3269 3461 AFB Carter Carburetor

**Duke W.** · March 22, 2011, 11:08 AM

Re: 3269 3461 AFB Carter Carburetor

Are you saying that the idle speed will not go lower than what you state with the idle speed adjustment screw - i.e. you run out of adjustment because the butterflies are at the limit of travel and will not close down further?

Where are the mixture screws set - number of turns out from the seats?

What is the VAC number, initial timing, and total idle timing?

Duke

**Stuart F.** · March 22, 2011, 02:23 PM

Re: 3269 3461 AFB Carter Carburetor

Tim;

Having trouble invisioning what you mean by two vacuum bleed holes below the air valve in the secondary. I don't recall seeing those in 3461's. Are you sure someone didn't Hot Rod the carb before you got it? Sounds like a trick for a big cam.

Hope you get some more input. Don't believe we settled the hot air tube vacuum bleed controversy yet. I'd like to see more guys post who have changed out their manifold heat tubes.

Stu Fox

**Timothy B.** · March 22, 2011, 05:33 PM

Re: 3269 3461 AFB Carter Carburetor

Stu,

I remember in the past I asked a carburetor restorer about these holes I am refering to and he said they are correct for the 3461. The holes are located just under the weight on the secondary air valve and are drilled into the secondary throttle shaft bore, approx (just guessing) 3/32 or 1/8" diameter.

I wish others would post if they have a 1962 or 63 AFB with these holes, it's definitely a filtered air leak as the vacuum travels around the shaft to find air here. When I squirted carburetor cleaner in this area the motor stumbles almost to a stop. If I cover the secondary with a rag tight the car will flood and leave gas visable in the secondary area because this air draw is forced to suck fuel through the secondary inital discharge and booster venturi.

After seeing your pictures of the restricted hot air tube located in the exhaust manifold I did some testing with my car almost blocking the passage but decided it's not my problem. I believe the hot air draw is good for the choke and the operation is normal.

Duke,

To answer your questions, the micture screws at this 6-800 idle speed are 2 1/4 turns each from seated. Vacuum advance is stock 201 tested with miti vac and inital timing @8*, total idle timing is 23*. I have cleaned the primary venturis checking the air, economiser and IFR.

The throttle blades are completely closed, idle screw backed out but the car will not idle below what's stated. If these small secondary holes are supposed to be there I am thinking this is done for the 340 hp to allow more air while maintaining correct primary throttle blade position. As you are aware the 1962 and 63 afb's are duel purpose and the camshafts are very different.

Do you recall these holes in your 3461??

**John H.** · March 22, 2011, 07:35 PM

Re: 3269 3461 AFB Carter Carburetor

I have a 3461 from my 63-340 HP and a 3721 in front of me right now, and am having a hard time locating the holes you are talking about. I did find two extra holes in the 3461 above the secondary blades in the bores , but not sure about the holes you are talking about. Sorry . - John

**Timothy B.** · March 23, 2011, 08:09 AM

Re: 3269 3461 AFB Carter Carburetor

John,

These may be the holes I am talking about, are they visible from inside the carburetor under the secondary air valve weight and drilled into the top of the secondary throttle shaft bore?

These holes are not present on the 3720-21 carburetors am I correct??

**Clem Z.** · March 23, 2011, 09:03 AM

Re: 3269 3461 AFB Carter Carburetor

pictures would help

**Timothy B.** · March 23, 2011, 09:25 AM

Re: 3269 3461 AFB Carter Carburetor

Here are pics of a 3721SB Carter with a pencil showing the location of the holes I am refering to that are drilled on the 3461 AFB. These holes are not on the carburetor shown or on the 3721S or SA model of which are unique to the 300hp engine.

I believe vacuum going around the throttle shaft is finding air through these holes and that calibrated air leak is to much for the 300hp engine.

This leak would be similar to opening the secondary throttle blades on a Holley to achieve a stable idle after setting the approx primary transfer slot exposure. I believe it may be done for the solid lifter cam which shares this carburetor.

All comments are appreciated, based on what everyone thinks I am probably going to remove the carburetor lid and plug these holes, what's the best way the seal them..

Attached Files

**Clem Z.** · March 23, 2011, 09:45 AM

Re: 3269 3461 AFB Carter Carburetor

Originally posted by Timothy Barbieri (6542)

Here are pics of a 3721SB Carter with a pencil showing the location of the holes I am refering to that are drilled on the 3461 AFB. These holes are not on the carburetor shown or on the 3721S or SA model of which are unique to the 300hp engine.

I believe vacuum going around the throttle shaft is finding air through these holes and that calibrated air leak is to much for the 300hp engine.

This leak would be similar to opening the secondary throttle blades on a Holley to achieve a stable idle after setting the approx primary transfer slot exposure. I believe it may be done for the solid lifter cam which shares this carburetor.

All comments are appreciated, based on what everyone thinks I am probably going to remove the carburetor lid and plug these holes, what's the best way the seal them..

are there flats on the shaft so the air can get thru to the throttle bore? how big are these holes ??

**Timothy B.** · March 23, 2011, 10:03 AM

Re: 3269 3461 AFB Carter Carburetor

Clem,

The shaft is round but IMO the vacuum will draw air through the holes and around the shaft. The secondary shaft has some play but I think it's normal machining tolerence.

