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Cold weather choke tuning

Matt Harwood

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Last night driving home, the transmission in my Cadillac CTS failed, so I limped home in 2nd gear and parked it. Still had to get to work this morning but Melanie left early to take one of the kids to the doctor, so I figured I'd drive the '41 Limited. Of course, the battery is dead, probably because of the incident in Cincinnati a few weeks ago--I'm not surprised that situation polished it off. Fortunately, there's an Auto Zone about 3/4 mile away, so I walked over, bought a battery, and walked home, which was a bit of a joy since the battery in this car probably weighs 40 pounds. Installed the battery and it cranked over but refused to start, just like in Cincinnati. I didn't want to flood it again, so I stopped.


By manually propping the chokes open, I eventually got it to fire and run roughly for a few minutes, then it smoothed out and ran properly.

How do I adjust these chokes so they'll cooperate with the cold weather, since it appears this will be my daily driver for the foreseeable future. It starts instantly and idles perfectly when it's warm, but will not start in the cold. Do I just prop the chokes in the open position for now?


Any thoughts on the choke? Thanks in advance.

Edited by Matt Harwood (see edit history)
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Well.... It is easier when it is a stock application, but when it isn't you have to determine it by experiment. Fortunately MOST automatic chokes do about the same thing.


1) First of all there is the fast idle. It follows the choke plate, and should not come off the bottom step until the choke is COMPLETELY open. If the choke is even a tiny bit angled, you should be on the low step of fast idle, and the screw is adjusted (hot) for some minor fast idle, for instance if the engine idles on hot slow idle at 500-550 RPM, it should probably be 700 or 800 or so on that first step. The next step (hot) is not adjustable separately, but is expected to be more of a ridiculous fast idle (1200 RPM or something)


2) Then there is the choke pulloff. This could be a piston or a diaphragm. When the engine fires, the choke should pop most of the way open instantly. From start to run is a much bigger difference than most people think. Thats why manual choke conversions usually suck. A factory manual choke will usually have some sort of device to let a bunch more air in the instant the engine starts. Conversions usually don't, and then it is up to the driver to do it, and he may not know, and even if he does, the linkage is often too balky. Anyway, the "engine started" position of the choke is a hard-ish limit set by the choke pulloff piston (or diaphragm), it only varies a little by how hard the choke thermostat is pushing. There is more variation when the car is almost warmed up, and the choke thermostat isn't pushing all the way to the limit anymore.


3) And now the choke thermostat. There has to be an adequate supply of heat to the thermostat to get enough movement. It is usually hot air, but on some cars the coil is just up against a heat riser passage. This is only gonna push hard when it is really cold. At room temp, the choke plate wont be completely open, but partly, and kind of loose and floppy. At something more like 40 degrees it would probably be completely shut prior to starting. You can sort of tell about the adjustment by how the car runs half warmed up. If too lean, it will run crappy and stall, if too rich it will stay on fast idle too long. That assumes the choke pulloff is set correctly, so on a custom application, a little experimenting is in order.


4) The choke unloader linkage. This should open the choke partway, or maybe even most of the way when you hold your foot all the way to the floor. The purpose here is to help the owner clear a flooded engine. Know it's flooded? Hold the pedal to the floor and crank.


That is pretty much the routine for almost every automatic choke from the 40s (or earlier) until the demise of carburetors. To start, you kick the throttle once (sets the choke and fast idle, gives a squirt of gas from the accelerator pump). wait 5 seconds or so if its cold, then start. It should fire up on high fast idle, and then you kick the throttle again and it should fall to low fast idle.


This brings me to a caveat: Buick Autostart. I don't know how much the routine changes. It would have to change some because you are starting the car with the throttle open, and almost no other car is meant to do that.


Another Buick caveat: Some straight eights have a drain in the manifold to drain out raw fuel on the ground when the manifold floods. There is a ball bearing that sucks up to plug the leak when the engine starts.


A hotspot of some kind under the carburetor(s) is basically always necessary to keep fuel from falling out of the mixture and pooling. Exhaust gets going faster, but hot water also works and is sometimes used.


Hopefully that gives you a few things to check. Good luck.



Edited by Bloo (see edit history)
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If you try to start the car now on gas you bought in the summer, that could be your whole problem. Summer gas sold in Cleveland is heavy on the heptane and octane for summer weather to prevent vapor lock, and winter gas has more pentane and hexane to evaporate more easily at lower temperatures. Once you burn off what's in your tank and buy the stuff they sell now when it's much colder, it will start much easier in the cold.


boiling point of gasoline:


pentane  97 F

hexane  156 F

heptane  209 F

octane 257 F

nonane 303 F


Edited by Morgan Wright (see edit history)
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