While these may be made to function, adjustment may be necessary on a much more frequent interval than the integral choke. Also to be considered is the thickness of the carburetor mounting gasket when the carburetor is removed and replaced; as the gasket thickness directly effects the adjustment of the choke. When properly adjusted, these do work well, as the choke is released based on engine temperature control.

Integral hot air chokes have a longevity issue due to the tube pressed into the exhaust burning through, or the blind chamber in the exhaust manifold burning through; but this is a long time between occurrences. The integral hot air choke works well, as the choke is released based on engine temperature control

Integral water chokes work well as the choke is released based on engine temperature control; until catastrophic failure due to corrosion.

Electric chokes are functionally the poorest form of automatic choke, as the choke is controlled by a pre-set time, based on the heating element rather than engine temperature control. They are not even consistent from engine to engine, as the voltage applied by the electrical system will have a bearing on the time. The only “pro” for the electric choke is that they may be used on modified engines with aftermarket manifolds that have no provision for either divorced or hot air integral choke. This issue has been addressed in recent years in the aftermarket, as a “tunable” voltage control is available. This improves the electric choke, but only in that the operator can set the timing. The choke still doesn’t know the engine temperature. If a control is available that senses engine temperature and choke plate opening position I am unaware of it.

Adjustment – all except electric chokes and the 1937-38 Delco unit

All chokes other than the above may be adjusted as follows: pick a time with the ambient temperature is between 65 and 70 degrees F. (68 degrees F. is the perfect temperature), and adjust the choke such that the choke plate on a cold engine just TOUCHES closed, with zero tension. The choke coil will then compensate for other temperatures

Testing – all except electric chokes and the 1937-38 Delco unit

We get calls daily from those who would replace a perfectly good choke unit because their choke doesn’t function. With the integral choke, the odds are at least 100 to 1 on a non-functional choke that the vacuum source is plugged rather than a bad choke unit. To test the unit, remove the choke coil from the engine or carburetor. Place the coil in an “indexed” position on your workbench. Measure the linear distance the coil must travel for the carburetor choke to move from the fully closed position to the fully open position. Now aim a portable hair dryer at the coil and watch it move. When it reaches its maximum deflection, measure the amount of travel and compare this with the needed travel you measured on the carburetor. If the choke coil travel equals or exceeds the carburetor travel, the choke unit does NOT need replacing.

Distribution

This article, as with all other articles found on this website, have been written to place in the public domain and copied if desired. We only ask that you credit The Carburetor Shop LLC when you copy the article.