By memory, the holes are sized 3/32 +- and I am curious if the 3269 afb used on the 62 has the same holes.

**Stuart F.** · March 23, 2011, 10:05 AM

Re: 3269 3461 AFB Carter Carburetor

Tim;

Have to admit I have never noticed these holes on a 3461S before, primarily because they seem to be in an unusual place for the purpose about which you are speculating. Certainly, an unusual way to afford an air draw to compensate for the Duntov on the 340 hp (L-76) application - an after thought of strange proportions. Wouldn't you think they would just drill the throttle plates to achieve this, as is done on many other carb applications. That way it would be a fixed/calibrated bleed, whereas, at the throttle shaft it seems like it would vary with wear, etc. I know the air valves are not a positive seal by any means in their position above the secondary throttle plates, so certainly enough air could bypass them if the plates were drilled.

I recall I had my 3461 off my engine and apart several times before it was stolen, but it never ran differently with it than it did with my 3720 or 21 with the obvious exception of jetting variations - never an idle speed or vacuum change/concern. I'm close to closing a deal to purchase a rebuilt 3461S now so I will look forward to checking it out.

Stu Fox

**Clem Z.** · March 23, 2011, 10:08 AM

Re: 3269 3461 AFB Carter Carburetor

Originally posted by Timothy Barbieri (6542)

Clem,

The shaft is round but IMO the vacuum will draw air through the holes and around the shaft. The secondary shaft has some play but I think it's normal machining tolerence.

By memory, the holes are sized 3/32 +- and I am curious if the 3269 afb used on the 62 has the same holes.

i would just fill the holes and the area with heavy grease and this would be easy to remove after you test. if not that clean the area good and use a piece of racer(duct) tape over the holes

**Stuart F.** · March 23, 2011, 10:22 AM

Re: 3269 3461 AFB Carter Carburetor

I agree with Clem. Just don't use anything to plug a hole in a carb that you wouldn't want the engine to ingest. I know from experience; I once locked a pin in secondary air valve assembly of a 4GC and my 58 290 hp ate it. Went into #8 and broke every ring land - not pretty.

Let us know how your test goes.

Stu Fox

**Timothy B.** · March 23, 2011, 10:39 AM

Re: 3269 3461 AFB Carter Carburetor

Clem,

Keep in mind these holes are eliminated for the 3720-3721 in 1964-65. As reported in one of my above posts, if I squirt carb cleaner here while the engine is at idle the extra fuel will almost kill the engine. I wish others would report if they have had the same issue with the 3461 and 300hp engine. Between the choke circuit, pcv circuit and these holes it's like flushing a toilet with air for the 300hp. If you don't mind the idle speed at 7-800+- I guess it's not a problem, just close the blades to shut.

My 67 with Holley 3810 does not have all these calibrated air leaks and the car will idle down to stall.

Stu,

I don't think you would notice a problem with idle speed using a 3721 because you can always open the primary throttle blade more to allow extra air for the 340hp cam. Doing this may put the primary throttle blade in a less disarable position possibly creating a stumble. If you expose and vacuum pulls to hard on the transfer slots you lean the curb idle A/F emulsion screws and off idle it may go lean. The starting point is like we have discussed before with the transfer slot just exposed and the primary blades just cracked open to allow small air flow through.

FWIW, I have found the best way to tune a holley is setting the primary throttle blade so the transfer slots are just visible (squares) then open the secondary blades to allow more air if needed. When the A/F emulsion screws are set this A/F to air relationship stays about the same as the blade exposes more of the transfer slot.

**Clem Z.** · March 23, 2011, 10:47 AM

Re: 3269 3461 AFB Carter Carburetor

Originally posted by Timothy Barbieri (6542)

Clem,

Keep in mind these holes are eliminated for the 3720-3721 in 1964-65. As reported in one of my above posts, if I squirt carb cleaner here while the engine is at idle the extra fuel will almost kill the engine. I wish others would report if they have had the same issue with the 3461 and 300hp engine. Between the choke circuit, pcv circuit and these holes it's like flushing a toilet with air for the 300hp. If you don't mind the idle speed at 7-800+- I guess it's not a problem, just close the blades to shut.

My 67 with Holley 3810 does not have all these calibrated air leaks and the car will idle down to stall.

Stu,

I don't think you would notice a problem with idle speed using a 3721 because you can always open the primary throttle blade more to allow extra air for the 340hp cam. Doing this may put the primary throttle blade in a less disarable position possibly creating a stumble. If you expose and vacuum pulls to hard on the transfer slots you lean the curb idle A/F emulsion screws and off idle it may go lean. The starting point is like we have discussed before with the transfer slot just exposed and the primary blades just cracked open to allow small air flow through.

FWIW, I have found the best way to tune a holley is setting the primary throttle blade so the transfer slots are just visible (squares) then open the secondary blades to allow more air if needed. When the A/F emulsion screws are set this A/F to air relationship stays about the same as the blade exposes more of the transfer slot.

having the idle transfer slot either open too far or completely closed will cause a "flat" spot.

3269 3461 AFB Carter Carburetor

3269 3461 AFB Carter Carburetor

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