What causes carb to move to fast idle position? Choke thermostat?

[86Woodgrain]

Hey all, I rebuilt the stock carb about a year ago, replaced all vacuum hoses, replaced choke pull-off and choke thermostat. Used factory Carter BBD service manuals and more to rebuild the carb and it was basically PERFECT at that time.

Fast forward to this year and it runs rough and rich while warming up and refuses to go into high idle position when I do the usual press it to the floor before starting. Now it is getting cold here, like -20 C so it's definitely cold.

I haven't researched a lot yet but is it the choke thermostat that causes the linkage to move to the fast idle position? If I can get it back to going onto fast idle at cold startup I think that will fix it. The choke thermostat was new last year and I put voltage to it to test it and it does move some. I don't know how far it should move though and how fast. I think I have about 60 seconds maybe for the choke thermostat to do it's job? Not sure.

I really need to get fast idle working because I have basically parked the car for the time being while I figure it out. It's OK it's not my daily driver, fortunately.

[TheBirdman]

Yep, the choke thermostat pulls on a linkage that moves the choke plate, and a little cam that sticks under the throttle linkage, to bump it up for high idle. Theres some complicated adjustments you can make to adjust the fast idle separate from the choke, but the easiest thing to do is loosen the 3 screws around the ring that holds the choke thermostat on, then turn the thermostat so that it pulls the choke linkage further.

[86Woodgrain]

Which way should I rotate it? I rotated it counter clockwise as you are facing it. I turned it until the choke was fully open when hot. I will test cold function tomorrow.

[TheBirdman]

I forgot, but as you twist it either way, you will see the choke blade closing and opening. Go the direction that closes it more to give it more choke and fast idle.

[86Woodgrain]

Then I turned it the wrong way.

[86Woodgrain]

I tried turning it both ways and am not satisfied with it. I ordered a new CV195 from RockAuto, I'll just try replacing it. The car runs so bad I'm willing to just do that to see.

I suspect the spring inside the choke thermostat just isn't moving as much as it did when it was new. Like I said I put a new one on a year ago and it was great until the summer then once the weather got bad the cold start warmup got terrible and I didn't touch anything.

[MIPS]

The electric choke can take several minutes to move from the fully closed to the full open position. Even longer if its super cold out. When in doubt, connect the heater terminal to battery positive and hold the throttle off the cam. Depending on how clean your choke linkage is the choke plate should slowly open on its own.

Initial choke pulloff adjustment requires a cold automatic choke coil and vacuum pulling the diaphragm in. the procedure can be found on page 1J-43 of the TSM in the Eaglepedia.  https://amceaglesden.com/guide/1J_-_Fuel_Systems

Edited: Another thing I ran into this winter was that even though I washed the carb last year I found this fall as it cooled off the choke would gum up and never pull far back enough with a pedal pump to get on the Fast Idle cam. The linkage had to be cleaned before it would work again.

[86Woodgrain]

Thanks for the reply, MIPS. I have the old choke thermostat which is removed and I applied 12V to it and I could see it move but not very much.

I'm wondering if that's just a "bug" of these electronic choke thermostats. I dunno. I tried to manually move the linkage but I really couldn't say if it was gummed up. I will look at the pivot points and such when I install the new thermostat, maybe hit with a little WD40 even though it's not a very good lubricant LOL.

[MIPS]

I *think* closed-to-open movement of the coil is about 90 degrees. Still, I heave heard of the coil getting weak after decades of cycling so it could be bad. \ /

Verify the grounding contact surface between the electric choke and the carb is clean and the carburetor is properly grounded. I don't know what replacement electric choke heater you used but I'm aware some versions do not ground through the housing but with a second terminal. Holley's do it that way.

Do not lubricate the choke linkage. If its dirty or gummy use a carb cleaner on it.
I speak here from recent experience when I had to get the Eagle home from a family event on the night of the 25th and in the -30 cold found 3-in-1 oil becomes molasses and will stick a choke completely open.

[86Woodgrain]

I have lubricated the linkage before.... hmmm

[86Woodgrain]

I removed the thermostat and blasted everything (linkage) with carb cleaner and put it back together 🤞

Today happened to be balmy warm, above 0, so I couldn’t tell how smoothly the linkage worked. Next week will dip back into the -15 to -21 C territory (normal).

[MIPS]

Around 0 with a cold engine, go out in the morning, pop the hood and take the air cleaner off. The choke should still be wide open from the last time the car was driven. Push the throttle linkage down and the choke should snap shut and the high idle screw will rest on the topmost step of the cam. Open the choke butterfly with your fingers and let go, The spring tension of the electric choke if correctly adjusted should again snap it shut. If it doesn't or it's slow to shut the linkage is not adjusted or binding somewhere.

Traditionally when you cold start a carbureted engine the first cold pedal pump would set the choke full closed (and you had the initial pulloff once you had vacuum) and once the engine started you gently tapped the gas once to take it off the top cam of the high idle and it would remain on the second step until the electric choke pulled it open further, then as the electric choke operated and the coil heated up it will land on each following step when you take your foot off the gas until eventually the choke completely unloads.

[86Woodgrain]

I went out this morning to see... it wasn't even cold, a mere -1C (30F)... kind of good news though...

So yesterday I put back in the factory original choke thermostat, but I had adjusted it less than I normally would have (leaner)...

I took off the air cleaner, pressed the gas from under the hood, the choke plate closed the correct amount and it did go to the fast idle cam step.

After 3-4 mins I tapped the gas and it came off fast idle. The choke plate was open a little more.

After 15-20 mins of warming up and driving the choke plate was about halfway between closed and full open position. I don't know if it opened enough as the engine was fully warm.

I will continue to experiment in the coming days, it's going to get proper cold by about Tuesday.

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The main thing I did differently this time is I marked on the carb with the thermostat removed the two positions of the choke itself, closed and open. Then I marked the thermostat where the spring hook is hot, room temp, and cold from the freezer. I matched those positions up as my starting point. It's kind of hard to explain so I'll add a pic to help explain what I did - don't worry I do have the choke wire hooked up even though it's not in the pic.
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My suspicion right now is that the linkage may have not been moving as freely as it was supposed to before (thanks for the cleaning tip), and I think I had the choke set way too rich. Having these new reference marks I think will help me dial it in better as I can visually see from the top outside where things should be at, at least to start.

Fingers crossed! I still have two other thermostats to try if needed but I figured I'd start with the original one first.

[AMC of Houston]

An electric choke coil should have gone wide open much more quickly I think.  Are you getting a full 12 volts to the choke coil?

[86Woodgrain]

I tested for 12V a year ago it was getting proper voltage.

I went out for more driving this aft and the choke plate was fully open after. Oddly I did notice a new bog off idle. It wasn’t there before. I’m not worried too much about that just yet because I’ve had that issue come and go with choke thermostat adjustments in the past.