Rebuilding an AFB Carb

[Bill Stoneberg]

I found this article today about rebuilding AFB Carbs.  Lots of pictures and a fairly generic description that will fit any AFB.

 

http://www.hotrod.com/articles/basics-restoring-carburetor-performance/

 

 

[carbking]

Interesting article. A few things not mentioned:

 

(1) The article mentioned removing the step-up piston, rod, and spring assembly first. They did not say this, but the very last step with the AFB is installing this combination. NEVER, yes never, attempt to place the air horn onto the lower body with the step-up assembly in place! Should you ignore this advice, I make step-up rods, but they are expensive, and you WILL be buying a set!

 

(2) The carb is actually a 700 CFM rather than a 750 CFM, but they probably have fewer Carter documents.

 

(3) A real "gotcha" with the AFB is the orientation of the secondary venturii clusters. They are physically, but NOT functionally, interchangeable. Check the orientation when removing, but check that a previous rebuilder had correctly oriented the secondary clusters. Inspection of the clusters will reveal a small air bleed in one side of each cluster. The air bleeds ALWAYS face toward the outside of the carburetor.

 

(4) One should always check the functionality of the accelerator pump inlet check valve.

 

(5) One should also check wear on the step-up pistons (which come in the better kits). Carter knew that aluminum rubbing against aluminum would create wear. Thus the Carter engineers designed the carburetor such that the carburetor casting is much harder than the pistons. In other words, the pistons are sacrificial. Replacing the pistons often will alleviate a "timing" issue within the carburetor. Vacuum leakage past the pistons can change the timing of the change in rod step diameter, and even cause an over-rich condition if the leakage is great, as the springs will have the rods on the wrong step in all but WOT conditions.

 

(6) The accelerator pump "S" link can be installed backwards. The pump will still function, but wear both the air horn and the accelerator pump.

 

Here is an AFB article I did many years ago (without pictures):

 

http://www.thecarburetorshop.com/AFBtuningtips.htm

 

Jon.

[Bill Stoneberg]

Jon,  thanks for adding to the conversation.  

In your article you state "Since everyone who works on AFB carburetors knows all there is to know concerning tuning the step-up rods, springs, and main metering jets; I will not bore you with explaining these. "

Considering I dont any pointer on this besides just leave the ones alone that are in your carb ?

Thanks for all you add to these forums.

[carbking]

Bill - that comment was made factiously.

 

If the carburetor is on the original engine, and the engine is more or less stock; then using the factory supplied rods, jets, and springs is an excellent idea (after all, engineers interested in keeping their jobs specified the calibrations).

 

HOWEVER: over time, the primary jets wear, the step-up piston springs fatigue, and I mentioned in my previous post about the sacrificial nature of the vacuum pistons. The secondary jets rarely, if ever, wear. I always suggest replacing the primary jets, the step-up pistons, and step-up piston springs during an AFB rebuild IF the carb has 50 or so thousand miles. I have seen a very few of the rods that wore in the area where they pass through the housing, and also in the horizontal leg that is attached to the step-up piston; generally after 250 or so thousand miles.

 

Now, if the engine is highly modified, is a horse of a different color!

 

And, as one might gather from my posts, I really do like the GENUINE Carter AFB carburetors!

 

I have posted before, in one form or another, but a bit about metering rod technology:

 

(1) The dynamic metering area on the primary side of the AFB is the area of the jet less the dynamic area of the rod currently in the jet.

(2) The metering rods come in 2-step, 3-step, and 4-step calibrations. Each step is a different diameter.

(3) Unless one REALLY understands what one is doing, DO NOT INTERCHANGE RODS WITH A DIFFERENT NUMBER OF STEPS!

(4) The step-up piston springs are calibrated to produce a certain force when depressed (pulled down by vacuum). Several different tension calibrations exist.

(5) The spring tension works against the engine vacuum to cause a specific step of the rod to be in the jet; thus the metering may be calibrated for different values of vacuum.

 

Jon.

