Re: Secondary butterflies

[Dan McCormack <mcwheels@xxxxxxxxxxxxxx>]

Sean I just found this on a Google search. Note the second sentence in the 
second paragraph.

Dan

The Carter AFB (Aluminum Four Barrel) was the second four barrel carburetor 
produced by Carter, after the WCFB.  AFB's were appeared in 1957 and were 
used as original equipment on most makes of car at one time or another up to 
1971.  The last notable OEM application was as a dual-quad setup on 1971 
Dodge 426 Hemi engines.

The Carter AVS (Air Valve Secondary) is a refinement of the basic AFB 
design. Whereas the AFB used a velocity valve, which is a counterweighted 
flap below the secondary venturi, the AVS used an air valve, which is a 
spring loaded flap above the venturi.  The purpose of the two is similar, 
but the air valve is adjustable from outside the carb.
When the throttle is opened so that the secondary throttle plates begin to 
open, vacuum develops under the air valve, slowly pulling it open.  In this 
way, the air flow is controlled and the mixture is kept from getting too 
lean if the throttle is opened suddenly.

Carter AVS carbs appeared in 1966 on the 275 HP version of the Chevrolet 
327, but were never again used on Chevies after that.  AVS carbs were found 
mostly on Mopar applications from 1968-1971. The most sought-after AVS carbs 
are the 800 cfm models found on 440 CID motors.



----- Original Message ----- 
From: "Sean B" <polecat2@xxxxxxxx>
To: <1962to1965mopars@xxxxxxxxxx>
Sent: Sunday, April 06, 2008 8:35 PM
Subject: RE: Secondary butterflies


> John,
>
> Thanks for the detailed explanation.  Do you know how the early-mid
> sixties AFB's worked, since they had no secondary air valve of any kind?
> I can't seem to find an answer.
>
> Sean
>
>
> althausjbl@xxxxxxx wrote:
> > Rich,
> > I know everyone has had their say on this, but the AVS is to prevent the
> > "bog" which isn't really a bog. but a lean stumble. These carbs do not
> > have a secondary accelerator pump so as the secondaries open they must
> > start drawing fuel to keep the mixture correct. Holley solved this
> > problem by going to a "double pumper" which has a accelerator pump on
> > the secondaries as well as on the primaries. The AVS allow the air flow
> > to slowly match to the fuel available rather than opening suddenly and
> > going lean. I'm sure that the weights are on the conservative side as
> > evidenced by the Edelbrock tech telling you to drill them to open the
> > sooner, but with out them you will get the lean stumble when suddenly
> > going WOT.? If your old enough to remember the old "screw in the
> > linkage" in the vacuum secondaries of Holleys you saw the same stumble
> > you will experience.? Long and short of this, leave the AVS in and work
> > on opening timing if your looking for the most from those carbs.
> >
> > John Althaus
> > 64 Savoy Hemi 4 spd


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