Choke Problems - Won't Open

[WarpathEngineering]

Can anyone explain to me in simple terms how to adjust the hot air choke on my quadrajet? For whatever reason it's not opening up and causing the truck to flood out. It's a rebuild from Guaranteed Carb and only has about an hour of run time on it so everything is correct, free and should work. The choke cover is blank and doesn't have the standard markings. I attempted adjust it cold to stay open about an 1/8" in but if I get that far the butterfly just flies open. Do I hold the choke closed and adjust lean until the linkage starts forcing the butterfly open or do I adjust from the lean sided and richen until the doors close to 1/8"? Totally F++kin' confused!

Before anyone asks has new: Plugs, wires, cap, rotor, coil, vacuum hoses, unused carb ports have new caps, open headers, fuel pump & hoses from steel line to engine. Engine was running albeit rich and somewhat rough before any modifications were made. Timing was not changed and hasn't been checked yet. Carb has not been adjusted in anyway other than attempting to adjust the choke.

[MountainGoat1966]

I always adjusted mine so when I turn the adjuster thingy, the choke plate just closes, no additional tension.
Then, while the engine is still cold, fire it up, and set the screw on the fast idle cam to about 1,000 to 1,200 RPM.
After the choke has fully opened, blip the throttle and it should drop down to normal idle speed.
If you live in sub zero weather most of the year, you may need a tad more tension on the choke plate.
Warm climate, mostly closed should do fine.
In the summer here, I don't need a choke at all to start my engine.
Unless it's a rare cold morning.

[MountainGoat1966]

Also, I forgot to mention ..........
Make sure the choke plate moves both ways when you turn the adjuster cover.
There is a slot on the backside and the bimetallic spring has to be engaged in that slot.

[WarpathEngineering]

Thanks, I'll give that a try in the morning.

[CSHADES]

I have a couple of things to add to this. Make sure that the vacuum choke pulloff is working. There is also a gauge to check the amount of pulloff it should be roughly 29 degrees. also make sure when the truck is running that the choke tube is getting hot. If it is not getting hot then the choke will never open. If it is not getting hot then it either is not pulling a vacuum thru the tube, the tube is plugged or the intake crossover is plugged and not getting hot.

[tucsonjwt]

Others posting here will disagree, but I never had an automatic choke work properly on any carbureted vehicle I have owned. Even GM dealers could not make them work properly consistently. I have always converted the automatic choke to a manual choke - a simple $12 solution and you always know when the choke is open or closed. Just make sure the choke valve is working freely - pull the throttle bracket back all the way, and with the choke spring removed - move it back and forth. You can search this forum for keyword choke and find my write up and pics on a manual choke conversion.

[WarpathEngineering]

So this morning I went out and took a look at the butterfly position and it was about half closed on a cold engine. I adjusted the choke to the last lean notch and fired her up. After 10-15 min she was still running on high idle (1500rpm). A short bump of the accelerator brought it down a little but not much. I adjusted the fast idle screw down to 1200 rpm and drove it about 5 miles to the exhaust shop and after a disappointing conversation with them about the regulations of installing catalytic converters on a 40 year old truck, I drove it back and parked it. It never came off high idle.

I think I've got a blocked stove tube and will check that first thing tomorrow morning. If it turns out that the stove tube is clogged and I can't free it up, are replacements available or do I have to fab a replacement? I'll also check to make sure that the air supply port on the carb is allowing air thru.

[MountainGoat1966]

You're getting closer.
If all else fails, go the manual choke route.
It's not OEM for that truck, but always dependable and predictable.
Just don't do like the little old ladies did in the 40's and 50's.
They would pull out the choke, hang their purse on it, and drive on.

[CSHADES]

With the stove tube off and the engine running, put your finger on the choke housing where the stove tube treads on. you should have a vacuum there. If you don not have vacuum it will never pull enough heat. you should be able to clean the tube out with a piece of wire and choke/carb cleaner. To clean the intake crossover you have to pull the intake manifold and basically chisel the carbon out of the passage.
Auto chokes can be made to work properly with a little effort. You just have to make sure every part is doing its job. Don't change it to a manual setup, because you or someone else driving it will sooner or later forget to open the choke all the way.

[tucsonjwt]

And if you forget to open the choke after starting the engine then the engine will run rough. So, you will think: "Why is my engine running rough? Oh yeah, I forgot to push in the choke knob." One episode like that and you will remember. This is not a difficult task to master.

[mMad Dawg]

Forget the choke stove and manual choke.Find an electric choke that replaces your old one.It will be less trouble. Just remember to hook the wire to come on with the key on.You can buy a new one if you cant find a used one.

[tucsonjwt]

Each to his or her own. In my experience, electric chokes are just as bad as hot air or divorced chokes. If the choke is adjusted to close all the way when cold, then it will not open all the way when hot. Or, if the choke is adjusted to open all the way when hot, then it will not close all the way when cold. I realize that I am not smart enough to know how to adjust a choke (along with many GM mechanics), so I will just tolerate the extra effort to pull a knob when driving my old truck.