Edited July 11, 2017 by carbking (see edit history)

[60FlatTop]

I never had enough fingers to put the air horn on with the step up rods in it. Pulling the rods and pistons is the first step. Putting them back in is the last.

 

On one '59 Cadillac I discovered that sometimes you have to beat the vacuum pistons out with a hammer if a fresh restoration was put together with junkyard parts. That one ended up having a sacrificial air horn

 

Bernie

[carbking]

Bernie - a long time ago, one of our suppliers was touting "tumbling" as a method of cleaning castings. They used diamond shaped steel pieces of varying sizes as a media for the tumbling process. As the supplier had a good reputation, I sent them the castings from a junk AFB for testing. When the castings came back, they were CLEAN!!! And every hole was smaller! The step-up pistons would not slide into the airhorn, the throttle shafts would not slide into the throttle body, and every cast number or letter on the carburetor was no longer readable.

 

Tumbling may work well on cast iron, but it certainly was not the cat's meow on aluminum.

 

Jon.

[carbking]

On ‎7‎/‎11‎/‎2017 at 0:17 AM, Bill Stoneberg said:

Jon,  thanks for adding to the conversation.  

In your article you state "Since everyone who works on AFB carburetors knows all there is to know concerning tuning the step-up rods, springs, and main metering jets; I will not bore you with explaining these. "

Considering I dont any pointer on this besides just leave the ones alone that are in your carb ?

Thanks for all you add to these forums.

 

Bill - got this question on another forum as well, so:

 

http://www.thecarburetorshop.com/Carter_metering_rod_technology.htm

 

Jon.

[60FlatTop]

Carburetors are my hobby. Two metering rods and two holes.

When I go to work there are more holes and they are different sizes.

 

So I do the areas with D squared X 1/4 PI (.7854) and do most in my head. I go for the soul of those metering rods.

Which are in a Packard carb that I should be reassembling right now!

Bernie

[Bill Stoneberg]

Thank you Jon for the information.

[XframeFX]

Jon, your statement on castings now has me leaning back towards a Genuine Carter AFB (original) rather than shortcutting  to a larger 750 CFM Performer with airhorn and throttle shafts from my 401 Buick 6XX CFM AFB to use the original air filter:

 

"(5) One should also check wear on the step-up pistons (which come in the better kits). Carter knew that aluminum rubbing against aluminum would create wear. Thus the Carter engineers designed the carburetor such that the carburetor casting is much harder than the pistons. In other words, the pistons are sacrificial. Replacing the pistons often will alleviate a "timing" issue within the carburetor. Vacuum leakage past the pistons can change the timing of the change in rod step diameter, and even cause an over-rich condition if the leakage is great, as the springs will have the rods on the wrong step in all but WOT conditions."

 

Genuine AFB's are pressure formed rather than vacuum formed? Also, a new larger carb will have me tuning to my engine defeating my intent to shortcut carburetor restoration. Using a 625 CFM Performer with my airhorn will kill only 1 bird with 1 stone and leave me with an inferior casting then.

Which way to go? Still deciding . . . . .

A genuine "Thanks" to all expertise  shared above!

John B.

 

[carbking]

John B (don't have a big boat around anywhere do you  ) the throttle shafts from your original Buick carb will NOT work in the clone 750. I am not certain about the airhorn without checking the prints, but while it will fit, I don't believe it will work well.

 

I personally believe you would be happier rebuilding your original carb.

 

Jon.

[XframeFX]

Actually, I plan to change the primary throttle shaft from a smaller 4331S AFB for the bell crank in an upcoming Dynaflow to ST-400 swap. This will allow me to use the variable pitch switch. While I'm at it, I thought I might as well up-size (new 750 clone) or rebuild the original 3503SA AFB.

With so many things going on with my project, I'm not in an experimental mood. Staying with the original carb. Decision made!

Thanks for helping my indecision!

[carbking]

The Buick carb primary side has 1 9/16 throttle plates.

 

The 750 clone has 1 11/16 primary throttle plates.

 

To use the throttle shaft from the Buick carb in the clone, one would need to acquire a slitting saw of the proper thickness for use in one's mill to modify the shaft.

 

Jon.