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automatic choke woes

andyblack

andyblack

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Now that there's finally some cooler weather happening, I've noticed that my engine just doesn't want to run at all when the choke is supposed to be on, like before the engine is warm. It starts up just fine and acts like it's going to run okay, but when I get on and off the gas a couple of times (like through one or two stop/go cycles), it just doesn't want to idle and I have to give it a good bit of throttle to keep it going. It blows a lot of "gas smoke" during this time. Once the engine gets to 180* or whatever it's normal operating temp is, it runs about like normal.

I suspected the automatic choke was the culprit. Before starting the engine, I could see that the butterfly for the choke (the small one in the front of the carb) was about half open and didn't seem to have much tension on it. I tightened the spring on the side of the carb so that the butterfly stayed closed and had a good bit of tension before the engine was started. I then proceeded to drive the truck to work this morning while it was 21* and it still ran terrible, just like before - no change at all.

Do I need to get the enigine warmed back up and check to see if the choke butterfly is open all the way? Could it be now staying closed for too long? Also I noticed a vacuum motor thing with linkage that's connected to the choke. How do I check to see if it's working properly?

These carbs... :doah:

This is a stock '85 305 with a stock Quadrajet and automatic choke (49 state model). It has one ground wire on the choke spring case and one electrical connector on the side. Not sure what it does.

Help would be appreciated. :D
 
I've had them where the high idle isn't enough and they just won't run till they are warm. Had to tweak it, not sure the correct term.

Have you checked the power wire for power yet?

Should be able to leave the key on and watch the choke slowly open as well.
 
Is the power wire the one wire that's connected to the side of it? I haven't checked any wire yet.

I'm pretty sure my high idle is pretty high. I don't have a tach but it sounds close to 1,500 rpm. I turned it up a while back to help it run better. Actually it runs a lot better when the trans is in Park or Neutral.
 
Agree with Bowtie.

Engine cold (very cold), take air cleaner off, and work throttle once. Choke should shut fully. Key in run, it should then open fully.

IIRC, the "choke pulloff" is there to pull the choke plate open a TINY bit once the engine starts.
 
Okay so you mean that I don't even have to start the engine - just turn the key to run and I should see the choke open up? About how long should this take?
 
Wait, that was wrong. Been a long time since I dealt with these, but now that I think about it, there is an oil pressure switch on the back of the motor that needs to shut for the coil to work right. (oil pressure from engine running means the choke gets power) Should be able to jumper those two wires though and then it will work with the key on. Just don't forget to remove the jumper once you are done testing. Might also want to check that the switch works properly, if it doesn't, neither will the choke.

Electric choke uses electricity to heat up, imagine that! :) I always thought it was a stupid idea in theory (even though it seems to work fine) because the engine might take more or less time to heat up based on whether it's idling or being driven, but the electric choke is more or less independent of engine temp.

I'm sure it's more complex than that, but that's the general concept.

FWIW, resist the temptation to make ANY adjustments to the choke assembly (screws, linkage, etc) until you figure out EXACTLY what is going on. In general, I'd say the choke is the most complicated piece most people will deal with on the Q-jets, and making adjustments before you know what is causing the issue will most likely make things even worse.
 
Good to know about the oil pressure.

I always thought it was a switched 12v power wire.
 
That oil pressure switch might depend on age and if the PO has messed with anything. I have worked on more than a few carbed trucks of various years that the choke wire is a 12+ that was wired directly to ignition. So, if you left the key forward but engine not running the choke would open anyways. And, also make sure the wire is getting the full 12+, as if its not it wont heat the spring right and the choke will never open fully.
 
You fellas are giving me some good info to work with after I get home today.

It definately wasn't closing all the way before. Now I'm wondering if it's opening all the way. :thinking:

Time to get out the ol' test light I suppose.
 
in the boats we wire them to the alternator outputs.. that way they only energize when the engine is running...
 
That's a great idea. I'll check this old thing out this evening and see what's up. Until then, I'll just keep on chugging along. :haha:
 
Honestly you shouldn't be jumping in the truck and taking off. If you want to do that you need fuel injection. Let the poor thing warm up for at least a few minutes before you take off.

