TAC Tech > Mechanical

The Choke Thread

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firebirdparts:
To my chagrin I find that our choke thread is not on the wayback machine.  It'll give me a chance to do a better job on it.  It had some great pictures in it that Joe Richter made, so I will look over in the 301 garage at their thread for that.  I always wanted to make a larger thread about carburetion, but it's such a big topic it gets away from you.  Q-jets are complicated.

About half our carb questions here in the mechanical forum were really about choke problems, so it's important to get the choke working, and it's also important to understand when it's not working that the carburetor was intended to operate at operating temperature.  You have to get the carburetor working correctly with the choke open and the engine fully warmed up.  Once you have that, then the choke is just a very simple add-on which dramatically messes up the air/fuel ratio when you need it to.

In the olden days, cars used a manual choke for cold startup.  You had to adjust it to get the engine to "run well", and that required you to open it a little as soon as the engine cranked up, and then you adjust it back open a little bit later on.  Manual chokes had a fast idle cam built in to help you get by while the engine is cold natured.  Automatic chokes incorporate all three of these functions.

Q-jets used a bimetal coiled spring to move the choke based on the temperature of the spring.  There are three forms of this:

* From the earliest Q-jets to maybe 1973 or so, they used a "divorced" choke with the spring mounted directly on the exhaust crossover
* In the late 70's, they used an "integral" choke with the spring contained in a housing screwed to the carburetor itself
* Around 1980, you start to see applications with an electric choke, which had an electric heater on the integral choke.Here's a divorced hot air choke


Here's the late 70's Trans Am hot air choke

The electric choke looks just the same, but it's got a wire running to it.

There is something mysterious that you need to know about the hot air chokes.  The integral chokes were designed to suck hot air through a "stove" and into the carburetor.  That is how the carb knows that the engine is warming up.  This flow of air is controlled by a tiny orifice in the carb which is replaceable, and it depends on a reasonably good seal around the choke housing to work.  There is a gasket for that in a rebuild kit.

It's necessary for the choke stove to be able to suck in free air.  The source of this free air looks like vacuum hose, and has been a source of confusion.  The air came from the air cleaner, just for safety's sake, and this hose was the one that was really short and always had to be disconnected when you took the air cleaner off.  You will find very little discussion of this anywhere by anybody.  The pickup point for this choke stove air is this thing:

Eventually people got tired of unhooking that hose and they came up with this:

This is on a 403 car.  You can see here what the integral choke stove looks like installed.  The carb is sucking air from the back of the air horn, down inside the exhaust crossover in a tube, and then into the choke.

Y88 TA:
Thanks for putting this up also. Good information.

pancho400cid:
Great info!

jvmagic:
Great write up @firebirdparts! 

The choke on my 79' 403 works as it should when the temps are below 70 (choke kicks in the morning and idles high; 1200rpm).  During late spring/summer, the choke doesn't engage (first thing in the morning the car starts but idle is at 750rpm).  I live in the bay area (San Jose, CA). 
I'm not sure if something recently got disconnected or if I need to adjust the choke. If I need to adjust, do I need to work the thermostat?   
Thanks in advance

brentco:
Another item that’s not frequently discussed about chokes — the function of the choke pull off. Most electric chokes have a vacuum operated pull off. On cold starts the choke plate is tightly closed while the engine is cranking (when it needs the most enrichment), and then as soon as the engine starts the pull off “pulls off” the choke plate a bit so that it’s slightly cracked open during warm up and some air can get in to feed the mixture.

Some aftermarket carbs with electric chokes, notably Demons, don’t have pull offs. The rear of the choke plate has a gap that’s supposed to provide enough air to feed the engine after it starts until the electric choke heats up and starts opening the choke plate. I’ve found on many engines this system just doesn’t work — the engine starts and then dies immediately. The cure is to either leave the key in the on position for a few minutes before cranking the engine, which makes the electric choke heat up and start opening so the plate is cracked open a bit at starting, or bend the choke linkage so that the plate is already cracked open about 1/4” at the start when the choke is fully closed (my preference).

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