Choke thermostat problem

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French demon

French demon

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Hi all

I installed a freshly rebuilt Carter BBS carb on my slant six and it works well
But, I have a problem with the choke thermostat.
I purchased one brand new on Ebay some months ago and of course, I installed it on the new carb.
I precise that I've never used this thermostat before.

ressort starter.jpg

,

Cold engine, it closes the choke flap with no problem.
But with a hot engine, the rod doesn't pull sufficiently on the flap and this one doesn't open a lot. It stays nearly closed.

How do these thermostats work? I suppose that when the exhaust manifold is hot, the spring fully retracts and pulls on the choke flap.
This one doesn't seem to work correctly and it's not adjustable.
So, what's the problem?
Are the thermostats for Carter BBS and Holley 1920 the same?

:thankyou:
 
Last edited:
There is usually an adjustment on the spring inside. The spring tension can be increased or decreased. Sounds like you should decrease the spring pressure.
 
Murray said:
There is usually an adjustment on the spring inside
Click to expand...
It is possible as Original ones were adjustable. Aftermarket,maybe.
Worst case if not adjustable is to bend the rod as needed making it shorter decreasing tension.
Adjust so that the choke flap just closes with light tension. All you can do is try.
 
Yes, it's aftermarket and it doesn't seem to be adjustable, but I will check.
Bending the rod is a also good idea.
I didn't think about that...
Will try this tomorrow.
Thanks a lot Murray and Darter :thumbsup: :thankyou:
 
Bending the rod is a bad idea, don't do that. It is the correct length and shape as it comes from the factory.

There is an adjustment on the other side of the housing, and the correct adjustment is specified in the service manual.

The choke thermostat only pushes the rod (to close the choke flap); it does not pull the rod (to open the flap), beyond the pull exerted by gravity when the thermostat's spring tension reduces as it warms up. The choke flap is pulled open by the choke pull-off (vacuum pod near the top of the carburetor, with a short, U-shaped link to the choke lever).

If you are using the wrong gasket between the carburetor and the intake manifold (too thin, 3mm instead of 8.5mm) the choke thermostat pushrod will effectively be too long.

(A № 1231 electric choke kit is a better setup than the factory-type thermostat)
 
Hello Dan
If I understand correctly, when cold, the thermostat spring push onto the choke flap to close it and when hot this push slowly decreases.
In parallel, the vaccum pod pulls onto the flap to open it.

In my case, the push doesn't decrease. At least, not a lot. The flap isn't fully open, but rather 1/4 open, and 3/4 closed.

I have the 1970 FSM (sadly not the 1971 one) and page 14-122 it talks only about "2 notches rich". I don't see any other adjustment.
I will check tomorrow if any adjustment is possible

About the gasket, effectively it's thin. Around 3mm. But I have 2 or 3 other gaskets. May I pile up them to obtain the right height?
 
That unit is opened by heat in the mounting surface. which is controlled by exhaust in the carb heater under the intake, which is governed by a flapper valve in the top of the exhaust passage nearby. If there is anything wrong in that multi-part system, you get this trouble.
Later models were electrically assisted.
 
French demon said:
Hello Dan
If I understand correctly, when cold, the thermostat spring push onto the choke flap to close it and when hot this push slowly decreases.
Click to expand...

Correct.

French demon said:
In parallel, the vaccum pod pulls onto the flap to open it.
Click to expand...

Yes, plus the choke flap is offset-mounted on the shaft, so when the engine starts, airflow past the flap pushes on the long/downhill side of the flap more than on the short/uphill side, thus pushing the flap open.

French demon said:
In my case, the push doesn't decrease. At least, not a lot. The flap isn't fully open, but rather 1/4 open, and 3/4 closed.
Click to expand...

The choke thermostat must be adjusted correctly, and you need the correct gasket thickness under the carburetor.

French demon said:
I have the 1970 FSM (sadly not the 1971 one)
Click to expand...

Close enough.

French demon said:
and page 14-122 it talks only about "2 notches rich".
Click to expand...

Yes. loosening the adjustment nut allows you to increase or decrease the spring pressure on the choke rod. There is a long central index mark, and shorter marks on both side of it. "2 notches rich" means the pointer on the movable part of the spring assembly should be aligned with the second short mark in the direction of more spring pressure.

French demon said:
About the gasket, effectively it's thin. Around 3mm. But I have 2 or 3 other gaskets. May I pile up them to obtain the right height?
Click to expand...
Yes, you can use 3 of those 3mm gaskets. Make sure they are all the correct kind, with two small bites out of the big central hole plus a 6.4mm hole off to the side which aligns with the 6.4mm hole in the intake manifold, next to the big main hole. Make sure all three gaskets are aligned correctly with the manifold and with each other.
 
Oh, one other thing, FD: if your car has automatic transmission, and the linkage between the carburetor and the gearbox was adjusted with the too-thin carb base gasket, then when you raise the csrb with the thicker gasket the car will upshift early and downshift late. A new linkage adjustment will fix it.
 
slantsixdan said:
Yes. loosening the adjustment nut allows you to increase or decrease the spring pressure on the choke rod. There is a long central index mark, and shorter marks on both side of it. "2 notches rich" means the pointer on the movable part of the spring assembly should be aligned with the second short mark in the direction of more spring pressure.
Click to expand...

Hi Dan
Effectively, there is a long central index and shorter marks on both side of it, between L and R for Lean and Rich.
But I don't see any pointer on the spring assembly.
It's blank
How may I adjust it?
20241209_182012.jpg


This thermostat is not junk, it's manufactured by Carter :)
20241209_182025.jpg
 
I'd just like to mention that any adjustments and set-up needs to be done when stone cold, I do it first thing in morning.
I was taught, read, and experienced that trying to set it with any warmth would be luck.
Good luck.
 
After sleeping on it a couple nights: maybe there is an index mark, but it is so far away from where it should be that it can't be seen through the window. If that can be checked without loosening the adjuster nut, great. If it can't, I would not loosen the adjuster nut until there is no other option but to try making an adjustment (that is: after the carburetor is elevated to the correct height).
 
No, it's easy to check the spring assembly from underneath and sadly, the spring housing is definitely blank... No index mark.

Before trying something else, I plan to rise the carb to the correct height.
First, by piling up the three thin gaskets and then by installing the NOS high gasket I ordered on Ebay (thanks to Townsend who gave me the link) as soon as I get it here.
Mopar gasket.jpg
 
From a old Carter catalog, 135 are the last digits of the 70-71 Carter part number.
Stamped number 48, last two digits of Chrysler part number for 70-71 choke: 2946548.
 
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