The Choke Thread

[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).

[DeVilliers]

Since this is the choke thread.........

Where is the choke handle located under the dash? I'm busy putting mine back, but it now looks like the cable is to short, which it can't be.
My location is then wrong.

Thanks

[roadking77]

I dont know motors but am pretty sure there is no manual choke knob at least on the later second gen trans ams. One may have been added by a previous owner though. I know a guy that put one on a 79 formula.

[5th T/A]

I am almost positive that there was never a manual choke on any Firebird from the factory, so you will have to find a suitable place under the dash to mount yours. Or convert it to an automatic choke.

[Wallington]

No manual chokes on US-based Quadrajets.

[nUcLeArEnVoY]

I'm pretty well-knowledged on how the choke works at this point after two years of ownership, but I'm facing a bit of a dilemma with mine that I'm hoping somebody can chime in on...

My car is a 1979 400/4-speed, though most earlier years work generally the same: the hot air automatic choke, the intake manifold exhaust crossover, and the EFE/Heat Riser valve all work together to provide the best possible warmup in cold climates and minimize the amount of time necessary to appropriately vaporize intake manifold fuel charge for best cold-weather running. The exhaust crossover serves as a passage in the center of the intake manifold that runs under the carburetor - this passage interfaces on either side with ports on the cylinder heads that are open to the exhaust chamber, and exchanges exhaust gases between both cylinder heads and is the basis for which the stock hot air choke works. The EFE/heat riser valve, meanwhile, is a valve flapper located in the driver side exhaust manifold just where it meets the exhaust pipe, and the valve, at least for my model year, is vacuum-actuated and regulated by a thermal vacuum switch in the intake. From 0-140 degrees coolant temp, the heat riser valve receives a vacuum signal which *CLOSES* it, cutting off driver side cylinder bank exhaust from leaving the driver side manifold and instead forcing it across the intake crossover to escape the passenger side manifold and exhaust. This extra exhaust being forced across the crossover provides extra heat to warm up the intake, as well as further assist in transferring additional heat to the hot air choke tube that reaches into the passenger side cylinder head crossover. Once the coolant temp exceeds the calibrated temp, the vacuum signal is then cut off to the EFE/Heat riser valve, which OPENS it and allows the exhaust from the driver side cylinder bank to escape through the driver side manifold as normal. All three of these things work together.

Here's my issue: I'm doing an exhaust manifold gasket change on my driver side due to a massive leak I've been tolerating for 2 years. Covers my 1 and 3 plugs and temp sensor in soot, it's loud as heck... I'm sick of it. Anyway, I pulled the original heat riser valve and come to find out some idiot in the past actually welded the darn thing CLOSED. Yes, you heard that right. CLOSED. So my car has been driving with an exhaust restriction for Lord knows how long. Not a complete restriction - some exhaust makes it past the closed valve, plus the huge exhaust leak, and plus the exhaust was still able to at least go across the crossover and out the passenger side. It's a good thing I granny drive the car, I very rarely exceed 3200 RPM since no matter how tempted I am, I can't ever feel 100% confident with an engine so old.

Anyway, needless to say I'm gonna ditch the stupid heat riser valve and just gut the innards to turn it into a spacer. My current exhaust setup is stock manifolds mated to a true dual without cat(s), so if I had to have this restriction, at least all the other components were ideal to prevent catastrophe lol.

So since I'll be deleting the heat riser valve, my concern is, if it will delay the hot air choke from opening up. I *LOVE* how good my hot air choke is. I live in South Florida, so it's not like I need it anyway, but my car will start up after 3-4 seconds of cranking with just one pump of the pedal even after sitting for a week, and just after 3 or 4 minutes of fast idle, I can blip the throttle and be off the choke. I went through a lot to restore function to my hot air choke, the entire time not knowing that my heat riser valve was stuck closed and supplying excess heat and exhaust in the crossover even when the car was already at operating temp. I DON'T want to switch back to electric.

Does ANYBODY still use the factory hot air choke but deleted their heat riser valve? Like maybe you use the hot air choke but have headers installed? I just want to know if the loss of the heat riser valve will make that much of a difference with how quickly the hot air choke opens. I'll still have an open crossover, it's just I'll be depending on exhaust gases to exchange across it via normal turbulence rather than being forced by the closed heat riser valve. Again, I DON'T want to go back to an electric choke.

[5th T/A]

Nuclear,

I don’t have actual experience of testing a choke without a functional heat riser valve. But the crossover from the intake will still warm up from heat absorbed from the cylinder heads, just slower. There is adjustability in the choke. Adjusting it to open sooner along with the mild temps in South Florida, I would think you can adjust the choke to give you decent drivability for the few minutes it will take the engine to warm up.