340 six pack choke issue

1970 t/a all stock. Hard to start cold fires right up after it’s warm. I’m pretty sure the choke isn’t working right being the cause. Everything seems to be adjusted to spec and I can make the choke close completely on a cold engine but it won’t automatically do it like it should when you first pump the gas to prime. The choke valve only closes slightly as if there’s not enough force. If I do it under the hood there doesn’t seem to be any real resistance and I’m able to close it completely once I open up the throttle valves. Any ideas? Can this be adjusted? Like I said when it’s warm and no choke involvement it cranks immediately.

Assuming you are working with a "stock" motor. Make sure that your exhaust crossover passages are open as their heat is required for the choke to function. I will try to walk you through this but a copy of the factory service manual would be very helpful You can download a copy at LINKY
1.Take the air cleaner assy. off and allow the motor to sit to cool down (preferably overnite)
2. Once cool, Block the primary throttle arm open 1/4" or so.
3. The choke butterfly should snap shut completely and have a slight spring tension when trying to open with your index finger.
4. If it does not close with tension the thermostatic spring or arm EDIT: RODneeds adjusting. (I'm not sure which is used for adjusting on your particular application) The amount of tension determines how long the choke stays engaged. (warmer temps require less tension as the motors warn up quicker).


The next 2 adjustments (A, B & C ) are interrelated and where the confusion with setting them up properly comes in.
1. Remove the apparatus holding the primary throttle open.
2. Give it a 2 or 3 pumps, ( may require more). Have a helper ready to cover the carb with a wet towel should it stack fire while Starting the motor.

Now for the interrelated adjustments. (can be confusing)
Once the motor starts, The choke pull off (vacuum operated) should open the choke enough ( usually about 1/4" or so gap) to allow enough air in while keeping the fast idle cam engaged and the motor idling at about 12-1500 RPM.

A. if it is running rough and loading up, try blipping the throttle, if it continues to load up, either the choke butterfly needs to be opened more (adjust the pull off slightly) or the fast idle speed needs to be lowered.
B. If it is idling to high,"blip the pedal once to see if it drops, If not, be prepared to shut it off as the "fast idle" is set too high, lower the setting.
C. If it won't stay running the fast idle needs adjusting, again shut it off and raise the fast idle speed


The three steps above A,B, & C affect each other so it is a balancing act between the Three to accomplish proper choke operation. Keep in mind the 1st section controls how long the choke stays on.
The factory manual will have recommended procedures and measurements to achieve the above. Their recommended adjustments will usually get you pretty close
Final testing needs to be done on a cold motor.
They are quite simple once you understand the various adjustments. The difficulty is getting all 3 where you want them and the fact that once the motor starts to warm up even a little you need wait until it is stone cold to proceed.

have fun

I have bee able to make them work with blocked off passages by loosening the initial spring tension and just using the engine heat for the thermo spring but it can become a bit more challenging and possibly require more pumping of the accelerator pedal to enrichen the mixture enough for the initial start.

The choke doesn’t seem to be closing at all on a cold engine. What do I need to adjust to get it to fall closed once the accelerator is pumped on a cold engine?

If it wont close you could have several problems. Something binding? Take the clip off and see what the choke does. If it closes then I suspect too thick of a base gasket or the wrong choke.

Some of the chokes are adjustable by loosening a screw on the thermostatic coil mounting post, rotating it, and tightening the screw back down. The choke should be removed to determine which you have as it may have been replaced with an aftermarket unit.

Others are only adjustable by bending the rod between the TS coil and choke butterfly.
Or as Nitrous mentioned you may have the wrong choke / rod

Adjust it only when it is cold out, find or buy a factory service manual and follow the factory method

It’s a factory setup. When I disconnect the clip nothing really happens, but I didn’t but press the accelerator so the plate wouldn’t move until I did anyway I don’t think. I’m thinking the spring isn’t strong enough to close the choke plate. It does have some bounce but not a lot.

On a cold motor (sitting overnite)
IF you remove disconnect the clip,
remove the rod from the choke
It should spring upwards.
If it does not, that is why the choke is not closing.
See above post.
from what I could see in your picture, the rod appears straight up which would indicate you may have an adjustable post, (see above)
Or you have the wrong Choke assembly and or Rod.

thinking way back in the cobwebs of my old noggin, i remember "some" [?] of those type of choke mechanisms having a plate with lines on it, and those were adjustable by loosening the retaining nut and slightly winding or unwinding the thermostatic spring.
problem is, i can't remember if that was a factory unit or a parts store replacement.
also, the ones i remember like described above, were on 4bbl intakes. as cheap as Ma was, i wouldn't be surprised the basic mechanism would be similar.
a picture of the spring would be helpful to see if my foggy memory is any way near close.
i'll see if i can find any choke mechanisms on my shelves. i'm curious now about this.

Looks like an original adjustable one. How’s it supposed to be adjusted? Is there a correct position or just play with it?

Lucky you, that is an adjustable thermostatic choke.
Now the next step is to put it back into place and see if you can pull up on the rod.
If you can pull it up to a position that will close the choke,
You will need to remove it and adjust the thermostatic coil.

If not, the rod is too short.
You might be able to gain the needed length by straightening some of the offset bends providing it does not lead to binding
OR
you will need to find the correct one or fab one up

i guess my old, dusty memory "remembered" correctly for a change.
i dug out a 69 FSM and it showed the same choke mechanism as the one you have, including the side shot of it with the nut and the adjusting "lines".
however, when it came to telling how to adjust the choke, it did not show how to adjust the thermostatic spring, although it did mention that the spring was part of making the choke work correctly.
it mainly focused on slightly bending the choke rod and getting the vacuum pulloff set correctly.
it looks like adjusting the spring is a trial and error effort. make sure to mark the spring, nut, adjuster slot and body lines as they are now if you want to try adjusting the choke spring. that will insure you can get back to your original setting if need be.
if you decide to try this, keep us updated on how it works out, please.

The manual may not have dove into the adjusting of the spring much as it will vary quite a bit depending on location and the time of year. IE: Phoenix, likely set it once and leave it. Michigan? summer will require less tension as the motor will warm up quicker and not require the choke vs winter which will require it to stay on longer