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56D500boy

Posted 2026-01-31 2:54 PM (#642786)
Subject: Question about the 1958-59 (60?) Carter AFB choke unloaders

Exner Expert 10K+

Posts: 11182
5000

Location: Lower Mainland BC

.
I recently "acquired" (bought) a Carter 2787S AFB from a fellow on the HAMB board. I just picked it up at my mail drop
on Thursday and have been investigating it since.

One of the things that has confused me a bit is the vacuum choke unloader. It appears to be a piston in a chamber at the
top edge of the carb with an arm (rod?) that extends from the chamber to the choke plate. Because these early AFBs did
not have an attached thermostatic choke, they used a bimetal spring coil that lived in a choke well at the intake manifold
cross over. The spring was attached to a long arm that extended up to a short arm at the end of the choke plate shaft to
close the choke when the engine was cold. Upon starting, the choke needs to be pulled off, at least a little. For these early
Mopar AFBs that was done by the choke pull-over/unloader piston.

I would like to investigate the piston (it was stuck when I received the carb) to determine if I need to replace it or just clean
it up. There is a plate at the external end of the piston chamber but I don't know if there are any tricks to remove the plate
without damaging it. None of the rebuild kits that I have found so far include that piston or plate (mostly geared to the external thermo-
coil type thermostats and their internal vacuum pull-off system).

I hoping for information about the pull off piston on my 2787S but it would be the same for a bunch of 58-59 (and maybe 60
Mopar Carter AFBs) like these (probably all on low deck B blocks, e.g. 350, 361 and 383)

2642S = 1958 LD2-D500, LD2 w/Power Package, LD3

2773S = 1958 LSI, w/Power Package; 1959 MD3-H manual trans, MS1, w/Power Package

2787S = 1959 MD3-H Automatic

2794S = 1959 Dodge D500, 1959 MS3 Automatic

This what I am talking about (on my "new" 2787S):

(NOTE: Everything moves now after a bit of love from WD40 and Lloyd's MOOVIT)

Edited by 56D500boy 2026-01-31 2:59 PM

(DavesCarter2827SAFB_ShowingVacuumChokeUnloader.jpg)

(DavesCarter2827SAFB_ShowingVacuumChokeUnloader_PistonCap.jpg)

Attachments
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DavesCarter2827SAFB_ShowingVacuumChokeUnloader.jpg (132KB - 114 downloads)
Attachments

DavesCarter2827SAFB_ShowingVacuumChokeUnloader_PistonCap.jpg (137KB - 96 downloads)

LostDeere59

Posted 2026-01-31 7:11 PM (#642789 - in reply to #642786)
Subject: RE: Question about the 1958-59 (60?) Carter AFB choke unloaders

Extreme Veteran

Posts: 433
100

Location: Hilltown, PA

It's been a very very long time since I had one of those in my hands, much less had one apart and performed a rebuild, so take everything I say with a grain of salt.

To my recollection that pull-off is what we used to call a "dry" piston, meaning that it had no seals or serviceable components - it was simply a metal piston with a reasonably close tolerance in a metal bore.

The vacuum source was obviously an internal passage in the carburetor, but it was routed to a place in the base plate or throat where its effect would be equal on both sides of the carb. It was essentially a non-adjustable vacuum leak that was taken into account with the rest of the carburetors calibration.

As far as servicing the piston you didn't remove the staked on cover and clean the piston and bore - back then we used chlorinated carb cleaners in buckets and you would simply leave the parts in the bucket overnight, and by morning Voila! - everything just like new. The cleaner was able to sufficiently access and dissolve deposits through the vacuum supply passage and between the piston and bore.

You didn't include a photo from the arm side of the piston, so I can't say one way or the other, but it is also possible that if you remove the cotter from the choke plate you can remove the arm and piston from the bore towards the choke plate . . .

Gregg

Edited by LostDeere59 2026-01-31 7:14 PM

56D500boy

Posted 2026-02-01 11:14 AM (#642793 - in reply to #642789)
Subject: RE: Question about the 1958-59 (60?) Carter AFB choke unloaders

Exner Expert 10K+

Posts: 11182
5000

Location: Lower Mainland BC

.
Thanks for that Gregg. I had just come up from the basement last night, figuring out that that cap was staked into the piston bore, when
I read your posting (above). Yup. Staked. Presumably with the intention that there was no need in the future to pull the piston out.

Presumably.

As for pulling the piston out towards the interior of the carb, NOPE. There is only a small slot for the piston rod/arm to slip out of the
bore and go on to the choke flap connection (cotter pin).

The photo below has the cotter pin out. I was feebly attempting to push the rod and piston towards the staked cap. No prying tools
involved, just my fingers. NOTHING. (Obviously)

Now my "problem" is knowing that if the piston is moving smoothly in the bore (now), will I resist taking that cap out and freeing
the piston and rod just so I know for 101% sure?

It will be tough to resist.

Edited by 56D500boy 2026-02-01 11:17 AM

(DavesCarter2827SAFB_ShowingVacuumChokeUnloader_PistonCap_Stakes.jpg)

(DavesCarter2827SAFB_ShowingVacuumChokeUnloader_FromInside.jpg)

Attachments
----------------

DavesCarter2827SAFB_ShowingVacuumChokeUnloader_PistonCap_Stakes.jpg (210KB - 127 downloads)
Attachments

DavesCarter2827SAFB_ShowingVacuumChokeUnloader_FromInside.jpg (150KB - 90 downloads)

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