AFB Carburetor Question

[carbuff]

See attached photo showing the front of my AFB Carburetor. The two smaller screws are idle jet screws. What is the larger screw between the idle jet screws? Can’t seem to find any info on it.

 

[Professor Fate]

Not an AFB, it's an AVS. Center screw is an air bleed control screw, although I've never seen a hex-head one before...
It basically leans the carb out just off-idle, see the FSM for adjustment procedure.

 

[cudamark]

Idle air screw....yes. It is an AFB though....not an AVS.

 

[cudamark]

By '66 they did away with it.

 

[carbuff]

Definitely an AFB. So opening that leans out the mixture?

 

[carbuff]

Said that wrong. So as you open that, you bleed off air and have a richer mixture?

 

[RustyRatRod]

I believe that is a 68 only carburetor. The center screw is for a final "fine" adjustment of the air/fuel mixture after you get the two outside screws set.

 

[carbuff]

I have done some research. I do not know the details of how it is done; but here is what I learned. All adjustment screws are “clockwise-lean”. The two idle screws adjust the flow of a predetermined air/fuel mixture to the emulsification process (which adds more air) which determines overall air/fuel mixture. Clockwise turn reduces flow of this air/fuel mixture thus leaning out overall air/fuel mixture. The air bleed screw actually adjusts the predetermined mixture by bleeding off air into the mixture. It has reverse threads so a clockwise turn opens the passage allowing more air into that predetermined mixture thus going leaner.

 

[Vaanth]

See attached photo showing the front of my AFB Carburetor. The two smaller screws are idle jet screws. What is the larger screw between the idle jet screws? Can’t seem to find any info on it.

View attachment 1716371950

The large screw is the By-pass idle circuit adjustment:

Adjustment:

Reference:

 

[Bewy]

Pretty sure that large screw is idle air bypass. It is rt. hand thread. Those carbs that had left hand thread screws that I have seen are AVS carbs, not AFB & the screw is near the top, not the bottom of the carb. Head of the screw is normal size, not enlarged.
My GTO uses the same type of AFB carb, does NOT have an idle speed screw on the driver's side of the carb body, although the casting is there.
The large centre screw adjusts the idle speed & outer screws trim the idle mixture. These carbs usually have thick[er] pri t/blades with machined sections.

 

[RustyRatRod]

The large screw is the By-pass idle circuit adjustment:
View attachment 1716372098

Adjustment:
View attachment 1716372100
View attachment 1716372099

Reference:
View attachment 1716372097

Thanks. I knew it had a name. I couldn't remember what it was. To the OP, this is very simple. Get a vacuum gauge and just start playing around with the screws. You will catch on really quickly what they do.

 

[Mattax]

In addition to the FSM for that has that carb, and the Carter pictured above, you can reference the

Master Technician Service Conference - Chrysler's Training for Mechanics

The Imperial Mailing List Online Car Club is dedicated to the preservation, appreciation and restoration of Imperials and Chrysler Imperials of all years, models and body-styles.

www.web.imperialclub.info

For an example with a single idle air adjustment screw see the 1970 Carb booklet p.6

same booklets and and films available in pdf and youtube at Mymopar.com - but no index there.