Idle jet

[rgalajda]

GTO Geoff wrote ( in Holley section )
Carter patented the 'Economizer' concept. Carter AFBs, AVS, TQ, Rochester QJ & 2bbls use it. Not sure about Stromberg & others. Also called the idle down channel [ IDC ]. It is a jet that controls [ volume ] the A/F mix available to the transfer slot. Sometimes the IDC needs to be increased with big cams etc to avoid surging at cruise. Extremely hard to access on Carters above. On the TQ, it is buried in the air horn, about 1 1/4" in & requires long drill bits. QJ is very easy to access. As far as I can tell, brand H & clones just have a large-ish hole in the met block & not really specifically sized, just made 'big' to cover all possibilities. It is my opinion that this omission by H to properly size the IDC orifice is what makes them gas guzzlers.

Below is picture of a couple primary clusters from AFB carbs. On the left picture I have a .045" pin gauge into the Economizer ( Idle channel restrictor ).This channel runs between the 1st idle air bleed (Air Bypass) and the 2nd idle air bleed that you can see in the 2nd picture. In front of the Economizer is the 1st idle air bleed ( Air Bypass ) which is .052".
I have read different articles or posts suggesting that if you have a lean idle/off idle bog (leaness) than you should drill the Economizer (idle channel restrictor ) larger.
So I have a 4760s Carter Competition Carb 750 cfm. I installed this on my 454 BBC with 227@.050" 107 LSA camshaft. This short LSA camshaft which creates a lumpy idle. Started the car and would not idle . Afr gauge showed 16:1 or worse lean air fuel ratio . Tuned the idle mixture screws out 4 full turns and it idled rough with a lean off idle bog.I knew the booster was original to the carb because I had two of these carbs with same measurements.
Since drilling the Economizer ( idle channel restrictor ) was not reversible and I didn't believe that would cure it ,this is what I did.
Idle jet was enlarged from .032" to .036" and the 2nd idle air bleed on the top ( on the right in picture ) being .056" was drilled and tapped for a set screw. The set screw was drilled to .046" ( reduced from .056" )
Reassembled carb , started engine and idled nicely with a 13.5:1 (aproximately) AFR reading. Drove away nicely.
So the Economizer ( idle channel restrictor ) is not hard to access butt may not be the one to alter, at least in my case.

[GTO Geoff]

Rg.
A couple of comments.
[1] In the 1st pic, the drill bit is sticking out of the idle by pass air bleed. The IDC is behind that brass insert. The IDC sits between the idle bypass air bleed & the air bleed on top of the booster.

[2] The IDC should not affect idle, unless there is an excessive of t/slot showing below the blades. Something sounds very odd that with that mild-ish cam you could get a good idle with the idle screws out 4 turns.

[3] If you have the genuine CS AFB carbs, the mixture screws threads are necked down to allow for larger amounts of fuel+air mix to reach the t/slots. You have to remove the screws to see this.

[rgalajda]

[rgalajda]

[3] If you have the genuine CS AFB carbs, the mixture screws threads are necked down to allow for larger amounts of fuel+air mix to reach the t/slots. You have to remove the screws to see this.
I believe these mixture screws were originally intended as mixture limiting screws. I believe on 60s ( CAP ) chrysler. Probably for California. I have no idea why they used them on the original Carter Competiton Carb.

"Carter 1/4-36 mixture screw.This particular carburetor screw uses an allen screw that sits in the recessed part of this screw. Remove the allen screw 1st."

In the picture below you can see where the allen screw can be screwed in beside the mixture screw to limit adjustment.

[GTO Geoff]

Rg,

Different authors/companies use different terms for the Economizer. Carter calls it Economizer, Rochester the Idle Channel Restriction on the QJ. In his QJ book, Ruggles calls it the IDC. In Roe's QJ book, he calls it the ICR.

The Comp Series 750 AFB I had did not have any king of Allen Key type set up near the mixture screws to limit travel. This was an original 4XXX series carbs. I believe some later model AFBs were released that had the 'Competition Series' sticker across the front of the airhorn, but were 9XXX series. Is this what you have?

The IDC/ICR restriction controls premixed air + fuel, which is delivered to the T slot. If you enlarge it, more A/F is available & vice versa. Usually a 0.040-0.060" hole size. At idle, with T slot position correct, the area of T slot above the blades is much greater than a 0.060" hole, so the ICR hole controls the flow. Enlarging this hole makes more A/F available to the T slot. So I could see this change affecting the idle mixture [ requiring adjustment with the mixture screws ] depending on the amount of T slot showing at idle. Hope this answers your question.

[rgalajda]

Let's try this again. I know you consider yourself to be a carter carb buff.

Your Quote: "Different authors/companies use different terms for the Economizer. Carter calls it Economizer, Rochester the Idle Channel Restriction on the QJ. In his QJ book, Ruggles calls it the IDC. In Roe's QJ book, he calls it the ICR."

We are not talking about Quadrajets. So Economizer (Idle Channel Restrictor) is correct for Carter AFB carburetor.

Your Quote: "The Comp Series 750 AFB I had did not have any king of Allen Key type set up near the mixture screws to limit travel. This was an original 4XXX series carbs. I believe some later model AFBs were released that had the 'Competition Series' sticker across the front of the airhorn, but were 9XXX series. Is this what you have?"

The picture was not my carb. It was a CAP ( Clean Air Package ) carb that used the set screw. I said my carb was 4760s, (below is a picture) not 9xxx.

Your Quote: "If you have the genuine CS AFB carbs, the mixture screws threads are necked down to allow for larger amounts of fuel+air mix to reach the t/slots. You have to remove the screws to see this."

This is not why the mixture screws are necked down. If you take some measurements you would see that the necked down area is not far enough in for fuel to pass that area. Even with the mixture screw all the way in, the channel fuel passage is above the needle portion ,not the necked down area.

1966 Plymouth Service Manual
On CAP carburetors do not remove the idle mixture adjusting screws from the throttle body. These screws have limited travel and will be broken if removed.

[GTO Geoff]

Well I am sorry to trouble you. Won't happen again.