[cadillac_al]

Does that choke adjustment move the choke blade in both directions? You can turn the adjustment one way and it closes and you turn it the other way and it opens right? I have seen the spring not seated in the slot correctly and it would only move the choke one way and not the other way. I have never had a hot air choke fail on me but I guess if the manifold heat passage gets plugged up enough it can cause a problem for other people. I have seen them pretty plugged up on my vehicles and the choke still worked fine. The manifold and carb both get hot enough to cook on so the spring should get hot no matter what. It should be warm enough in 5 miles for the choke to be open and off of high idle.

[Wgesnerjr]

I can barely remember from way back when I was a teenager, but in 1985, my Trans Am had the same problem. Turned out it was the vacuum Choke Pull off. Even though it was a stove type choke, there was a vacuum pot on top of the carb that pulled the blades open. Once I had that replaced, it functioned perfectly.

I remember the guy that finally fixed it, used a vacuum hand pump and watched for the arm to move. When he pumped one up and it stayed still, he knew that was the problem.

Google "QuadraJet Choke Pull Off" and see if you have one of these on your carb. If so, try a vacuum test. It worked for me all those years ago, might work for you too.

[CSHADES]

All of them have choke pulloffs that is opens the choke when it is first started. If everything is working correctly and adjusted correctly auto chokes do work. Yes, I was a GM tech when chokes were on everything in the 80's.
I understand if the manual choke is all the way on when you are running it but what about when it is open 1/2 or 3/4? you can easily forget to finish opening it. If you are the only one driving your truck then it is probably no big deal but I can bet if there are multiple users of the vehicle someone will forget to finish opening the manual choke. Just my opinion, its your truck do what ever you want.

[gmachinz]

Weird....we have a divorced choke setup stock on a 1977 GMC Sierra and it works like it's supposed to. It basically has a metal coil which works directly off manifold heat-and aftermarket intakes do not have a flat spot to retain it....and I convinced my father in law to keep his cast iron Q-Jet intake just because of it. It's cool to retain factory components whenever possible imo so it's worth putting a little time and money into getting it to work properly.

[truckin 79]

On my 79 Chevy 350 there is not a front choke pull off but there is a rear choke pull off. So some truck Q-jets are different then car Q-jets. One other thing to remember is the choke spring is a bimetallic spring so outside temperatures will effect how much spring is coiled or loose. So I usually adjust carb choke early morning(coldest part of day) or late afternoon (warmest part of day) because outside tempature will affect how far open or closed choke butterfly is. Also remember there are thermal vacuum switches to consider as well.

[WarpathEngineering]

Originally Posted by cadillac_al

Does that choke adjustment move the choke blade in both directions? You can turn the adjustment one way and it closes and you turn it the other way and it opens right? I have seen the spring not seated in the slot correctly and it would only move the choke one way and not the other way. I have never had a hot air choke fail on me but I guess if the manifold heat passage gets plugged up enough it can cause a problem for other people. I have seen them pretty plugged up on my vehicles and the choke still worked fine. The manifold and carb both get hot enough to cook on so the spring should get hot no matter what. It should be warm enough in 5 miles for the choke to be open and off of high idle.

BINGO!

So I disconnected the choke stove tubing from the back of the carb, it's air source, and pulled the choke apart. With a piece of vacuum line attached to the carb side, I blew threw it and it came out inside the choke body. Everything is good as far as the choke stove tubing is concerned. I next put a piece of fuel line on the vertical port just behind the secondary air doors and tried to blow thru it to see if it exhausted at the port that the choke stove tubing was connected on the original carb.....no air flow. Next I re-installed the choke and attempted to adjust it. The one thing I found funny about this carb from the start was that I could adjust spring tension only in one direction. I could force the butterfly closed but I would have to completely circle the choke adjustment disk to force in the other direction. The bi-metallic spring was not originally installed in between the fingers of the choke pivot. With that fixed, I set the choke to the neutral position & started the truck. I have vacuum at the rear of the choke stove tubing as expected.

Question now is where do I get an free air source since the port on the back of the carb is dead? I don't want to leave it open to free air for fear of sucking crap into the carb. The picture of the carb below is not mine and not the one installed on my truck however, it shows the upper port on the back of the carb and the vertical air intake just above & to the left that it should flow to. For whatever reason the two do not pipe together.

[cadillac_al]

That's cool that you are getting closer. I have never messed with that rear port on the carb because I never had a problem. I'm not sure if it is open or not. I have run carbs with the choke tubes rusted clean through with the tubes just looking at each other and my choke seemed to work good enough. I still think the heat from the engine will warm the spring enough. If I remember I will take a look at a couple carbs tomorrow to refresh my memory on that rear port.