Question on the differences between various AFB carbs for the same engine application.

[Miranthis]

I know that the carbs are/were tuned by Carter and Mopar to each engine. I have a 66 273 4bbl manual trans and it has the AFB 4119s. The Auto trans used the 4120S. Is the only difference the dashpot? How about the metering rods as I understand the A/T version called for a different idle value. Also, at least in the 66 273 ( i can only assume other years as well) the CAP package cars had also had 2 additional carb numbers I assume at least different in the CAP plumbing necessary, and maybe other things. So, I can see why you need 2 (or 4) carbs for the 'same' engine in a year.

But, once they had a year dialed in, why change the carb choice each year? Not much if anything else changed between the 65 and 66 Commando 273, so why the carb changes? I guess my question might be what the year-to-year differences between the 65 and 66 273 Commando AFB carb choices are (3853S vs. 4119s in the M/T and 3854S vs. 4120s in the A/T cars)? The minimal published specs are the same (venturi sizes, jets, metering rods, etc.), so why use a new carb? Something had to change to warrant a new carb number each year, and I wonder what?

 

[chewy]

3853S vs. 4119s are similar with manual secondaries. Choke butterflies are different

3854S vs. 4120s are similar with vacuum secondaries. Edit: Correction. They are manual secondaries as well. Choke butterflies are different.

I gave a 4119 to a member that I had picked up for parts, specifically the choke butterflies, but they were different than my 65 3853.

 

[67Dart273]

When you get into "all" of the different AFBs there are many differences. Some of them had different heat crossover passages to match the manifold, and cause a vacuum leak if mismatched. Have you seen the ones that instead of a mechanical idle screw, have the big idle "air leak" screw in between the two idle jets? That works just like a modern EFI idle solenoid--a controlled air leak to control idle speed

 

[Bewy]

[1] Carter made 505 versions of the AFB.
[2] Some differences would be internal, not visible to the naked eye, such the Economiser jet inside the pri boosters.
[3] All Carter 4 bbl carbs, AFB, AVS & TQ had mechanical secondaries. There was never a Carter vac sec carb. Many models used velocity valves or air valves to control overall secondary flow.
[4] Some AFBs did not have an idle speed screw that was mechanically linked to the pri throttle shaft. These carbs had a large brass screw, centre front, that controlled idle air flow & this controlled idle speed.

 

[Dana67Dart]

so why use a new carb? Something had to change to warrant a new carb number each year, and I wonder what

My best guess is two fold.


One is government intervention, and the other is education. Each year 100,000 + cars were built and driven and issues would arise. The issue if wide spread enough would get addressed the next year.

 

[413]

Look in the factory service manual, some differences shown in there.

You remember the throttle stud is long for an automatic and short for a manual trans. There is a one difference.

Then there is the idle and fast idle speeds. These are set up before they go on the engine. So in hopes when it fires up it needs no screws turned by the mechanics ar the assy plant.

Then the various dash pots and connections.

The 66-67 era has CAP carbs numbers. Then for 1970 was the ECS carbs with different bowl vents and hose connections. Then in 1971 the 383 holleys had different numbers for N96 hoods or not.

The different air adjust screws were around 68-69. All these little differences mean a different part number. And these differences mean little to us swapping carbs later on. It can be done with success.

 

[Miranthis]

[1] Carter made 505 versions of the AFB.
[2] Some differences would be internal, not visible to the naked eye, such the Economiser jet inside the pri boosters.
[3] All Carter 4 bbl carbs, AFB, AVS & TQ had mechanical secondaries. There was never a Carter vac sec carb. Many models used velocity valves or air valves to control overall secondary flow.
[4] Some AFBs did not have an idle speed screw that was mechanically linked to the pri throttle shaft. These carbs had a large brass screw, centre front, that controlled idle air flow & this controlled idle speed.

I went on a bit of a rabbit hole journey with that vacuum secondary post, so glad it was not just me. Hell, we all make mistakes, but as I only have the 4119s with its mechanical secondaries I was without the knowledge to know better. I did see a teardown video on a 4120 and saw the internal dashpot and that is what got me thinking about the differences and reasons for it. I suppose I always knew that the idle values were different for the 66 as when I look up my M/T stuff I also see the A/T stuff, just decided to figure out the "why". I am restricting my search to the 66 that I have and how it differs within the 66 model year and between the 66 and the 65 and 67 273 Commandos that are nominally the "same" engine (until you get to the details, cosmetic and mechanical, as there are many differences)

 

[66fs]

I'll give a detailed answer in a couple days when I get back home. 273 AFBs are kinda unique.

 

[Miranthis]

I'll give a detailed answer in a couple days when I get back home. 273 AFBs are kinda unique.

I noticed that as I have been watching some of the rebuild videos. They do differ from the generic Edelbrock/Carter Performance ones that usually are the subject of the videos probably due to availability and the resulting larger target audience.

I was lucky in that after 35 years of storage the inside of my carb was not that bad. no issues with any fasteners and it appears to be in good order. Just a little bit of gunk in the secondary where the weights move. Just want to do a better job of cleaning out the bowls and then the outside to make it look pretty ( ).

 

[Miranthis]

I just noticed in Roger Kizer's video on replacing the acc. pump in a 65 4bbl Commando 273 (at least Roger's A/T model with the AFB 3854S) that the large hose connection for the crankcase vent is on the BACK of the base whereas my 66 Commando 273 with the AFB 4119S (for an M/T) has it on the FRONT of the base.

 

[cudamark]

When you get into "all" of the different AFBs there are many differences. Some of them had different heat crossover passages to match the manifold, and cause a vacuum leak if mismatched. Have you seen the ones that instead of a mechanical idle screw, have the big idle "air leak" screw in between the two idle jets? That works just like a modern EFI idle solenoid--a controlled air leak to control idle speed

Yup, '65 used the big air idle screw adjustment. "66-7 just had the linkage idle adjustment.

 

[66fs]

65 273 AFBs had the large brass idle speed screw in the front. 66 273 AFBs had the normal AFB configuration. 67 273 AFBs were a slightly tweaked 66. Choke assy were the same on all 273 AFB’s. All 273 AFBs use the same diameter primary and secondary blades. On auto trans AFBs the primary boosters are different, notched, as well as accelerator nozzles. Secondary boosters are all the same as well as the velocity valves. I never messed with the California 273 AFBs, so not much help there.