I'm trying to gat an idea of what's wrong with the choke mechanism on my new '64 Dart with 38,000 miles.

When I got it, the PO said "The choke doesn't come off and it runs rich." I observed that the choke pull-off wasn't pulling off and helped him drive it onto the trailer by unclipping the choke rod from the carb, allowing it to open fully.

When I got it home, I tested the old choke pull-off (holed diaphragm) and ordered a new choke pull-off.
The new unit came, I tested it, compared it with the old one (dimensionally identical), and installed it, miraculously managing not to lose any microscopically tiny clips.
When testing it, I found that the choke closed completely when cold, the pull-off opened it an appropriate amount once the engine was running, and the choke rod pulled back as the car warmed up, but the choke rod (from the bimetallic spring in the exhaust manifold well) stopped pulling back at a certain point, feeling like it had reached a dead-end or a stop inside its housing, and the choke wouldn't open all the way.
With the choke rod disconnected, the choke fell open fully just like it's supposed to.

I took the choke spring housing apart, thinking there was something wrong with it, but nothing seemed amiss, aside from the fact that while the spring pushed the choke closed, it couldn't pull it open. I thought that the end of the spring might be broken, but was advised by those who know, that it was normal.

Since I had taken the choke spring housing completely apart, obliterating any adjustments in the process, I decided to splurge on a used choke spring unit, just so I'd know I had a good one.
The unit came today, It was very nice, and almost not-rusty., and clearly in excellent condition.
I installed it, and – – wait for it – – It worked exactly the same, coming to a hard stop when the engine warmed up, preventing the choke from opening all the way.

I photographed the rear-side of the carburetor (it's hard to see back there, what with the hood coming down to the cowl and all) and compared it to the illustrations in the FSM, and all of the linkage is connected identically, and the choke lever tab is in the same position with regard to the choke plate as in the illustrations.

So... that leaves me stuck. I'm really not sure what's going on, but it looks like something is installed wrong somewhere.
I just can't see where.

I haven't had this carb off, or done anything to mess with it, except for installing a new pull-off.
I have no idea what anyone else might have done, but I'm the fourth owner, and the car is very original, so nobody has done anything wild.

I do plan to rebuild the carb (and disassemble the distributor, and adjust the valves, and replace the fuel pick-up and rubber lines, and re-do all the brakes, and install a dual master, and...), but for now, I'd like to be able to get the car to where my wife can drive it out of the barn if she needs to, because telling her to "just disconnect the choke lever once it startts getting warm" won't go over well.
I can install a manual choke on it, and have done so on many other cars, and have a couple of period-correct manual choke kits hanging from the joists, but I don't want to do that because this car is so original, and I'd rather keep it that way.

Anyone got any ideas?

– Eric

Pics. Your new and old choke thermostat assemblies including the pushrod, and your exhaust manifold, and your carburetor. Pics, pics, pics.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Sorry Dan.

The upload resolution on this server... blah, blah, blah... It just hurts my brain to have to make them so yucky.

At work now, I'll degrade a few and send them along after I get home.

It's a really vexing problem. The exact kind that always has an embarrassingly simple solution.

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Do not read below if you need to be efficient:


[digression]

By the way, how common is it for these things to blow head gaskets or crack heads at low mileage?
I had to pack it in last night when I was messing around (Dinner. Harumph.) and won't be able to check compression and cooling system pressure, and maybe adjust valves, until Tuesday (if I'm lucky).
Exhaust was just a little bit too misty at idle after warm-up, and it seemed to hang in the barn a bit more than I would expect. No smell of antifreeze, but that's not determinative.
Cooling system doesn't develop pressure, but it might not on a cool day at idle.
Uncharacteristically, it kept me up part of the night, contemplating cylinder wall cracks (which should pressurize the cooling system, but, well, you know...).
I don't want to find an ulterior reason why the PO was so cheerful and helpful getting the car onto the trailer and helping me back out of his driveway.
I know there's no answer until I do a compression test, I'm just looking for reassurance that this never happens on these, which I know you can't give me.

I'll advise you when I do find out something objective.

Also: Holy Cow, the timing jumps around about 20 degrees at idle. Makes me think it's a loose points plate.
I won't be disassembling the distributor just yet, but I reviewed the FSM and it's just like every other distributor in the history of mankind, so no biggie. I'm just curious what I'll find.

[/digression]


- Eric

Not common.

If there's antifreeze in the cooling system, and it's being burned, you will smell it. You'll also see it as silica fouling on the spark plugs.



These cars use a partial-pressure cap. See here

More likely a distributor weight missing its spring, or a badly loose timing chain.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Didn't think so. Steel shim head gaskets have very little room for leakage, other than the odd case of rusting.


That's been my previous experience.
At the very least one very clean plug.

But I haven't pulled all the plugs yet (I know: Lazy.).



I'm familiar with them.
Also, this car has an aftermarket one that looks like it's been through a war, so I'll test it when I break out the pressure tester.


Ah. I could see that.

We'll see when I unstick it. Not happening right away, but it's starting to make my brain itch, so who knows?

- Eric

For me, the worst part of that is figuring out exactly what's gone wrong; why the thing isn't doing what it should (or is doing what it shouldn't, etc), with a perfect unbroken chain of cause and effect and a neat list of parts, tools, and operations required to fix it…

…and then that turns out not to be it at all. Reality has a nasty habit of poking ugly holes in beautiful ideas.

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一期一会
Too many people who were born on third base actually believe they've hit a triple.

Agreed.

I usually formulate several alternative hypotheses, and have the tools available (except that ONE thing that I KNEW was right there last week...), but hold off on buying any parts until I am sure of the problem, because so much of the time, once it's apart, I find I'm completely wrong.

– Eric

Just to post a conclusion, for those who may, by some stroke of fate, read this in the future:

After some off-line correspondence with Dan, it became clear that the problem was that the exhaust manifold had been replaced with one from the 1970s, which, apparently, has its choke stove pocket in a very slightly different position than the 1964 manifold, which made the choke rod the wrong length.

Using both the thick and thin carburetor base gaskets combined, instead of the single thin gasket that was on there, created enough distance for the length to be perfect.

The car still runs like crap cold, but its timing is unstable and I doubt it's had a carburetor rebuild in over half a century, so we'll see how it runs after I take the distributor apart and rebuild the carb.

As always, all praise to Dan, who, through experience and acuity, knew right away what the problem was.

– Eric