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Cad carb question


mbenseignant@gmail.com

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Rereading Hendry's history of Cadillacs, he says they used a similar carburetor over a 30 year period: from the early 4 cylinder cars through the thirties. I have one of these Johnson carbs on my 1924 V8. A question: what is the purpose of the auxiliary throttle? There is the usual pedal-operated butterfly, but between this and the intake manifold is a second, spring-operated butterfly. It is not solid but partially cut out, and depending on the year, even had some holes drilled in it. The manual says a tool was used to regulate it. I leave mine so slack it will open with just a breath of flow. But I wonder if the car would run just as well with this throttle removed, or left open. Has anybody actually experimented with the tension and judged the effects on car operation? Or can anyone explain the reasoning behind it. Thanks!

By the way, join the Yahoo group: EarlyV8Cadillac if you are interested in the Cadillacs from 1915 through 1930.

Regards, Mike Barnes, EarlyV8Cadillac list owner.

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Is it to "fast idle" the engine when cold? That is how my Model A Ford operates. Retard spark, richen fuel mixture, advance the throttle a few clicks, complete circuit( contact), pull out choke, crank over engine. When the engine starts, release the choke and let the engine fast idle for a few moments. OR, an early attempt at cruise control. LOL

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If I understand your description of the secondary throttle butterfly valves correctly, something similar was used on the 1956 Packard Caribbeans with dual Rochester 4-bbls and the air-flow operated butterfly valves were incorporated only on the secondary venturis. The purpose was to prevent the engine from bogging down from over carburetion (from an extremely fast drop in manifold vacuum) when the throttle was suddenly and abruptly opened fully at low engine speeds.

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The Johnson carburetor used by Cadillac had lots of "features" (none of which prevented leakage of the carburetor).

The auxiliary AIR VALVE's function was to provide additional air/fuel when the engine was under load. This allowed a small primary air intake for good idle air velocity.

The auxiliary THROTTLE VALVE's function is as stated by Owen_Dyneto. As there is a huge difference in the amount of primary air and secondary air, the auxiliary throttle valve was an attempt to prevent hesitation.

The adjustment of the auxiliary throttle valve is accomplished by use of a special tool (which I have never seen). The special tool was simply a weight that attached to the edge of the auxiliary throttle valve to preload the spring and cause the valve to open to a predetermined range. I have a sketch (without dimensions or weight) of the tool, but have never seen one.

Jon.

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Thank you carbking and Owen_Dyneto. I will repost this useful info to the Yahoo site devoted to early Cadillac V8s: EarlyV8Cadillac, in case anybody is interested in checking it out.

By the way, I guess the Johnson of the Johnson carb was engineer Frank Johnson, who Hendry mentions frequently in the early years of his history of the Cadillac car. Johnson being one of Leland's trusted team. But I don't believe the book ever credits Johnson with carburetor design particularly.

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