I've driven a lot of carbd cars and a lot of higher horse power carbd cars and adjusted correctly to the 9s I've never driven a car that you could jump in, start it up, and have the choke magically make it run right.

The fast idle is there to warm the car up faster, not to bandaid bad driveability of taking off in a stone cold carbd car.

O and BTW that's horrible for your motor too. Good way to wash out the cylinders.
 
I usually do let it warm up a little because I know it's bad for it to just jump in and go. I let it run for about a minute this morning and it idled good at that fast speed like it usually does. It's all over once I hit the gas the first time though. Maybe tomorrow I'll try to get out there earlier. It's hard to do though because I have to be at work at 6:00. :rolleyes:
 
Even if the choke is otherwise working properly, if it's not starting closed all the way, it will be "wrong" all through the warmup cycle of the motor, as the mix will be wrong (too much air and not enough fuel).

Need to figure out why it's not shutting fully. That has nothing to do with the electrical side of the choke, "setting" (when you hit the pedal with a cold motor) is all mechanical. You might check and see if there are any written instructions online, on how to check/adjust the Q-jet chokes properly, specifically your model with the electric choke.
 
Yeah I kind of thought about that. There does seem to be a lot of slop in the linkage that would (theoretically) make it easy for something to bind up and not let the choke do its thing.
 
From my experience the linkage is usually pretty sloppy, most of it is just looped through a hole, it was never "tight" even from the factory. I've not seen any that actually bound up (I'm sure it can happen) and for awhile, I was dealing with a LOT of q-jets, so it's probably not likely.

If it's not shutting all the way, I *think* the only adjustment (if the choke spring is tight) is to bend the choke rod that hooks to the choke plate. I'm not saying to do that, but with the engine off, choke coil tension is what pulls the choke shut, the only thing that is adjustable in that equation is coil tension and the rod length.

If rod length is a suspected issue, I'd see if it opens up fully ok. But since you adjusted the coil tension, it might be so much that it never opens fully based on that, not based on the rod length. Has someone else monkeyed with the carb already? If you adjusted the coil, the rivets must be drilled out?

Be careful, those choke plates are weak bakelite, they don't take much screwdriver force before they break. Awhile back they were extremely expensive to replace, maybe they are cheaper now, just in case it does break.
 
I did get it pretty tight last night when I was messing with it. Maybe I got it too tight? If I run the engine until it's all the way up to normal temp, the choke should be open right? I'll check that this afternoon, but it seems somewhat likely.

My dad messes with the choke sometimes when he comes to my house and I start complaining about it. It always seems to work right after he gets through with it. For a while at least. I'm not sure what he does. I do remember him telling me to adjust the choke before I ever started the engine to make sure it was as cold as it could get.
 
Proper operation (ignoring the idle portion of the choke) on a cold engine is simply that you step on the pedal and the choke coil snaps the choke plate shut. You start the vehicle, the choke pulloff pulls the plate open a TINY bit, and as the choke coil heats up the plate gradually opens until it is fully vertical. Most all of that should be readily observable with the air cleaner off.

Problem with messing with the choke and not getting it 100% right is that it will normally either start great when cold, but run terrible once warmed up, or conversely run terrible when cold but when warmed up, be fine.

As I recall the choke actually has some sort of circuitry in it, I have no idea if it's got a way to compensate for varying starting engine temperatures or what...but I do know that if the engine isn't dead cold, it's likely you won't get a good "read" of whether the choke is truly operating correctly or not.
 
Interesting. Guess I'll learn as I go. Thanks for the info everybody. Hopefully this will get me pointed in the right direction at least.

I just drove home and took off the air filter lid and the choke seems to be wide open as it should be. So I'm pretty sure it's getting fully open and fully closed now, it's just the in between while the engine is warming up that's messing with me. I suppose it could be something besides the choke. This is a 30 year old engine with who knows how many miles and not much compression left anyway.

On a side note: is it possible to convert this carb over to a manual choke?
 
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