[vertbird]

Guys, I have a problem I need some help with. It's on a 65 Bonneville that has a Carter AFB. I've had it apart, cleaned it up, but on disassembly I failed to clarify the secondary venturi configuration and the correct gasket. The only difference on the venturi is the little hole I've hi-lighted. Can you please tell me if the picture I have would have the correct venturi position and should I be using gasket a, b or c? I have added just basic picture of entire carb lower body for reference. Thank you!!

[steve25]

Gasket C is the correct one.

[vertbird]

Thanks Steve, do I have the Venturi's correct or are they reversed?

[shaker455]

Looks like your rear clusters are on the wrong sides.
look on either side of the booster inside the ring,, you will see a hole or brass tube.

They have to be on the inboard side

[vertbird]

So this little hole should be on the inboard (toward the center) of the carb?

[shaker455]

Yes, if you were looking down on the carb from the front that would be on the right.
And if you look at the carb there is a passage / groove that matches the one in your cluster

[vertbird]

I'm not usually this much of an idiot but if the booster hole is on the inner side, then the slot in the body doesn't line up?

[shaker455]

You got the orientation right now so assemble and back into service

[vertbird]

Ok, will do, thanks!

[389]

I've always wanted to modify an AFB carb, make it a double pumper with a dual feed.

[steve25]

The thing to do to those secondary clusters is to pick up a nice chunk of air flow by thinning down those booster supports by like 1/16” on the left and right side of each one.

[694.1]

Originally Posted by steve25

The thing to do to those secondary clusters is to pick up a nice chunk of air flow by thinning down those booster supports by like 1/16” on the left and right side of each one.

Intriguing. Just slim them down? Would you see a diff on the Dyno?

[Mike Davis]

Originally Posted by 694.1

Intriguing. Just slim them down? Would you see a diff on the Dyno?

May not see much on the dyno. I am running 750AFB's on my 66. By thinning down the booster support leg, milling off the choke horns I picked up almost 45 cfm. My carbs flowed around 745, now after the mods they flow 795. The car did pick up .10 afterwards.

[Kenth]

Originally Posted by steve25

Gasket C is the correct one.

WRONG!
Gasket b is correct for the secondary venturi cluster.
Gasket c is for the primary venturi cluster.

FWIW

[vertbird]

Hey Kenth, I've already used "c" gaskets and reassembled. It runs ok but bogs some on the secondaries, is it possible that the gasket could be responsible and I should have used "b"?

[Kenth]

You should not expect the carburetor to work as intended with the wrong gaskets under the venturies.

[steve25]

Someone needs to explane to me how it is that adding a second fuel feed to be of these Carbs all of a sudden makes another accelerator pump show up as stated in post #10?

[carbking]

Someone may have looked at a mid-1960's Lincoln carburetor that some call a "double pumper"; because it has two "accelerator pumps".

IT DOESN'T!

Lincoln from about 1963 through 1968 used Carter AFB carburetors with an internal dashpot. The internal dashpot LOOKS like a second accelerator pump. However, if one looks closely at the "pump", one will see three calibrated bleed holes in the stem of the "pump"'; one at one end and two at the other end of a vertical internal passage containing a bleed ball. The dashpot is connected via steel linkage to the primary throttle shaft, and the "pump" pushing against the fuel in the "pump well" prevents the primary throttle from closing until the calibrated bleeds allow the fuel beneath the dashpot to flow through. Like other dashpots, the purpose is to slow the closing of the throttle, preventing stalls under certain conditions.

There is no internal passage from the dashpot cylinder to the accelerator pump discharge jets, and I see no easy way of machining one. Not saying it could not be done, but it certainly would not be easy, and why bother???

A "double pumper" AFB would be a solution to a non-existent problem! Carter used a number of different diameter, and different functional length accelerator pumps. If one needs additional fuel, one simply either extends the functional length of the pump, or machines the pump cylinder and installs a larger diameter pump, or both; to increase the volume of fuel in the pump well displaced by the pump.

And not that it really matters for the purpose of the question, but there were two different dashpots: 64-196 and 64-333. I have been making them both for more than 40 years, as they need to be in the rebuilding kits; and I have never seen a FLAPS kit that contained one.

Jon.

[61-63]

You can also just drill out the accelerator pump tubes if you want a bigger shot when you depress the gas pedal, but don't go too big. I haven't measured them all but the ones on the early sd carbs are .027 or something like that. I've drilled them out to .031 and you would be surprised at how that increases the pump shot and it DOES increase the way the engine jumps when you blip the throttle arm.

Af far as the engine bogging when an AFB goes into the secondaries lightening the secondary air valves is one way of fixing that. Also on dual quad sets the secondary air valve in the secondary carb should weigh less than the one in the primary carb, and both should have a little weight taken off if the carbs are run in tandem.

[carbking]

As John (61-63) stated, the pump jet may be drilled oversize.

Pump jets on Pontiac carbs ranged at least from 0.025 to .033 (I also have not checked them all).

Be advised that drilling out the jet does NOT change the volume of the pump squirt, rather the time it takes for the squirt to be dispersed. Drilling without corresponding volume increase might result in an initial surge, following be a delayed hesitation.

John - no offense meant, but we have found that the way to eliminate secondary hesitation is to start with Pontiac air valves, which are HEAVIER than Chevrolet and with a smaller attack angle.

Since I have not tried to make Chevrolet AFB's work on a Pontiac, I have no knowledge if lightening the weights would help with the more aggressive attack angle used by Chevrolet.

We did put a set of 625 Comp Series on a racing only 455 that improved E.T. by removing the lighter Comp Series air valves (Chevrolet), and replacing them with the heavier Pontiac air valves with the smaller angle.

I really don't want to say that either way is the best; the above is just the experience I have.

This fits into the category of individual testing.

EDIT: the telephone rang, and I took an order, and didn't finish this post. The race set built mentioned earlier was done about 30 years ago, and now I remember a set of 750's we built for a SD swiss cheese car that also benefited from the heavier valves. This was maybe 40 years ago. What I am trying to say in too many words, is that cams, tires, head design, etc. have changed; and for this reason the experience we had with these 30~40 years ago, may no longer be valid. Individual testing is my recommendation!

Jon.